JP2019074595A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator capable of improving high frequency characteristics and widening an optical band. An optical modulator 100 comprises a substrate 1, an electro-optical material film formed on the substrate 1, and a waveguide layer 2 including first and second optical waveguides 10A and 10B adjacent to each other. A buffer layer 4 that covers at least the upper surfaces of the first and second optical waveguides 10A and 10B, a first signal electrode 7 that faces the first optical waveguide 10A via the buffer layer 4, and a first signal electrode. At least one that electrically connects the first ground electrode 8A and 8B provided so as to be sandwiched between the first ground electrode 8A and 8B and the first ground electrode 8A and the second ground electrode 8B straddling the first signal electrode 7. It is provided with two connecting conductors 9. [Selection diagram] Fig. 2

Description

  The present invention relates to an optical modulator used in the optical communication and optical measurement fields, and more particularly to an electrode structure of a Mach-Zehnder type optical modulator.

  With the spread of the Internet, the amount of communication has dramatically increased, and the importance of optical fiber communication has greatly increased. Optical fiber communication converts an electrical signal into an optical signal, transmits the optical signal through an optical fiber, and is characterized by a wide band, low loss, and resistance to noise.

  As a system for converting an electrical signal into an optical signal, a direct modulation system using a semiconductor laser and an external modulation system using an optical modulator are known. Although direct modulation does not require an optical modulator and is low cost, high speed modulation is limited, and external modulation is used for high speed and long distance applications.

  As an optical modulator, a Mach-Zehnder type optical modulator in which an optical waveguide is formed in the vicinity of the surface of a lithium niobate single crystal substrate by Ti (titanium) diffusion is put to practical use (see, for example, Patent Document 1). A Mach-Zehnder type optical modulator has an optical waveguide having a structure of a Mach-Zehnder interferometer that divides light emitted from one light source into two, passes through different paths, and then superimposes again to cause interference (Mach-Zehnder Optical modulators (optical waveguides) are used, and high-speed optical modulators of 40 Gb / s or more are commercialized, but a long length of around 10 cm is a major drawback.

  On the other hand, Patent Documents 2 and 3 disclose a Mach-Zehnder type optical modulator using a lithium niobate film with c-axis alignment. An optical modulator using a lithium niobate film can be made much smaller in size and lower in driving voltage than an optical modulator using a lithium niobate single crystal substrate.

  In the optical modulator, in order to realize the current speedup from 32 Gbaud to 64 Gbaud, an optical band of 35 GHz or more is required, and broadening of the optical band is desired. It is necessary to improve the high frequency characteristics in order to realize the broadening of the optical band.

  In order to improve the high frequency characteristics of the optical modulator, for example, Patent Document 4 discloses that a ground electrode provided on both sides of a signal electrode for driving an optical waveguide and a housing of a ground connection member are connected by wire bonding. Have been described. Further, Patent Document 5 includes not only the top surface but also the side surface and the bottom surface of a lithium niobate substrate on which a control electrode is formed, in an optical waveguide device including a waveguide for guiding the light wave and a control electrode for controlling the light wave. It is described that almost the entire surface of the optical waveguide element is coated with a conductive material to form a ground electrode. Also in Patent Document 6, the high frequency characteristics are improved by covering appropriate regions on the side and back of the substrate with a metal film, and connecting the metal film to a metal casing by soldering to ensure the stability of grounding. It is described.

Patent 4485218 gazette JP, 2006-195383, A JP, 2014-6348, A Patent No. 4056545 JP-A-5-158002 Unexamined-Japanese-Patent No. 10-239648

  However, the conventional optical modulator described in Patent Document 4 only partially connects the ground function of the ground electrode because only a part of the ground electrode is connected to the metal casing by wire bonding. However, the entire surface of the ground electrode can not be reinforced uniformly. Further, the conventional light modulators described in Patent Documents 5 and 6 require special processing for forming an electrode film not only on the top surface but also on the side surface and the bottom surface, and require a very large number of electrode materials. In the case of using Au as an electrode material, it is disadvantageous in cost and not practical. Furthermore, with the recent miniaturization of the optical modulator, it is very difficult to secure a soldering space for connecting the ground electrode to the metal housing.

  Therefore, an object of the present invention is to provide an optical modulator capable of improving the high frequency characteristics to widen the optical band.

  The inventors of the present invention conducted intensive studies to improve the high frequency characteristics of the optical modulator, and as a result, when connecting conductors are provided to electrically connect the ground electrodes on both sides across the signal electrodes. It has been found that the high frequency characteristics of the modulator can be improved. Since a strong electromagnetic field is generated around the signal electrode constituting the coplanar traveling wave electrode, it is considered that if the conductor is disposed above the traveling wave electrode, the electromagnetic field is disturbed and the high frequency characteristics of the optical modulator are deteriorated. The However, if it is a connecting conductor which electrically connects between the two ground electrodes on both sides across the signal electrode, it is newly found that the high frequency characteristics are rather improved, and the present invention can be made. .

