Light modulation element

Abstract

[Technical Problem] An optical modulator is provided that can suppress electrode loss at low frequencies below 50 GHz and suppress radiation loss at high frequencies above 50 GHz. [Solution] An optical modulator (1) includes a substrate (2) and at least one interaction portion (MZ) disposed on the substrate (2). The interaction portion (MZ) includes: first and second optical waveguides (10a, 10b) adjacent to each other formed on the substrate (2), and first and second signal electrodes (7a, 7b) disposed opposite the first and second optical waveguides (10a, 10b), respectively. No ground electrode is configured in a region (NZ) near the interaction portion (MZ), and a ground electrode is configured near at least one of an input portion and a terminal portion electrically connected to the first and second signal electrodes (7a, 7b).

Description

Technical Field

[0001] The present invention relates to an optical modulation element, and in particular to an electrode structure of a Mach-Zehnder type optical modulation element. Background Art

[0002] With the rapid growth of the internet and the rapid increase in communication volume, fiber-optic communications have become increasingly important. Fiber-optic communications convert electrical signals into optical signals and transmit them through optical fibers. They offer high bandwidth, low loss, and strong noise immunity.

[0003] Known methods for converting electrical signals into optical signals include direct modulation using semiconductor lasers and external modulation using optical modulators. Direct modulation requires no optical modulator and is cost-effective, but has limitations in high-speed modulation. Therefore, external modulation is used for high-speed, long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder optical modulator, in which an optical waveguide is formed by diffusing Ti (titanium) near the surface of a lithium niobate single crystal substrate, has been put into practical use (see, for example, Patent Document 1). A Mach-Zehnder optical modulator utilizes an optical waveguide (Mach-Zehnder optical waveguide) with a Mach-Zehnder interferometer structure—a technique in which light emitted from a single light source is split into two beams that pass through different paths and then reunite to produce interference. High-speed optical modulators exceeding 40 Gb / s are already commercially available, but they suffer from a significant drawback of being approximately 10 cm in length.

[0005] In contrast, Patent Documents 2 and 3 disclose Mach-Zehnder optical modulators using a c-axis-oriented lithium niobate film. Compared to optical modulators using a lithium niobate single crystal substrate, optical modulators using a lithium niobate film can achieve significant miniaturization and lower driving voltage.

[0006] Figure 12 Figure (a) shows the cross-sectional structure of a conventional optical modulator 20A described in Patent Document 2. A pair of optical waveguides 22a and 22b composed of lithium niobate films are formed on a sapphire substrate 21. A signal electrode 24a and a ground electrode 24b are disposed above the optical waveguides 22a and 22b, respectively, with a buffer layer 23 interposed therebetween. This optical modulator 20A is a so-called single-drive type element having a single signal electrode 24a. The signal electrode 24a and the ground electrode 24b are symmetrically arranged, ensuring that the electric fields applied to the optical waveguides 22a and 22b are equal in magnitude and opposite in sign, preventing wavelength chirp in the modulated light. However, due to the small area of ​​the ground electrode 24b, the optical modulator does not operate at high frequencies.

[0007] Figure 12(b) shows the cross-sectional structure of a conventional optical modulator 20B described in Patent Document 3. Two signal electrodes 24a1 and 24a2 are arranged above a pair of optical waveguides 22a and 22b formed of lithium niobate films, sandwiched between a buffer layer 23. Three ground electrodes 24c, 24d, and 24e are spaced apart from the signal electrodes 24a1 and 24a2. By applying voltages of equal magnitude and opposite sign to the two signal electrodes 24a1 and 24a2, the electric fields applied to the pair of optical waveguides 22a and 22b are equal in magnitude and opposite in sign, preventing wavelength chirp in the modulated light. Furthermore, the chirp amount can be adjusted by adjusting the voltage applied to the pair of optical waveguides 22a and 22b. Furthermore, since the area of ​​the left and right ground electrodes 24c and 24d is sufficiently large, the structure is capable of operating at high frequencies. However, the light modulation element 20B is a dual-drive type element having two signal electrodes 24 a 1 and 24 a 2 , and therefore the electrode structure becomes complicated.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent No. 4485218

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-195383

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2014-6348 Summary of the Invention

[0013] Technical problem to be solved by the invention

[0014] In optical modulators, increasing the speed from the current 32 Gbaud to 64 Gbaud requires widening the bandwidth to over 35 GHz, and increasing the speed to 96 Gbaud requires widening the bandwidth to over 50 GHz. To achieve this wide bandwidth, (1) reducing electrode loss at high frequencies, (2) matching the speed of light and microwaves, and (3) impedance matching are crucial, with (1) being particularly important. This is because, at high frequencies, current flows only near the electrode surface due to the skin effect, increasing electrode loss.

[0015] exist Figure 12 In the conventional optical modulation element 20B shown in (b), increasing the cross-sectional area of ​​the signal electrodes is effective in reducing electrode loss. To this end, the thickness T of the signal electrodes 24a1 and 24a2 needs to be increased, or the width W of the signal electrodes 24a1 and 24a2 needs to be widened.

[0016] However, increasing the thickness T of signal electrodes 24a1 and 24a2 raises the issue of speed matching due to a decrease in the effective dielectric constant of microwaves, and impedance matching due to a decrease in impedance. Furthermore, increasing the width W of signal electrodes 24a1 and 24a2, while the effective dielectric constant and impedance of microwaves do not change significantly, the efficiency of the electric field applied to the optical waveguide deteriorates, leading to an increase in the half-wave voltage. Therefore, with existing electrode structures, achieving broadband bandwidths above 35 GHz, corresponding to the high-speed transmission of 64 Gbaud, is difficult. In particular, in high-speed communications exceeding 64 Gbaud, the dramatic increase in radiation loss at high frequencies above 50 GHz becomes a problem.

[0017] Furthermore, when multiple optical waveguides are arranged side by side to form an optical waveguide array for miniaturization and multiplexing of optical modulators, the proximity of the optical waveguides between adjacent channels increases ripple and crosstalk noise in the EO characteristics, deteriorating high-frequency characteristics.

[0018] Therefore, an object of the present invention is to provide an optical modulation element capable of suppressing electrode loss at low frequencies below 50 GHz and suppressing radiation loss at high frequencies above 50 GHz.

[0019] Technical means to solve the problem

[0020] To address the aforementioned issues, the optical modulator of the present invention is characterized in that it includes: a substrate; and at least one interaction portion disposed on the substrate, the interaction portion comprising: first and second optical waveguides formed adjacent to each other on the substrate; and first and second signal electrodes disposed opposite the first and second optical waveguides and to which differential signals are applied, wherein no ground electrode is disposed in a region near the interaction portion, and a ground electrode is disposed in a region near at least one of an input portion and a terminal portion electrically connected to the first and second signal electrodes.