  The present invention is based on such technical findings, and an optical modulator according to the present invention comprises a substrate and an electro-optical material film formed on the substrate, and the first and second light guides adjacent to each other A waveguide layer including a waveguide, a buffer layer covering at least upper surfaces of the first and second optical waveguides, a first signal electrode facing the first optical waveguide through the buffer layer, and Electrically connecting the first ground electrode and the second ground electrode across the first signal electrode and the first and second ground electrodes provided so as to sandwich the 1 signal electrode; And at least one connection conductor.

  According to the present invention, the ground function of the ground electrode can be reinforced uniformly in the plane. Therefore, the high frequency characteristics of the optical modulator can be improved, and the optical band can be broadened.

  In the present invention, the connection conductor is preferably a bonding wire. According to this, the connection conductor for electrically connecting the first ground electrode and the second ground electrode across the first signal electrode can be easily realized at low cost. Further, the signal transmission quality can be maintained without the connecting conductor adversely affecting the applied electric field of the optical waveguide.

  In the present invention, the number of bonding wires is 3 dB (half value) of the lowest dip frequency appearing in an Electro-Optical (hereinafter referred to as EO) frequency response characteristic representing the efficiency for converting an electrical signal to an optical signal. Preferably, it is set to be higher than the frequency to be reduced. Although the dip that appears in the EO frequency response characteristics adversely affects the width of the light band, it is possible to enhance the ground function of the ground electrode and shift the lowest dip frequency to the high frequency side by increasing the number of bonding wires. It is possible to prevent the narrowing of the light band.

  In the present invention, the first ground electrode is provided on the opposite side of the second optical waveguide as viewed from the first optical waveguide, and the second ground electrode is formed via the buffer layer. It faces the second optical waveguide, and the distance between the first signal electrode and the first ground electrode is wider than the distance between the first signal electrode and the second ground electrode preferable. According to this configuration, the difference in magnitude of the applied electric field to the pair of optical waveguides caused by the difference in magnitude between the signal electrode and the second ground electrode is minimized, thereby reducing the wavelength chirp of the modulated light. can do. Further, by providing the first ground electrode, radiation loss can be reduced and good high frequency characteristics can be obtained. Further, by making the distance between the signal electrode and the first ground electrode wider than the distance between the signal electrode and the second ground electrode, the magnitude of the electric field applied to the pair of optical waveguides due to the influence of the first ground electrode To reduce the wavelength chirp of the modulated light.

  In the present invention, the first signal electrode includes a first lower layer facing the first optical waveguide via the buffer layer, and a first upper layer provided above the first lower layer. A second lower electrode portion facing the second optical waveguide through the buffer layer, and a second lower electrode portion provided above the second lower electrode portion. It is preferable that the width of the lower surface of the second lower layer portion be narrower than the width of the second upper layer portion. According to this configuration, it is possible to further reduce the difference in magnitude of the electric field applied to the pair of optical waveguides due to the influence of the second ground electrode, and to reduce the wavelength chirp of the modulated light.

  In the present invention, the width of the lower surface of the first lower layer portion is preferably narrower than the width of the first upper layer portion. According to this configuration, the electric field can be concentrated on the first optical waveguide, and the electric fields applied to the first and second optical waveguides can be balanced.

  An optical modulator according to the present invention is provided above the buffer layer, and includes an electrode layer including the first signal electrode and the first and second ground electrodes, and between the buffer layer and the electrode layer. And an insulating layer provided, the insulating layer having first and second openings respectively located above the first and second optical waveguides, and the first upper layer portion The first lower layer portion is formed in the electrode layer, the first lower layer portion is embedded in the first opening, and the second upper layer portion is formed in the electrode layer, and the second lower layer portion is formed. Preferably, the portion is embedded in the second opening. According to this configuration, it is possible to easily realize a two-layer electrode structure in which the width of the lower layer portion is narrower than the width of the upper layer portion.

  In the present invention, the first and second optical waveguides have at least one linear portion and at least one curved portion, and the lower surface of the first lower layer portion of the first signal electrode is the buffer. The lower surface of the second lower layer portion of the second ground electrode facing the straight portion of the first optical waveguide through a layer is formed of the second optical waveguide through the buffer layer. It is preferable that it opposes the said linear part. According to this configuration, the optical waveguide can be folded back, and the element length can be shortened. In particular, in the case of using an optical waveguide formed of a lithium niobate film, the effect of the present invention is remarkable because the loss is small even if the radius of curvature is reduced to, for example, about 50 μm.