[0021] The present invention not only reduces ripple and crosstalk noise at low frequencies below 50 GHz, but also suppresses radiation loss at high frequencies above 50 GHz. This makes it possible to provide an optical modulation element capable of high-speed communications exceeding 64 Gbaud.

[0022] In the present invention, it is preferred that the input portion include first and second signal electrode pads, and the ground electrode includes a first ground electrode pad adjacent to the first signal electrode pad and a second ground electrode pad adjacent to the second signal electrode pad. This structure can reduce radiation loss and leakage loss on the input portion side of the first and second signal electrodes.

[0023] In the present invention, it is preferred that the first ground electrode pad and the second ground electrode pad are electrically connected. In this case, the first ground electrode pad and the second ground electrode pad can be electrically connected via a first short-circuit pattern provided on the substrate, or via a ground line within a drive circuit that applies the differential signal to the first and second signal electrode pads. This further stabilizes the reference potential on the input side of the first and second signal electrodes, enabling the realization of an optical modulator with excellent high-frequency characteristics.

[0024] In the present invention, preferably, the terminal portion includes first and second terminal electrode pads, and the ground electrode includes a third ground electrode pad adjacent to the first terminal electrode pad and a fourth ground electrode pad adjacent to the second terminal electrode pad. This structure can reduce radiation loss and leakage loss on the terminal portion side of the first and second signal electrodes.

[0025] In the present invention, the third and fourth ground electrode pads can be electrically connected via a second short-circuit pattern on the substrate, or via a ground line within a terminator connected to the first and second terminal electrode pads. Alternatively, the third ground electrode pad can be connected to the first ground electrode pad via a third short-circuit pattern on the substrate, and the fourth ground electrode pad can be connected to the second ground electrode pad via a fourth short-circuit pattern on the substrate. This stabilizes the reference potential on the terminal side of the first and second signal electrodes, enabling the realization of an optical modulator with excellent high-frequency characteristics.

[0026] In the present invention, the region near the interaction portion is preferably an area within a range of five times or less the distance between the first and second optical waveguides from the center of the interaction portion. In the absence of a ground electrode in the region within a range of five times or less the distance between the waveguides from the center of the interaction portion, ripple and crosstalk noise can be suppressed at low frequencies of 50 GHz or less.

[0027] In the present invention, the substrate is preferably a single crystal substrate, and the first and second optical waveguides are formed of a lithium niobate film formed in a ridge shape on the substrate. Furthermore, the interaction portion preferably includes: a waveguide layer including the first and second optical waveguides and formed on the main surface of the substrate; a buffer layer formed at least on the upper surface of the first and second optical waveguides; and an electrode layer including the first and second signal electrodes and formed on the upper surface of the buffer layer, the first and second signal electrodes facing the first and second optical waveguides, respectively, across the buffer layer. This enables the realization of a compact optical modulation element with excellent high-frequency characteristics.

[0028] Effects of the Invention

[0029] According to the present invention, it is possible to provide an optical modulation element capable of suppressing electrode loss at low frequencies of 50 GHz or lower and suppressing radiation loss at high frequencies of 50 GHz or higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic plan view showing the structure of the light modulator according to the first embodiment of the present invention. Figure 1 (a) shows only the optical waveguide. Figure 1 (b) shows the entire optical modulation element including the traveling wave electrodes.

[0031] Figure 2 It is along Figure 1 (a) and (b) are schematic cross-sectional views of the light modulation element 1A taken along the X1-X1 line.

[0032] Figure 3 (a) and (b) represent the transmission loss (S dd21 ) is a coordinate diagram of the frequency characteristics of .

[0033] Figure 4 It is a schematic plan view showing the structure of a light modulation element according to a second embodiment of the present invention.

[0034] Figure 5 It is a schematic plan view showing the structure of a light modulation element according to a third embodiment of the present invention.

[0035] Figure 6 It is a schematic plan view showing the structure of a light modulation element according to a fourth embodiment of the present invention.

[0036] Figure 7 It is a schematic plan view showing the structure of a light modulation element according to a fifth embodiment of the present invention.

[0037] Figure 8 It is a schematic plan view showing the structure of a light modulation element according to a sixth embodiment of the present invention.

[0038] Figure 9 It is a schematic plan view showing the structure of a light modulation element according to the seventh embodiment of the present invention.

[0039] Figure 10 It is a schematic plan view showing the structure of an optical modulation element according to an eighth embodiment of the present invention.

[0040] Figure 11 It is a schematic plan view showing the structure of a light modulation element according to a ninth embodiment of the present invention.

[0041] Figure 12 (a) and (b) are schematic cross-sectional views showing the structure of a conventional light modulation element. DETAILED DESCRIPTION

[0042] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0043] Figure 1 is a schematic plan view showing the structure of the light modulator according to the first embodiment of the present invention. Figure 1 (a) shows only the optical waveguide. Figure 1 (b) shows the entire optical modulation element including the traveling wave electrodes.

[0044] like Figure 1 As shown in Figures (a) and (b), this optical modulation element 1A includes: a Mach-Zehnder optical waveguide 10 formed on a substrate 2 and comprising first and second optical waveguides 10a and 10b arranged parallel to each other; a first signal electrode 7a arranged along the first optical waveguide 10a; a second signal electrode 7b arranged along the second optical waveguide 10b; a first bias electrode 9a arranged along the first optical waveguide 10a; and a second bias electrode 9b arranged along the second optical waveguide 10b. The first and second signal electrodes 7a and 7b, together with the first and second optical waveguides 10a and 10b, constitute the interaction zone MZ of the Mach-Zehnder optical modulation element.

[0045] The Mach-Zehnder optical waveguide 10 has a Mach-Zehnder interferometer structure. It comprises first and second optical waveguides 10a and 10b, which are split from a single input waveguide 10i by a beam splitter 10c. These first and second waveguides 10a and 10b are combined into a single output waveguide 10o by a beam combiner 10d. Input light Si is split by the beam splitter 10c and propagates through the first and second waveguides 10a and 10b, respectively. After this, the light is combined at the beam combiner 10d and output from the output waveguide 10o as modulated light So.

[0046] The first and second signal electrodes 7a and 7b are linear electrode patterns that overlap with the first and second optical waveguides 10a and 10b in a plan view. Both ends of the electrodes are extended to near the outer periphery of the substrate 2. Specifically, one end of the first and second signal electrodes 7a and 7b is extended to near the edge of the substrate 2 via lead portions 7a1 and 7b1, and electrically connected to first and second signal electrode pads 7a3 and 7b3 provided near the edge of the substrate 2. Furthermore, the other end of the first and second signal electrodes 7a and 7b is extended to near the edge of the substrate 2 via lead portions 7a2 and 7b2, and electrically connected to first and second terminal electrode pads 7a4 and 7b4 provided near the edge of the substrate 2.

[0047] First and second signal electrode pads 7a3 and 7b3 on one end of first and second signal electrodes 7a and 7b serve as signal input terminals and are connected to a drive circuit. First and second terminal electrode pads 7a4 and 7b4 on the other end of first and second signal electrodes 7a and 7b are connected to each other via terminal resistor 12. Thus, first and second signal electrodes 7a and 7b function as differential, coplanar traveling-wave electrodes.

[0048] The first and second bias electrodes 9a and 9b are provided independently of the first and second signal electrodes 7a and 7b to apply a DC voltage (DC bias) to the first and second optical waveguides 10a and 10b. One end 9a1 and 9b1 of the first and second bias electrodes 9a and 9b serve as the input end for the DC bias. In this embodiment, the regions where the first and second bias electrodes 9a and 9b are formed are located closer to the output end of the Mach-Zehnder optical waveguide 10 than the regions where the first and second signal electrodes 7a and 7b are formed. However, they can also be located closer to the input end. Alternatively, the first and second bias electrodes 9a and 9b can be omitted, and a modulated signal pre-superimposed with a DC bias can be input to the first and second signal electrodes 7a and 7b.

[0049] A differential signal (modulation signal) with the same absolute value but different signs is input to the first and second signal electrode pads 7a3 and 7b3. The first and second optical waveguides 10a and 10b are made of a material with an electro-optical effect, such as lithium niobate. Therefore, the electric field applied to the first and second optical waveguides 10a and 10b causes the refractive indexes of the first and second optical waveguides 10a and 10b to change by +Δn and -Δn, respectively, thereby changing the phase difference between the pair of optical waveguides. Signal light modulated by this phase difference is output from the output waveguide 10o.

[0050] As described above, the optical modulation element 1A of the present embodiment is a dual-drive type element composed of a pair of signal electrodes. Therefore, the symmetry of the electric field applied to the pair of optical waveguides can be improved, and wavelength chirp can be suppressed.

[0051] In this embodiment, no ground electrode is provided in the vicinity NZ of the interaction zone MZ. The vicinity NZ of the interaction zone MZ is defined as a region extending to the left and right from the center of the interaction zone MZ in the width direction and less than five times the distance W0 between the first and second optical waveguides 10a and 10b (W1 ≤ 5W0). The ground electrode (GND) is an electrode that serves as a potential reference point.

[0052] Typically, the ground electrode is provided near the first and second signal electrodes 7a and 7b constituting the interaction zone MZ (see Figure 12(b), etc.). However, as optical modulators become smaller, it becomes difficult to ensure a sufficient width or area for the ground electrode. An insufficiently large ground electrode is a major factor in deteriorating high-frequency characteristics. However, by excluding the ground electrode from the vicinity NZ of the interaction zone MZ, as in this embodiment, even with miniaturization, high-frequency characteristics are not deteriorated, and the interaction zone MZ can be reused more easily.

[0053] For ease of explanation, Figure 1 In the figure, the gap W0 between the first and second optical waveguides 10a and 10b is shown as being significantly wider than it actually is. The actual waveguide gap W0 is very narrow, ranging from 5 to 50 μm. Meanwhile, the lead portions 7a1 and 7a2 at one and the other ends of the first signal electrode 7a, as well as the lead portions 7b1 and 7b2 at one and the other ends of the second signal electrode 7b, are longer than 100 μm. Therefore, the first and second signal electrode pads 7a3 and 7b3, the first and second ground electrode pads 8a1 and 8b1, the first and second terminal electrode pads 7a4 and 7b4, and the third and fourth ground electrode pads 8a2 and 8b2 are located outside the vicinity NZ of the interaction zone MZ and are therefore located away from the interaction zone MZ.

[0054] To stabilize the reference potential, a ground electrode may be provided on the back side of the substrate 2. As described above, a ground electrode should not be provided in the vicinity NZ of the interaction zone MZ. However, this limitation does not apply to the back side of the substrate 2. In other words, a ground electrode may be provided in the vicinity NZ of the interaction zone MZ on the back side of the substrate 2.

[0055] On the other hand, first and second ground electrode pads 8a1 and 8b1 are provided near the input portions of first and second signal electrodes 7a and 7b, outside of the vicinity zone NZ of the interaction zone MZ. First ground electrode pad 8a1 is provided adjacent to first signal electrode pad 7a3, which constitutes the input portion of first signal electrode 7a, and second ground electrode pad 8b1 is provided adjacent to second signal electrode pad 7b3, which constitutes the input portion of second signal electrode 7b.

[0056] Similar to the input portions of first and second signal electrodes 7a and 7b, third and fourth ground electrode pads 8a2 and 8b2 are provided near the terminal ends of first and second signal electrodes 7a and 7b, located outside of the vicinity zone NZ of the interaction zone MZ. Third ground electrode pad 8a2 is provided adjacent to first terminal electrode pad 7a4, which constitutes the terminal end of first signal electrode 7a, and fourth ground electrode pad 8b2 is provided adjacent to second terminal electrode pad 7b4, which constitutes the terminal end of second signal electrode 7b.

[0057] In this embodiment, the first ground electrode pad 8a1 and the second ground electrode pad 8b1 are electrically connected via a short-circuit pattern 8c1 (first short-circuit pattern) provided in the same electrode layer as the first and second ground electrode pads 8a1 and 8b1. The third ground electrode pad 8a2 and the fourth ground electrode pad 8b2 are also electrically connected via a short-circuit pattern 8c2 (second short-circuit pattern) provided in the same electrode layer as the first and second ground electrode pads 8a1 and 8b2. Short-circuit patterns 8c1 and 8c2 are formed along the edge of substrate 2. Therefore, the first and second signal electrode pads 7a3 and 7b3 and the first and second terminal electrode pads 7a4 and 7b4 are positioned inward of the edge of substrate 2. By short-circuiting two adjacent ground electrode pads, the reference potential on the ground electrode pads can be stabilized, improving high-frequency characteristics.

[0058] Figure 2 It is along Figure 1 (a) and (b) are schematic cross-sectional views of the light modulation element 1A taken along the X1-X1 line.

[0059] like Figure 2 As shown, the optical modulation element 1A of this embodiment has a multilayer structure in which a substrate 2 , a waveguide layer 3 , a protective layer 4 , a buffer layer 5 , and an electrode layer 6 are stacked in this order.

[0060] The substrate 2 is, for example, a sapphire substrate, and a waveguide layer 3 made of an electro-optical material such as lithium niobate is formed on the main surface of the substrate 2. The waveguide layer 3 includes first and second optical waveguides 10a and 10b formed by a ridge 3r. The width W of the ridge 3r forming the first and second optical waveguides 10a and 10b is 10 For example, it may be 1 μm.