  In the present invention, the first and second optical waveguides connect first to third linear portions parallel to each other, and the other end of the first linear portion and one end of the second linear portion. A substantially S-shaped waveguide including a first curved portion, and a second curved portion connecting the other end of the second linear portion and one end of the third linear portion, and the connecting conductor is It is preferable to be provided at a position straddling the first signal electrode covering at least the second linear portion of the first optical waveguide. According to this configuration, not only can the optical waveguide be folded twice to shorten the element length, but also the ground function of the ground electrode can be strengthened to improve the high frequency characteristics. Therefore, it is possible to provide a very compact light modulator with a wide light band.

  The optical modulator according to the present invention further includes a second signal electrode facing the second optical waveguide through the buffer layer, and the second ground electrode is the same as the second optical waveguide when viewed from the second optical waveguide. The first and second ground electrodes are provided on the opposite side to the first optical waveguide, and the first and second ground electrodes are provided so as to sandwich the first and second signal electrodes, and the connection conductor is provided It is preferable that the first ground electrode and the second ground electrode are electrically connected across the first and second signal electrodes. According to this configuration, in the dual drive type optical modulator, the ground function of the ground electrode can be strengthened, and the occurrence of dip in the frequency response characteristic can be suppressed. Therefore, it is possible to provide an optical modulator capable of broadening the optical band.

  According to the present invention, it is possible to provide an optical modulator capable of improving the high frequency characteristics and broadening the optical band.

FIG. 1 is a plan view of an optical modulator 100 according to a first embodiment of the present invention, in which (a) shows only the optical waveguide and (b) shows the entire optical modulator 100 including traveling wave electrodes. Is illustrated. FIG. 2 is a schematic cross-sectional view of the light modulator 100 taken along the line AA 'in FIGS. 1 (a) and 1 (b). FIG. 3 is a schematic cross-sectional view showing the configuration of an optical modulator 200 according to the second embodiment of the present invention. FIG. 4 is a plan view of an optical modulator 300 according to a third embodiment of the present invention, wherein (a) shows only the optical waveguide, and (b) shows the entire optical modulator 300 including the traveling wave electrode. Is illustrated. FIGS. 5 (a) and 5 (b) are schematic cross-sectional views respectively showing the configurations of the light modulators 400A and 400B according to the fifth and sixth embodiments of the present invention. FIG. 6 is a view showing the structure of an evaluation model of a traveling wave electrode, wherein (a) is a schematic sectional view and (b) is a schematic plan view. FIG. 7 is a graph showing the S21 characteristic of the evaluation model shown in FIG. FIG. 8 is a diagram showing the relationship between the evaluation model of the traveling wave electrode and the lowest frequency of the dip appearing in the S21 characteristic, where (a) is a schematic plan view of the evaluation model and (b) is the S21 characteristic of the evaluation model. FIG. FIG. 9 is a graph showing the relationship between the number of bonding wires and the lowest dip frequency of the S21 characteristic. FIG. 10 is a schematic plan view showing the configuration of an evaluation model of an optical modulator having an S-shaped optical waveguide, in which (a) shows straight portions 10e ₁ , 10e ₁ on both ends sandwiching a central straight portion 10e ₂ . 10e ₃ only structure in which a bonding wire 9, (b) shows the middle of the straight portion 10e ₂ only the structure in which the bonding wires 9, respectively. 11 (a) is a graph showing the S21 characteristic of the evaluation model of FIG. 10 (a), and FIG. 11 (b) is a graph showing the S21 characteristic of the evaluation model of FIG. 10 (b).

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

  FIG. 1 is a plan view of an optical modulator 100 according to a first embodiment of the present invention, in which (a) shows only the optical waveguide and (b) shows the entire optical modulator 100 including traveling wave electrodes. Is illustrated.

  As shown in FIGS. 1 (a) and 1 (b), this optical modulator 100 is a Mach-Zehnder optical waveguide having first and second optical waveguides 10A and 10B formed on a substrate 1 and provided parallel to each other. A waveguide 10, a signal electrode 7 provided to overlap the first optical waveguide 10A, and first and second ground electrodes 8A and 8B provided on both sides thereof so as to sandwich the signal electrode 7; There is. The first ground electrode 8A is provided on the opposite side of the second optical waveguide 10B as viewed from the first optical waveguide 10A, and the second ground electrode 8B is provided to overlap the second optical waveguide 10B. It is done.

  The Mach-Zehnder optical waveguide 10 is an optical waveguide having a structure of a Mach-Zehnder interferometer, and includes first and second optical waveguides 10A and 10B branched from a single input optical waveguide 10i by a branching unit 10c. The first and second optical waveguides 10A and 10B are combined into a single output optical waveguide 10o via the coupler 10d. The input light Si is demultiplexed by the demultiplexing unit 10c, travels through the first and second optical waveguides 10A and 10B, and then is multiplexed by the multiplexing unit 10d, and output as the modulated light So from the output optical waveguide 10o Be done.

The signal electrode 7 is located between the first and second ground electrodes 8A and 8B in a plan view. One end 7i of the signal electrode 7 is a signal input end, and the other end 7o of the signal electrode 7 is connected to the first and second ground electrodes 8A and 8B via the termination resistor 12, respectively. Thus, the signal electrode 7 and the first and second ground electrodes 8A and 8B function as coplanar traveling wave electrodes. Details will be described later, the signal electrodes 7 and the second ground electrode 8B is a two-layer structure, the lower portion 7 _L of the signal electrodes 7 shown by a broken line overlaps the first optical waveguide 10A in a plan view, also lower portion 8B _L of the second ground electrode 8B shown by a broken line is overlapped with the second optical waveguide 10B in plan view.

  Three bonding wires 9 are connected between the first ground electrode 8A and the second ground electrode 8B so as to straddle the signal electrode 7. One end of the bonding wire 9 is connected to the top surface of the first ground electrode 8A, and the other end of the bonding wire 9 is connected to the top surface of the second ground electrode 8B. In order to make the in-plane potential of the ground electrode as even as possible, it is preferable that the three bonding wires 9 be discretely arranged over the entire length of the optical waveguide.

  The material of the bonding wire 9 is not particularly limited. For example, gold or aluminum used for a semiconductor or the like may be used, or another metal material may be used. Also, the diameter of the bonding wire 9 is not particularly limited.

  The number of bonding wires 9 is not particularly limited as long as it can enhance the ground function of the first and second ground electrodes 8A and 8B, and the grounding function can be enhanced as the number of bonding wires 9 increases. It is desirable to strengthen efficiently with a small number. For that purpose, it is preferable to set the number of bonding wires 9 so that the lowest frequency of dip appearing in the EO frequency response characteristic of the optical modulator is higher than the frequency which decreases 3 dB (half value) from the reference value. The light band is defined as a frequency that is 3 dB (half value) lower than the reference value with the EO frequency response characteristic. The reference value is the value of the EO frequency response characteristic at a certain frequency, and the frequency is determined to be, for example, 1 GHz or 2 GHz by the standard. Therefore, the optical band is a frequency higher than the frequency at which the reference value is obtained, but the optical band is also lowered when a large dip in gain occurs at a frequency lower than the optical band. The lowest frequency of dip tends to shift to the higher frequency side as the number of bonding wires is increased to strengthen the ground function. Therefore, by setting the number of bonding wires to an appropriate number, it is possible to prevent the deterioration of the light band by making the lowest dip frequency higher than the frequency that reduces 3 dB (half value) from the reference value.

  An electrical signal (modulated signal) is input to one end 7i of the signal electrode 7. Since the first and second optical waveguides 10A and 10B are made of a material having an electro-optical effect such as lithium niobate, the first and second optical waveguides are generated by the electric field applied to the first and second optical waveguides 10A and 10B. The refractive indexes of the waveguides 10A and 10B change as + Δn and −Δn, respectively, and the phase difference between the pair of optical waveguides changes. The signal light modulated by the change in the phase difference is output from the output optical waveguide 10o.

  As described above, since the optical modulator 100 according to the present embodiment is a single drive type configured with one signal electrode 7, the area of the second ground electrode 8A can be sufficiently ensured, and the operation at high frequency is performed. It is possible. Further, by disposing the first ground electrode 8A on the opposite side of the signal electrode 7 to the second ground electrode 8B, the radiation loss can be reduced, and better high frequency characteristics can be obtained.

  FIG. 2 is a schematic cross-sectional view of the light modulator 100 taken along the line AA 'in FIGS. 1 (a) and 1 (b).

As shown in FIG. 2, the optical modulator 100 according to the present embodiment has a multilayer structure in which a substrate 1, a waveguide layer 2, a protective layer 3, a buffer layer 4, an insulating layer 5 and an electrode layer 6 are stacked in this order. Have. The substrate 1 is, for example, a sapphire substrate, and a waveguide layer 2 made of a lithium niobate film is formed on the surface of the substrate 1. The waveguide layer 2 has first and second optical waveguides 10A and 10B formed of a ridge portion 2r. The width W ₀ of the first and second optical waveguides 10A and 10B can be, for example, 1 μm.

The protective layer 3 is formed in a region not overlapping with the first and second optical waveguides 10A and 10B in plan view. The protective layer 3 covers the entire top surface of the waveguide layer 2 in the region where the ridge portion 2 r is not formed, and the side surface of the ridge portion 2 r is also covered by the protective layer 3. Scattering loss caused by the roughening of the The thickness of the protective layer 3 is substantially the same as the height of the ridge portion 2 r of the waveguide layer 2. Although the material of the protective layer 3 is not particularly limited, for example, silicon oxide (SiO ₂ ) can be used. It is also possible to omit the protective layer 3 and directly form the buffer layer 4 on the top surface of the waveguide layer 2.