[0061] The protective layer 4 is formed in an area that does not overlap with the first and second optical waveguides 10a and 10b when viewed from above. The protective layer 4 covers the entire area of ​​the upper surface of the waveguide layer 3 where the ridge 3r is not formed. The side surfaces of the ridge 3r are also covered by the protective layer 4, thereby preventing scattering loss caused by the roughness of the side surfaces of the ridge 3r. The thickness of the protective layer 4 is approximately the same as the height of the ridge 3r of the waveguide layer 3. The material of the protective layer 4 is not particularly limited; for example, silicon oxide (SiO2) can be used.

[0062] The buffer layer 5 is formed on the upper surface of the ridge portion 3r of the waveguide layer 3 to prevent light propagating through the first and second optical waveguides 10a and 10b from being absorbed by the first and second signal electrodes 7a and 7b. The buffer layer 5 is preferably made of a material with a lower refractive index than the waveguide layer 3 and high transparency. For example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc. can be used. The thickness of the buffer layer 5 on the upper surface of the ridge portion 3r can be approximately 0.2 to 1 μm. More preferably, the buffer layer 5 is made of a material with a high dielectric constant. In this embodiment, the buffer layer 5 covers not only the upper surfaces of the first and second optical waveguides 10a and 10b, but also the entire base surface, including the upper surface of the protective layer 4. However, the buffer layer 5 can also be patterned to selectively cover only the vicinity of the upper surfaces of the first and second optical waveguides 10a and 10b. Alternatively, the protective layer 4 can be omitted, and the buffer layer 5 can be formed directly on the entire upper surface of the waveguide layer 3.

[0063] In order to reduce the light absorption of the electrode, the thicker the buffer layer 5 is, the better. In order to apply a high electric field to the optical waveguide, the thinner the buffer layer 5 is, the better. Since the light absorption of the electrode and the applied voltage of the electrode have a trade-off relationship, it is necessary to set an appropriate film thickness according to the purpose. The higher the dielectric constant of the buffer layer 5, the lower the VπL (an indicator of electric field efficiency) can be, so it is preferred. The lower the refractive index of the buffer layer 5, the thinner the buffer layer 5 can be, so it is preferred. Generally, materials with high dielectric constants also have high refractive indexes. Therefore, it is important to consider the balance between the two and select materials with high dielectric constants and low refractive indexes. As an example, Al2O3 has a relative dielectric constant of about 9 and a refractive index of about 1.6, which is a preferred material. LaAlO3 has a relative dielectric constant of about 13 and a refractive index of about 1.7. In addition, LaYO3 has a relative dielectric constant of about 17 and a refractive index of about 1.7, which are particularly preferred materials.

[0064] A first signal electrode 7a and a second signal electrode 7b are provided on the electrode layer 6. To modulate light propagating within the first optical waveguide 10a, the first signal electrode 7a is provided so as to overlap with the ridge portion 3r corresponding to the first optical waveguide 10a and is opposed to the first optical waveguide 10a via the buffer layer 5. To modulate light propagating within the second optical waveguide 10b, the second signal electrode 7b is provided so as to overlap with the ridge portion 3r corresponding to the second optical waveguide 10b and is opposed to the second optical waveguide 10b via the buffer layer 5.

[0065] like Figure 2 As shown, the electrode structure is bilaterally symmetrical in a cross section perpendicular to the direction of travel of the first and second optical waveguides 10a and 10b. Therefore, the magnitudes of the electric fields applied from the first and second signal electrodes 7a and 7b to the first and second optical waveguides 10a and 10b can be made as equal as possible, thereby reducing wavelength chirp.

[0066] Waveguide layer 3 can be made of any electro-optical material, but is preferably composed of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optical constant, making it suitable as a material for optical devices such as light modulators. The following describes in detail the structure of this embodiment, in which the waveguide layer 3 is made of a lithium niobate film.

[0067] There is no particular limitation on the substrate 2, as long as the refractive index is lower than that of the lithium niobate film. It is preferably 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 has the property of being easily formed as a c-axis oriented epitaxial film for single crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-dimensional symmetry, so it is ideal that the underlying single crystal substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferred.

[0068] Here, an epitaxial film refers to a film whose crystal orientation is uniformly aligned with the underlying substrate or base film. When the film's in-plane is defined as the XY plane and the film thickness is defined as the Z axis, the crystals are uniformly aligned along the X, Y, and Z axes. For example, confirmation of an epitaxial film can be achieved by first confirming the peak intensity at the orientation position using θ-2θ X-ray diffraction and then confirming the poles.

[0069] Specifically, in the first step of θ-2θ X-ray diffraction measurement, the peak intensities of all planes other than the target plane (the target plane) must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the target plane. For example, in a c-axis-oriented epitaxial film of lithium niobate, the peak intensities of all planes other than the (00L) plane must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the (00L) plane. (00L) is a general term for planes equivalent to (001) or (002).

[0070] In the second-step pole measurement, it is necessary to observe the pole. Under the conditions for peak intensity at the orientation position confirmed in the first step, only orientation in one direction can be indicated. Even if the conditions in the first step are met, if the crystal orientation is inconsistent within the plane, the X-ray intensity will not increase at a specific angle position, and the pole cannot be observed. LiNbO3 has a trigonal crystal structure, so there are three poles in a single crystal of LiNbO3 (014).

[0071] It is known that lithium niobate films grow epitaxially in a so-called twinned state—a state formed by symmetrically joining crystals rotated 180° about the c-axis. In this case, since the two are joined with three poles of symmetry, there are six poles. Furthermore, when a lithium niobate film is formed on a (100)-plane silicon single crystal substrate, 4 × 3 = 12 poles are observed due to the substrate's four-fold symmetry. Furthermore, in the present invention, epitaxially grown lithium niobate films in a twinned state are also included in epitaxial films.

[0072] The composition of the lithium niobate film is LixNbAyOz. A represents an element other than Li, Nb, and O. x is 0.5 to 1.2, preferably 0.9 to 1.05. y is 0 to 0.5. z is 1.5 to 4, preferably 2.5 to 3.5. Elements representing 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, and combinations of two or more are also acceptable.

[0073] Ideally, the lithium niobate film has a thickness of 2 μm or less. This is because a film thickness greater than 2 μm makes it difficult to form a high-quality film. On the other hand, if the lithium niobate film is too thin, the film's ability to confine light weakens, allowing light to leak into the substrate 2 or buffer layer 5. Furthermore, even when an electric field is applied to the lithium niobate film, the change in the effective refractive index of the optical waveguide (10a, 10b) may decrease. Therefore, ideally, the lithium niobate film has a thickness of at least approximately 1 / 10 the wavelength of the light used.

[0074] As a method for forming a lithium niobate film, it is ideal to utilize a film forming method such as a sputtering method, a CVD method, or a sol-gel method. The c-axis of lithium niobate is oriented perpendicular to the main surface of the substrate 2, and by applying an electric field parallel to the c-axis, the optical refractive index changes in proportion to the electric field. When sapphire is used as a single crystal substrate, the lithium niobate film can be epitaxially grown directly on the sapphire single crystal substrate. When silicon is used as a single crystal substrate, the lithium niobate film is formed by epitaxial growth via a coating layer (not shown). As a coating layer (not shown), a material having a lower refractive index than that of the lithium niobate film and suitable for epitaxial growth is used. For example, when Y2O3 is used as a coating layer (not shown), a high-quality lithium niobate film can be formed.