The buffer layer 4 prevents the light propagating in the first and second optical waveguides 10A and 10B from being absorbed by the signal electrode 7 and the second ground electrode 8B. It is formed on the upper surface. As the buffer layer 4, a material having a refractive index smaller than that of the waveguide layer 2, for example, silicon oxide (SiO ₂ ) or aluminum oxide (Al ₂ O ₃ ) can be used, and the thickness is 0 It may be about 2 to 1 μm. In the present embodiment, the buffer layer 4 covers not only the upper surfaces of the first and second optical waveguides 10A and 10B but also the entire surface of the base surface including the upper surface of the protective layer 3. It may be patterned so as to selectively cover only the vicinity of the upper surface of the waveguides 10A and 10B.

  The insulating layer 5 is provided to form a step on the lower surface of the traveling wave electrode. An opening (slit) is formed in a region overlapping the first and second optical waveguides 10A and 10B of the insulating layer 5, and the upper surface of the buffer layer 4 is exposed. By partially embedding the electrode layer 6 in this opening, a step is formed on the lower surface of the signal electrode 7 and the second ground electrode 8B. The thickness T of the insulating layer 5 is preferably 1 μm or more. If the thickness of the insulating layer 5 is 1 μm or more, the effect of providing a step on the lower surface of the signal electrode 7 and the second ground electrode 8B can be obtained.

  The electrode layer 6 is provided with a signal electrode 7, a first ground electrode 8A and a second ground electrode 8B. The signal electrode 7 is provided so as to overlap the ridge portion 2 r corresponding to the first optical waveguide 10 A in order to modulate the light traveling in the first optical waveguide 10 A, and the first optical waveguide via the buffer layer 4 is provided. It is facing 10A. The second ground electrode 8B is provided so as to overlap the ridge portion 2r corresponding to the second optical waveguide 10B in order to modulate the light traveling in the second optical waveguide 10B. And the optical waveguide 10B of FIG. The second ground electrode 8B is provided on the opposite side of the first ground electrode 8A as viewed from the signal electrode 7.

  A bonding wire 9 is wired across the signal electrode 7 between the first ground electrode 8A and the second ground electrode 8B. Thus, by electrically connecting the first and second ground electrodes 8A and 8B, the imbalance of the in-plane potential of the ground electrode can be eliminated, and the high frequency characteristics of the optical modulator can be improved. Can.

The waveguide layer 2 is not particularly limited as long as it is an electro-optic material, but is preferably made of lithium niobate (LiNbO ₃ ). Lithium niobate has a large electro-optical constant and is suitable as a constituent material of an optical device such as an optical modulator. Hereinafter, the configuration of the present invention in the case of using a lithium niobate film as the waveguide layer 2 will be described in detail.

  The substrate 1 is not particularly limited as long as it has a lower refractive index than a lithium niobate film, but a substrate capable of forming a lithium niobate film as an epitaxial film is preferable, and a sapphire single crystal substrate or a silicon single crystal substrate is preferable. . The crystal orientation of the single crystal substrate is not particularly limited. Lithium niobate films have the property of being easily formed as c-axis oriented epitaxial films on single crystal substrates of various crystal orientations. Since the c-axis oriented lithium niobate film has 3-fold symmetry, it is desirable that the underlying single crystal substrate also have the same symmetry, and in the case of a sapphire single crystal substrate, c-plane, In the case of a silicon single crystal substrate, a substrate of (111) plane is preferable.

  Here, the epitaxial film is a single crystal film in which crystal orientations are aligned by crystal growth on a base single crystal substrate or a single crystal film. That is, an epitaxial film is a film having a single crystal orientation in the film thickness direction and in the film surface direction, and when the film surface direction is the XY plane and the film thickness direction is the Z axis, the crystal is All are aligned in the X-axis, Y-axis and Z-axis directions. Whether the film is an epitaxial film can be proved, for example, by confirming the peak intensity and the pole point at the alignment position in 2θ-θ X-ray diffraction.

  Specifically, first, when the measurement by 2θ-θ X-ray diffraction is performed, the peak intensity of all except the target surface is 10% or less, preferably 5% or less of the maximum peak intensity of the target surface. It needs to be. For example, in the c-axis-oriented epitaxial film of lithium niobate, the peak intensity other than the (00 L) plane is 10% or less, preferably 5% or less, of the maximum peak intensity of the (00 L) plane. (00L) is a display that collectively refers to equivalent surfaces such as (001) and (002).