[0075] Another known method for forming a lithium niobate film is to thin, grind, or slice a lithium niobate single crystal substrate. This method has the advantage of achieving properties similar to those of a single crystal and can be applied to the present invention.

[0076] The width W7 of the first and second signal electrodes 7a and 7b is larger than the ridge width W of the first and second optical waveguides 10a and 10b formed of the lithium niobate film formed in a ridge shape. 10In order to concentrate the electric field from the first and second signal electrodes 7a, 7b on the first and second optical waveguides 10a, 10b, the width W7 of the first and second signal electrodes 7a, 7b is preferably the same as the ridge width W of the second optical waveguide 10b. 10 The distance between the first and second signal electrodes 7a and 7b is 1.1 to 15 times, more preferably 1.5 to 10 times. Furthermore, the signal electrode spacing refers to the distance from the width center of the first signal electrode 7a to the width center of the second signal electrode 7b. Furthermore, the waveguide spacing refers to the distance from the width center of the first optical waveguide 10a to the width center of the second optical waveguide 10b. In this embodiment, the signal electrode spacing and the waveguide spacing are substantially equal.

[0077] No ground electrode is provided on the electrode layer 6 in the vicinity NZ of the interaction zone MZ. 1a 、D 1b Providing a ground electrode increases ripple and crosstalk, deteriorating high-frequency characteristics. This deterioration in high-frequency characteristics is believed to be due to the fact that, with the miniaturization of optical modulators, the width and area of ​​the ground electrode cannot be sufficiently ensured, leading to unstable reference potential. By omitting the ground electrode, as in this embodiment, ripple and crosstalk can be reduced, improving high-frequency characteristics.

[0078] The first to fourth ground electrode pads 8a1, 8b1, 8a2, and 8b2, as well as the short-circuit patterns 8c1 and 8c2, are provided on the electrode layer 6, along with the signal electrode pads 7a3 and 7b3 of the first and second signal electrodes 7a and 7b, and the first and second terminal electrode pads 7a4 and 7b4. These ground electrodes are located outside the vicinity NZ of the interaction zone MZ. Therefore, they do not cause ripple or crosstalk at low frequencies below 50 GHz, and can also reduce radiation loss at high frequencies above 50 GHz.

[0079] Figure 3 (a) and (b) represent the transmission loss (S dd21 ) is a coordinate diagram of the frequency characteristics of .

[0080] The transmission loss of the optical modulation element having the conventional electrode structure in which the ground electrode is arranged in the region NZ near the interaction part MZ is as follows: Figure 3 As shown in the curve GL1 in (a), ripples or crosstalk occur at low frequencies below 50 GHz. In contrast, in the case of an optical modulator in which the ground electrode is excluded from the entire substrate surface including the vicinity NZ of the interaction portion MZ, as shown in FIG. Figure 3 (a) and Figure 3As shown by curve GL2 in (b), ripple and crosstalk do not occur at low frequencies below 50 GHz. However, since no ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b, transmission loss increases at high frequencies above 50 GHz.

[0081] On the other hand, the transmission loss of the optical modulation element of the present invention is as follows: Figure 3 As shown in the curve GL3 in (b), it is not only possible to suppress ripple or crosstalk at low frequencies below 50 GHz, but also to reduce transmission loss at high frequencies above 50 GHz.

[0082] As described above, the optical modulator 1A of this embodiment does not include a ground electrode in the region NZ near the interaction zone MZ. This reduces electrode loss and enables low-voltage operation by improving the efficiency of the electric field applied to the optical waveguide. Furthermore, the optical modulator 1A of this embodiment includes a ground electrode near at least one of the input and terminal portions of the first and second signal electrodes 7a and 7b, located away from the region NZ near the interaction zone MZ. This reduces radiation loss at high frequencies above 50 GHz, thereby providing an optical modulator with excellent high-frequency characteristics.

[0083] Figure 4 It is a schematic plan view showing the structure of a light modulation element according to a second embodiment of the present invention.

[0084] like Figure 4 As shown, this optical modulation element 1B is characterized in that the first ground electrode pad 8a1 on the input side of the first signal electrode 7a and the third ground electrode pad 8a2 on the terminal side are electrically connected via a short-circuit pattern 8c3 (third short-circuit pattern) that extends widely around the interaction zone MZ. Furthermore, the second ground electrode pad 8b1 on the input side and the fourth ground electrode pad 8b2 on the terminal side are electrically connected via a short-circuit pattern 8c4 (fourth short-circuit pattern). Furthermore, the first and second signal electrode pads 7a3 and 7b3 are positioned near the edge of the substrate 2, and the first and second terminal electrode pads 7a4 and 7b4 are also positioned near the edge of the substrate 2. The remaining structure is the same as that of the first embodiment.

[0085] Short-circuit patterns 8c3 and 8c4, like the first to fourth ground electrode pads 8a1, 8b1, 8a2, and 8b2, are located outside the vicinity NZ of the interaction zone MZ. Therefore, no ground electrode is provided in the vicinity NZ of the interaction zone MZ. According to this embodiment, similar to the first embodiment, radiation loss at high frequencies of 50 GHz and above can be reduced.

[0086] In this embodiment, the first and second ground electrode pads 8a1 and 8b1 on the input side are not connected by a short-circuit pattern, and the third and fourth ground electrode pads 8a2 and 8b2 on the terminal side are not connected by a short-circuit pattern. However, this configuration can be similar to that of the first embodiment. In this case, the first and second signal electrode pads 7a3 and 7b3 and the first and second terminal electrode pads 7a4 and 7b4 are arranged away from the edge of the substrate 2.

[0087] Figure 5 It is a schematic plan view showing the structure of a light modulation element according to a third embodiment of the present invention.

[0088] like Figure 5 As shown, this optical modulation element 1C is characterized in that the first ground electrode pad 8a1 and the second ground electrode pad 8b1 on the input side of the first and second signal electrodes 7a and 7b are electrically connected via contact plugs 8d1 and 8d2 penetrating the substrate 2 and a short-circuit pattern 8c5 formed on the back surface of the substrate 2. Furthermore, the third ground electrode pad 8a2 and the fourth ground electrode pad 8b2 on the terminal side of the first and second signal electrodes 7a and 7b are electrically connected via contact plugs 8d3 and 8d4 penetrating the substrate 2 and a short-circuit pattern 8c6 formed on the back surface of the substrate 2. The short-circuit patterns 8c5 and 8c6 can be formed as a single ground pattern or can be formed across the entire back surface of the substrate 2. Furthermore, the first and second signal electrode pads 7a3 and 7b3 are positioned near the edge of the substrate 2, and the first and second terminal electrode pads 7a4 and 7b4 are also positioned near the edge of the substrate 2. The remaining structure is the same as that of the first embodiment.

[0089] Short-circuit patterns 8c5 and 8c6, like the first to fourth ground electrode pads 8a1, 8b1, 8a2, and 8b2, are located outside the vicinity NZ of the interaction zone MZ. Therefore, no ground electrode is provided in the vicinity NZ of the interaction zone MZ. According to this embodiment, similar to the first embodiment, radiation loss at high frequencies of 50 GHz and above can be reduced.

[0090] Figure 6 It is a schematic plan view showing the structure of a light modulation element according to a fourth embodiment of the present invention.

[0091] like Figure 6As shown, this optical modulation element 1D is characterized in that the first ground electrode pad 8a1 and the second ground electrode pad 8b1 are electrically connected via a ground line within the driver circuit 200 connected to the input portions of the first and second signal electrodes 7a and 7b. Furthermore, the third ground electrode pad 8a2 and the fourth ground electrode pad 8b2 are electrically connected via a ground line within the terminator 300 connected to the terminal portions of the first and second signal electrodes 7a and 7b. Furthermore, the first and second signal electrode pads 7a3 and 7b3 are positioned near the edge of the substrate 2, and the first and second terminal electrode pads 7a4 and 7b4 are positioned near the edge of the substrate 2. The remaining structure is the same as that of the first embodiment.

[0092] The ground line within the drive circuit 200 and the ground line within the terminator 300 are located outside the region NZ near the interaction zone MZ. Therefore, no ground electrode is provided in the region NZ near the interaction zone MZ. Similar to the first embodiment, this embodiment reduces radiation loss at high frequencies of 50 GHz or higher.

[0093] Figure 7 It is a schematic plan view showing the structure of a light modulation element according to a fifth embodiment of the present invention.

[0094] like Figure 7 As shown, the light modulator 1E is a modified example of the light modulator 1D of the fourth embodiment, and is characterized in that Figure 6 The driving circuit 200 and terminator 300 are flip-chip mounted on the substrate 2. The remaining structure is the same as that of the fourth embodiment. Therefore, the optical modulator 1E of this embodiment not only achieves the same effects as the fourth embodiment but also achieves miniaturization of the entire optical modulator.

[0095] Figure 8 It is a schematic plan view showing the structure of a light modulation element according to a sixth embodiment of the present invention.

[0096] like Figure 8 As shown, this optical modulation element 1F is characterized by a Mach-Zehnder waveguide 10 composed of a combination of straight and curved sections. Specifically, the Mach-Zehnder waveguide 10 includes first, second, and third straight sections 10e1, 10e2, and 10e3 arranged parallel to each other; a first curved section 10f1 connecting the first straight section 10e1 and the second straight section 10e2; and a second curved section 10f2 connecting the second straight section 10e2 and the third straight section 10e3. The first and second curved sections 10f1 and 10f2 shift the direction of travel of the optical waveguide by 180 degrees, forming concentric semicircular shapes. The remaining structure is the same as that of the first embodiment.

[0097] In this embodiment, the majority of the first straight portion 10e1, the second straight portion 10e2, the entire first and second curved portions 10f1 and 10f2, and a portion of the third straight portion 10e3 of the Mach-Zehnder waveguide 10, together with the first and second signal electrodes 7a and 7b, form an interaction zone MZ. Input light Si is input to one end of the first straight portion 10e1, travels from one end to the other end of the first straight portion 10e1, then turns back at the first curved portion 10f1 and travels from one end to the other end of the second straight portion 10e2 in the opposite direction to the first straight portion 10e1. It then turns back at the second curved portion 10f2 and travels from one end to the other end of the third straight portion 10e3 in the same direction as the first straight portion 10e1.

[0098] While the long length of Mach-Zehnder optical modulators poses a significant practical challenge, bending the optical waveguide as shown in the figure significantly reduces the length, resulting in significant miniaturization. In particular, optical waveguides formed from lithium niobate films exhibit low loss even when the radius of curvature of the curved portion is reduced to, for example, approximately 50 μm, making them suitable for this embodiment.

[0099] In this embodiment, no ground electrode is provided in the region NZ near the interaction zone MZ. Placing an insufficiently large ground electrode near the first and second signal electrodes 7a and 7b that constitute the interaction zone MZ can degrade high-frequency characteristics. In particular, when the optical waveguide has first and second bends 10f1 and 10f2, and ground electrodes are provided near these bends, high-frequency signal leakage increases at the bends, potentially degrading high-frequency characteristics. However, by omitting the ground electrode, as in this embodiment, high-frequency characteristics are not degraded by the ground electrode, and miniaturization and multiplexing of the optical modulator are facilitated.

[0100] In this embodiment as well, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b, located outside the neighborhood NZ of the interaction zone MZ. Specifically, first and second ground electrode pads 8a1 and 8b1 are provided near the input portions of the first and second signal electrodes 7a and 7b, away from the interaction zone MZ. The first ground electrode pad 8a1 is provided adjacent to the first signal electrode pad 7a3, which constitutes the input portion of the first signal electrode 7a. The second ground electrode pad 8b1 is provided adjacent to the second signal electrode pad 7b3, which constitutes the input portion of the second signal electrode 7b. The first and second ground electrode pads 8a1 and 8b1 are connected to each other via a short-circuit pattern 8c1.

[0101] Similar to the input portions of the first and second signal electrodes 7a and 7b, third and fourth ground electrode pads 8a2 and 8b2 are provided near the terminal ends of the first and second signal electrodes 7a and 7b, away from the interaction zone MZ. The third ground electrode pad 8a2 is provided adjacent to the first terminal electrode pad 7a4, which constitutes the terminal end of the first signal electrode 7a. The fourth ground electrode pad 8b2 is provided adjacent to the second terminal electrode pad 7b4, which constitutes the terminal end of the second signal electrode 7b. The third and fourth ground electrode pads 8a2 and 8b2 are connected to each other via a short-circuit pattern 8c2.

[0102] As described above, the optical modulator 1F of this embodiment does not have a ground electrode in the vicinity NZ of the interaction zone MZ. However, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b located outside the vicinity NZ. This reduces radiation loss at high frequencies of 50 GHz and above, thereby providing an optical modulator with excellent high-frequency characteristics.

[0103] Figure 9 It is a schematic plan view showing the structure of a light modulation element according to the seventh embodiment of the present invention.