Second, it is necessary for pole measurement to be visible in pole measurement. Under the conditions for confirming the peak intensity at the first orientation position described above, only the orientation in one direction is shown, and even if the first condition described above is obtained, the crystal orientation is aligned in the plane. If not, the X-ray intensity does not increase at a specific angular position, and no pole point is seen. Since LiNbO ₃ has a trigonal crystal structure, the number of pole points of LiNbO ₃ (014) in a single crystal is three. In the case of a lithium niobate film, it is known that epitaxial growth is performed in a so-called twin state in which crystals rotated 180 ° around the c-axis are symmetrically coupled. In this case, since three pole points are symmetrically coupled in two, there are six pole points. When a lithium niobate film is formed on a (100) plane silicon single crystal substrate, 4 × 3 = 12 pole points are observed because the substrate is four-fold symmetric. In the present invention, a lithium niobate film epitaxially grown in a twin crystal state is also included in the epitaxial film.

  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. As elements of A, K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc. There may be a combination of two or more.

  The film thickness of the lithium niobate film is preferably 2 μm or less. If the film thickness is larger than this, it becomes difficult to form a high quality film. On the other hand, when the film thickness of the lithium niobate film is too thin, confinement of light in the lithium niobate film becomes weak, and light leaks to the substrate or the buffer layer to be guided. Even when an electric field is applied to the lithium niobate film, the change in the effective refractive index of the optical waveguides (1a, 1b) may be small. Therefore, it is desirable that the lithium niobate film has a thickness of about 1/10 or more of the wavelength of light to be used.

As a method of forming a lithium niobate film, it is desirable to use 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 perpendicularly to the main surface of the substrate 1, 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 the single crystal substrate, a lithium niobate film can be epitaxially grown directly on the sapphire single crystal substrate. When silicon is used as a single crystal substrate, a lithium niobate film is formed by epitaxial growth via a cladding layer. As the cladding layer, one having a refractive index lower than that of a lithium niobate film and suitable for epitaxial growth is used. For example, when Y ₂ O ₃ is used as a cladding layer, a high quality lithium niobate film can be formed.

  As a method of forming a lithium niobate film, a method is also known in which a lithium niobate single crystal substrate is thinly polished or sliced. This method has the advantage of obtaining the same characteristics as a single crystal, and can be applied to the present invention.

The signal electrode 7 has a two-layer structure, and includes an upper layer portion 7 _H formed in the electrode layer 6 and a lower layer portion 7 _L embedded in an opening (first opening) penetrating the insulating layer 5. There is. The lower layer portion 7 _L of the signal electrode 7 is provided at an end portion of the upper layer portion 7 _H of the signal electrode 7 close to the second ground electrode 8 B. Therefore, the lower layer portion _{7 L} of the lower surface (first lower _{surface) S 11} of the signal electrode 7 is provided on the second ground electrode 8B nearer the upper part the lower surface of the _{7 H} (second bottom _{surface) S 12} . With this configuration, the first lower surface S ₁₁ of the signal electrode 7 is in contact with the upper surface of the buffer layer 4 above the first optical waveguide 10A, the first optical waveguide 10A via the buffer layer 4 Are facing each other. The second lower surface S ₁₂ of the signal electrode 7 is located above the first lower surface S _11, not in contact with the buffer layer 4.

Signal width _{W 11} of the lower portion _{7 L} of the lower surface _{S 11} of the electrode 7 is narrower than the upper portion ₇ width (signal full width of the electrode 7) _{W 1} of the _H. The lower layer portion _7L is formed only in the vicinity of the region overlapping the first optical waveguide 10A in plan view, and is not formed in the other regions. Therefore, the width _{W 11} of the first lower surface _{S 11} of the signal electrode 7 is the degree slightly wider than the width _{W 0} of the first optical waveguide 10A. In order to concentrate the electric field to the signal electrode 7, the width _{W 11} of the first lower surface _{S 11} of the signal electrode 7 is preferably 1.1 to 15 times the width _{W 0} of the optical waveguide 10A, 1. More preferably, it is 5 to 10 times.

The second ground electrode 8B is also a two-layer structure, and an upper layer portion 8B _H formed in the electrode layer 6, and a lower layer portion 8B _L embedded in the opening (second opening) that penetrates the insulating layer 5 Have. Lower portion 8B _L of the second ground electrode 8B is provided at an end portion of the signal electrode 7 side of the upper portion 8B _H of the second ground electrode 8B. Therefore, the lower surface (first lower _{surface) S 21} of the upper layer portion 8B _H of the second ground electrode 8B is provided to the signal electrode 7 nearer the lower portion lower surface of the 8B _L (second bottom _{surface) S 22} . With this configuration, the first lower surface S ₂₁ of the second ground electrode 8B is in contact with the upper surface of the buffer layer 4 above the second optical waveguide 10B, the second optical via the buffer layer 4 It faces the waveguide 10B. The second lower surface S ₂₂ of the second ground electrode 8B is positioned above the first lower surface S _21, not in contact with the buffer layer 4.

The width W (the width of the second lower surface S ₂₁ ) W ₂₁ of the lower layer portion 8 B _L of the second ground electrode 8 B is the width W (the entire width of the second ground electrode 8 B) W of the upper layer portion 8 B _{H It} is narrower than _two . Lower portion 8B _L of the second ground electrode 8B is formed only in the vicinity of a region overlapping with the second optical waveguide 10B in plan view, it is not formed in the other region. Therefore, the width _{W 21} of the first lower surface _{S 21} of the second ground electrode 8B is a degree slightly larger than the width _{W 0} of the second optical waveguide 10B. Therefore, the width _{W 21} lower part of the 8B _L in the X direction of the second ground electrode 8B is smaller than the X direction width _{W 22} of the upper layer portion 8B _H. In order to concentrate the electric field on the second optical waveguide 10B, the width _{W 21} of the first lower surface _{S 21} of the second ground electrode 8B is 1.1 to 5 times the width _{W 0} of the optical waveguide 10B Is preferable, and 1.5 to 3 times is more preferable.

Insulating layer 5 which is present in the lower inter-electrode region G ₂ between the signal electrodes 7 and the second ground electrode 8B, at least a portion of the buffer layer 4 and the protective layer 3 may be removed. In this case, only the insulating layer 5 may be removed, only the insulating layer 5 and the buffer layer 4 may be removed, and the insulating layer 5, the buffer layer 4 and the protective layer 3 may be removed. The effective refractive index of the traveling wave electrode can be lowered by removing a part of the protective layer 3, the buffer layer 4, and the insulating layer 5, and the effective refractive index of the traveling wave electrode matches the effective refractive index of light. The speed matching can be made better.

  The first ground electrode 8A is provided on the opposite side of the signal electrode 7 to the second ground electrode 8B. The first ground electrode 8A has a single-layer structure consisting of only a conductor provided in the electrode layer 6, but may have a two-layer structure as well as the signal electrode 7 and the second ground electrode 8B.

The width W ₂ of the upper layer portion 8 _BH of the second ground electrode 8 B is wider than the width W ₁ of the upper layer portion 7 _H of the signal electrode 7. It is preferred also wider than the width W ₁ of the upper portion 7 _H of the width W ₃ is also the signal electrode 7 of the first ground electrode 8A. By making the area of each of the first and second ground electrodes 8A and 8B larger than the area of the signal electrode 7, radiation loss can be reduced, and good high frequency characteristics can be obtained.

First and second optical waveguides 10A, the sectional structure shown in FIG. 2 10B to the vertical cut, (the width of the inter-electrode region _{G 3)} Interval _{W G3} between the signal electrode 7 and the first ground electrode 8A is widely set than the signal electrodes 7 and the distance W _G2 between the second ground electrode 8B _(the width of the inter-electrode region G _2). The distance between the two electrodes means the shortest distance between the two in the X direction. When the distance W _G3 between the signal electrode 7 and the first ground electrode 8A is smaller than the distance W _G2 between the signal electrode 7 and the second ground electrode 8B, the pair of light guides is affected by the first ground electrode 8A. The difference in magnitude of the electric field applied to the waveguide increases, which causes the wavelength chirp. However, the distance W _G3 between the signal electrode 7 and the first ground electrode 8A corresponds to the signal electrode 7 and the second ground electrode 8B. By setting the distance between the first and second ground electrodes 8A and 8B to be wider than the interval W _{G2 of} the first and second optical waveguides 10A and 10B, the influence of the first ground electrode 8A on the electric field applied to the pair of optical waveguides can be reduced. The wavelength chirp can be reduced by adjusting to the same size as much as possible.

In the present embodiment, the width _{W 11} of the lower surface _{S 11} of the signal electrode 7 is wider than the width _{W 21} of the lower surface _{S 21} of the second ground electrode 8B _{(W 11>} W ₂₁₎ is preferably. When the first ground electrode 8A is provided next to the signal electrode 7 as described above, good high frequency characteristics can be obtained by reducing the radiation loss, but since the electrode structure becomes asymmetric, A problem arises. If no is provided a first ground electrode 8A, and a width _{W 21} of the lower portion 8B _L between the width _{W 11} of the lower portion _{7 L} of the signal electrodes 7 and the second ground electrode 8B same _(W 11 ₌ W ₂₁ , The magnitude of the electric field applied to the pair of optical waveguides can be made substantially the same, but when the first ground electrode 8A is provided next to the signal electrode 7 as described above, W _{11 =} only the W ₂₁ can not be substantially the same size of the electric field applied to the first and second optical waveguides 10A, 10B. However, in the case of W ₁₁ > W ₂₁ , it is possible to suppress the influence of the first ground electrode 8 A and make the electric fields respectively applied to the pair of optical waveguides to have substantially the same magnitude, thereby wavelength chirping. It can be prevented.