[0104] like Figure 9 As shown, the optical modulator 1G is characterized by having a dual-channel array structure obtained by arranging two interaction parts of a Mach-Zehnder optical modulator, and using the first and second interaction parts MZ1 and MZ2 to perform quadrature phase modulation (QPSK) or quadrature amplitude modulation (xQAM) on the input light Si. That is, the optical modulator 1G of this embodiment is an IQ optical modulator composed of the first and second interaction parts MZ1 and MZ2, and a phase shifter 10g is provided on the output side of the second interaction part MZ2. The structures of the interaction parts MZ1 and MZ2 are similar to Figure 1 The same is true for the single interaction portion MZ of the Mach-Zehnder optical modulation element shown. Different differential signals are applied to the pair of RF signal input terminals (first and second signal electrode pads 7a3, 7b3) of the first and second interaction portions MZ1 and MZ2, respectively.

[0105] An input waveguide 10i, consisting of a single optical waveguide, is split into four (two pairs) by a two-stage splitting unit 10c. Specifically, the first and second optical waveguides 10a and 10b form the first interaction zone MZ1, and the first and second optical waveguides 10a and 10b form the second interaction zone MZ2. On the output side of the optical waveguides, they are combined by a two-stage combining unit 10d to form a single output waveguide 10o.

[0106] In this embodiment, no ground electrode is provided in the area NZ near the first and second interaction zones MZ1 and MZ2. In particular, no ground electrode is provided in the inter-channel region between the second signal electrode 7b of the first interaction zone MZ1 and the first signal electrode 7a of the second interaction zone MZ2. When miniaturizing and multiplexing optical modulators, it is difficult to secure a ground electrode of sufficient width or area in the inter-channel region. Insufficient ground electrodes can lead to deterioration in high-frequency characteristics. However, by omitting the ground electrode, as in this embodiment, high-frequency characteristics are not degraded even when the optical modulator is miniaturized, making multiplexing of the optical modulator easier.

[0107] On the other hand, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b, away from the vicinity NZ of the first and second interaction zones MZ1 and MZ2. Specifically, first and second ground electrode pads 8a1 and 8b1 are provided near the input portions of the first and second signal electrodes 7a and 7b, away from the first and second interaction zones MZ1 and MZ2. The first ground electrode pad 8a1 is provided adjacent to the first signal electrode pad 7a3, which constitutes the input portion of the first signal electrode 7a, and the second ground electrode pad 8b1 is provided adjacent to the second signal electrode pad 7b3, which constitutes the input portion of the second signal electrode 7b. The first and second ground electrode pads 8a1 and 8b1 are connected to each other via a short-circuit pattern 8c1.

[0108] Similarly to the input portions of the first and second signal electrodes 7a and 7b, third and fourth ground electrode pads 8a2 and 8b2 are provided near the terminal ends of the first and second signal electrodes 7a and 7b, away from the vicinity NZ of the first and second interaction zones MZ1 and MZ2. The third ground electrode pad 8a2 is provided adjacent to the first terminal electrode pad 7a4, which constitutes the terminal end of the first signal electrode 7a. The fourth ground electrode pad 8b2 is provided adjacent to the second terminal electrode pad 7b4, which constitutes the terminal end of the second signal electrode 7b. The third and fourth ground electrode pads 8a2 and 8b2 are connected to each other via a short-circuit pattern 8c2.

[0109] As described above, the optical modulator 1G of this embodiment does not have a ground electrode in the vicinity NZ of the first and second interaction zones MZ1 and MZ2. However, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b located outside the vicinity NZ. This reduces radiation loss at high frequencies of 50 GHz and above, thereby providing an optical modulator with excellent high-frequency characteristics.

[0110] Figure 10It is a schematic plan view showing the structure of an optical modulation element according to an eighth embodiment of the present invention.

[0111] like Figure 10 As shown, the optical modulator 1H is characterized by having a four-channel array structure obtained by arranging the four interaction parts of the Mach-Zehnder optical modulator, and using the four interaction parts MZ1, MZ2, MZ3, and MZ4 to perform polarization multiplexing quadrature phase modulation (DP-QPSK). Therefore, the first and second interaction parts MZ1 and MZ2 constitute the first IQ modulator, and the third and fourth interaction parts MZ3 and MZ4 constitute the second IQ modulator, and each output is output through the polarization multiplexing waveguide 10h. The structure of each interaction part MZ1 to MZ4 is similar to Figure 1 The single interaction portion MZ of the light modulation element 1A shown is similar.

[0112] In this embodiment, no ground electrode is provided in the area NZ near the first to fourth interaction zones MZ1, MZ2, MZ3, and MZ4. In particular, no ground electrode is provided in the inter-channel region between the second signal electrode 7b and the first signal electrode 7a, which are adjacent to each other and belong to different interaction zones. When optical modulators are miniaturized and multiplexed, it is difficult to secure a ground electrode of sufficient width or area in the inter-channel region. Insufficient ground electrodes can lead to deterioration in high-frequency characteristics. However, by omitting the ground electrode, as in this embodiment, high-frequency characteristics are not degraded even when the optical modulator is miniaturized, making multiplexing of the optical modulator easier.

[0113] On the other hand, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b, away from the vicinity NZ of the first to fourth interaction zones MZ1, MZ2, MZ3, and MZ4. Specifically, first and second ground electrode pads 8a1 and 8b1 are provided near the input portions of the first and second signal electrodes 7a and 7b, away from the first to fourth interaction zones MZ1, MZ2, MZ3, and MZ4. The first ground electrode pad 8a1 is provided adjacent to the first signal electrode pad 7a3, which constitutes the input portion of the first signal electrode 7a. The second ground electrode pad 8b1 is provided adjacent to the second signal electrode pad 7b3, which constitutes the input portion of the second signal electrode 7b.

[0114] Similarly to the input portions of the first and second signal electrodes 7a and 7b, third and fourth ground electrode pads 8a2 and 8b2 are provided near the terminal ends of the first and second signal electrodes 7a and 7b, located away from the first to fourth interaction zones MZ1, MZ2, MZ3, and MZ4. The third ground electrode pad 8a2 is provided adjacent to the first terminal electrode pad 7a4, which constitutes the terminal end of the first signal electrode 7a. The fourth ground electrode pad 8b2 is provided adjacent to the second terminal electrode pad 7b4, which constitutes the terminal end of the second signal electrode 7b. The third and fourth ground electrode pads 8a2 and 8b2 are connected to each other via a short-circuit pattern 8c2.

[0115] As described above, the optical modulator 1H of this embodiment does not have a ground electrode in the vicinity NZ of the first to fourth interaction zones MZ1, MZ2, MZ3, and MZ4. However, ground electrodes are provided near the input and terminal portions of the first and second signal electrodes 7a and 7b, which are separated from the vicinity NZ. This reduces radiation loss at high frequencies of 50 GHz and above. Consequently, it is possible to provide an optical modulator with excellent high-frequency characteristics.

[0116] Figure 11 It is a schematic plan view showing the structure of a light modulation element according to a ninth embodiment of the present invention.

[0117] like Figure 11 As shown, the light modulation element 1I is characterized in that Figure 10 In the optical modulation element with a four-channel array structure shown, the first to fourth interaction portions MZ1 to MZ4 are formed by a combination of straight portions and curved portions, that is, a combination of the sixth embodiment and the eighth embodiment.

[0118] In this embodiment, the outputs of the respective channels are not combined but outputted individually. Therefore, the Mach-Zehnder optical waveguide 10 includes first to fourth output waveguides 10o1, 10o2, 10o3, and 10o4.

[0119] As described above, when the optical waveguide has a bent portion in a multi-channel structure, the problem of crosstalk becomes more pronounced. However, since no ground electrode is provided near the signal electrode, crosstalk can be reduced.

[0120] Preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention, which are naturally included in the scope of the present invention.

[0121] For example, in the above embodiment, ground electrodes are provided near both the input portion and the terminal portion of the first and second signal electrodes 7a and 7b in the vicinity NZ away from the interaction portion MZ. However, they do not necessarily have to be provided on both sides, and a ground electrode may be provided only near one of the input portion and the terminal portion.

[0122] Furthermore, while the above embodiment illustrates an optical modulation element having a pair of optical waveguides formed from a lithium niobate film epitaxially grown on substrate 2, the present invention is not limited to this structure. Optical waveguides can also be formed using electro-optical materials such as barium titanate or lead zirconate titanate. However, optical waveguides formed from lithium niobate films can be narrowed, which significantly increases the layout of the ground electrode, resulting in greater benefits for the present invention. Furthermore, semiconductor materials or polymer materials exhibiting electro-optical effects can also be used as waveguide layer 3.

[0123] In addition, Figure 1 For convenience, the structure of the lead portion is shown as a signal electrode bent at a right angle. However, the present invention is not limited to this. From the perspective of high-frequency characteristics, a lead portion may also be used, such as a signal electrode bent into an arc shape. In addition, the curvature of the signal electrode when bent into an arc shape can be appropriately set according to the width of the signal electrode, etc.

[0124] Description of Reference Numerals

[0125] 1A~1I optical modulation elements

[0126] 2 substrates

[0127] 3 waveguide layers

[0128] 3r ridge

[0129] 4 protective layers

[0130] 5 buffer layer

[0131] 6 electrode layers

[0132] 7a First signal electrode

[0133] 7b second signal electrode

[0134] 7a1, 7a2 Lead-out portion of the first signal electrode

[0135] 7a3 first signal electrode pad

[0136] 7a4 first terminal electrode pad

[0137] 7b second signal electrode

[0138] 7b1, 7b2 Lead-out portion of the second signal electrode

[0139] 7b3 second signal electrode pad

[0140] 7b4 second terminal electrode pad

[0141] 8a1 First ground electrode pad

[0142] 8b1 second ground electrode pad

[0143] 8a2 third ground electrode pad

[0144] 8b2 fourth ground electrode pad

[0145] 8c1~8c6 short circuit pattern

[0146] 8d1~8d4 contact plugs

[0147] 9a First bias electrode

[0148] 9a1 One end of the first bias electrode

[0149] 9b Second bias electrode

[0150] 9b1 One end of the second bias electrode

[0151] 10 Mach Zehnder optical waveguide

[0152] 10a First optical waveguide

[0153] 10b Second optical waveguide

[0154] 10c beam splitter

[0155] 10d beam combining section

[0156] 10e1 first straight line

[0157] 10e2 second straight line portion

[0158] 10e3 third straight line

[0159] 10f1 first bend

[0160] 10f2 second bend

[0161] 10g phase shifter

[0162] 10i input waveguide

[0163] 10o output waveguide

[0164] 12 terminal resistors

[0165] 20A, 20B light modulation elements

[0166] 21 Sapphire substrate

[0167] 22a, 22b optical waveguide

[0168] 23 buffer layer

[0169] 24a, 24a1, 24a2 signal electrodes

[0170] 24b grounding electrode

[0171] 200 drive circuit

[0172] 300 terminal

[0173] D0 electrode separation area

[0174] D 1a 、D 1b nearby areas

[0175] MZ interaction

[0176] MZ1 first interaction site

[0177] MZ2 second interaction part

[0178] The third interaction site of MZ3

[0179] The third interaction site of MZ4

[0180] The vicinity of the NZ interaction region

Claims

1. A light modulating element, characterized in that: include: substrate; and at least one interaction portion provided on the substrate, The interaction part includes: forming first and second optical waveguides adjacent to each other on the substrate; and first and second signal electrodes are provided opposite to the first and second optical waveguides and to which differential signals are applied, There is no ground electrode in the vicinity of the interaction portion. A ground electrode is arranged near at least one of an input portion and a terminal portion electrically connected to the first and second signal electrodes. The vicinity of the interaction portion is a region within a range of 5 times or less the distance between the first optical waveguide and the second optical waveguide from the center of the interaction portion.

2. The light modulation element according to claim 1, wherein The input portion has first and second signal electrode pads, The ground electrode includes a first ground electrode pad adjacent to the first signal electrode pad and a second ground electrode pad adjacent to the second signal electrode pad.

3. The light modulation element according to claim 2, wherein The first ground electrode pad is electrically connected to the second ground electrode pad.

4. The light modulation element according to claim 3, wherein The first ground electrode pad and the second ground electrode pad are electrically connected via a first short-circuit pattern on the substrate.

5. The light modulation element according to any one of claims 2 to 4, wherein The first ground electrode pad and the second ground electrode pad are electrically connected via a ground line in a driving circuit that applies the differential signal to the first and second signal electrode pads.

6. The light modulation element according to any one of claims 2 to 4, wherein The terminal portion has first and second terminal electrode pads, The ground electrode includes a third ground electrode pad adjacent to the first terminal electrode pad and a fourth ground electrode pad adjacent to the second terminal electrode pad.

7. The light modulation element according to claim 6, wherein The third ground electrode pad and the fourth ground electrode pad are electrically connected via a second short-circuit pattern on the substrate.

8. The light modulation element according to claim 6, wherein The third ground electrode pad is connected to the first ground electrode pad via a third short-circuit pattern on the substrate. The fourth ground electrode pad is connected to the second ground electrode pad via a fourth short-circuit pattern on the substrate.

9. The light modulation element according to claim 6, wherein The third ground electrode pad and the fourth ground electrode pad are electrically connected via a ground line in a terminator connected to the first and second terminal electrode pads.

10. The light modulation element according to any one of claims 1 to 4, wherein The substrate is a single crystal substrate, The first and second optical waveguides are composed of a lithium niobate film formed in a ridge shape on the substrate.

11. The light modulation element according to any one of claims 1 to 4, wherein The interaction part includes: a waveguide layer including the first and second optical waveguides and formed on the main surface of the substrate; a buffer layer formed on at least upper surfaces of the first and second optical waveguides; and an electrode layer comprising the first and second signal electrodes and formed on an upper surface of the buffer layer, The first and second signal electrodes are respectively opposed to the first and second optical waveguides via the buffer layer.

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