  As described above, in the optical modulator 100 according to the present embodiment, the bonding wire 9 connecting the first ground electrode 8A and the second ground electrode 8B across the signal electrode 7 is provided. The high frequency characteristics can be improved by enhancing the ground function of the optical fiber, and the widening of the optical band can be realized.

Further, in the optical modulator 100 according to the present embodiment, since the widths W ₂ and W _{3 of} the first and second ground electrodes 8A and 8B are wider than the width W ₁ of the signal electrode 7, radiation loss can be reduced. Characteristics can be obtained. In addition, by making the second ground electrode 8B a two-layer structure and making the width W ₂₁ of the lower surface S ₂₁ of the lower layer portion 8B _L narrower than the width W ₂ of the upper layer portion 8B _H , an electric field is applied to the second optical waveguide 10B. The concentration can be concentrated, and the difference in magnitude of the electric field applied to the pair of optical waveguides can be reduced to reduce the wavelength chirp of the modulated light. Furthermore, the width _{W 11} of the lower surface _{S 11} of the signal electrode 7 as well as wider than the width _{W 21} of the lower portion 8B _L of the second ground electrode 8B, the interval between the signal electrode 7 and the first ground electrode 8A _{W G3} Is made wider than the distance W _G2 between the signal electrode 7 and the second ground electrode 8B, thereby further reducing the difference in magnitude of the electric field applied to the pair of optical waveguides under the influence of the first ground electrode 8A. And the wavelength chirp of the modulated light can be further reduced.

  FIG. 3 is a schematic cross-sectional view showing the configuration of an optical modulator 200 according to the second embodiment of the present invention.

As shown in FIG. 3, the feature of the optical modulator 200 according to the present embodiment is that the upper layer portion 7 _H and the lower layer portion 7 _L of the signal electrode 7 have the same width W ₁ , and both have the second ground electrode 8 B. The lower layer portion 8B _L has a point (W ₁ > W ₂₁ ) that is larger than the width W ₂₁ . The other configuration of the light modulator 200 is the same as the light modulator 100 according to the first embodiment. In the present embodiment, since the cross-sectional area of the signal electrode 7 is reduced, the electrode loss is slightly increased as compared with the first embodiment, but the other features are not changed. Therefore, this embodiment can also achieve the same effect as that of the first embodiment.

  FIG. 4 is a plan view of an optical modulator 300 according to a third embodiment of the present invention, wherein (a) shows only the optical waveguide, and (b) shows the entire optical modulator 300 including the traveling wave electrode. Is illustrated.

As shown in FIGS. 4A and 4B, the feature of the optical modulator 300 according to the present embodiment is that the Mach-Zehnder optical waveguide 10 is configured by a combination of a linear portion and a curved portion. More specifically, the Mach-Zehnder optical waveguide 10 includes first to third straight portions 10e ₁ , 10e ₂ , 10e ₃ , a first straight portion 10e _1, and a second straight portion, which are disposed parallel to each other. 10e ₂ and the first curved portion 10f ₁ that connects has a second straight portion 10e ₂ of the second curved portion 10f ₂ that connects the third straight portion 10e _3, thereby substantially S A letter-shaped optical waveguide is constructed.

In the optical modulator 300 according to the present embodiment, for example, the cross-sectional structure of the linear portions 10 e ₁ , 10 e ₂ , and 10 e ₃ of the Mach-Zehnder optical waveguide 10 along the line AA ′ in FIG. It is configured to have the cross-sectional structure shown. That is, the signal electrodes 7 covers the upper side of the first optical waveguide 10A in the first to third linear portions _{10e 1,} 10e 2, 10e _3, the first lower surface _{S 11} of the signal electrode 7, the buffer layer The first optical waveguide 10A faces the first optical waveguide 10A in the first to _third linear portions 10e ₁ , 10e ₂ , and 10e ₃ through the fourth. The second ground electrode 8B covers the second optical waveguide 10B in the first to third linear portions _{10e 1, 10e 2, 10e 3} , the first lower surface _{S 21} of the second ground electrode 8B is It faces the second optical waveguide 10B in the _{first to} third linear portions 10e ₁ , 10e ₂ , 10e ₃ via the buffer layer 4. The first lower surface _{S 21} of the first lower surface _{S 11} and the second ground electrode 8B of the signal electrodes 7, that covers the whole of the first to third linear portion _{10e 1,} 10e 2, 10e ₃ preferred, for example, the first may cover only the linear portion 10e _1.

In this embodiment, the input light Si is input to a first end of the linear portion 10e _1, toward the other end progresses from the first end of the linear portion 10e _1, folded in the first bending section 10f ₁ Te is the first linear portion 10e ₁ toward the other end from the second end of the linear portion 10e ₂ travels in the reverse direction, further the second third is folded back at the curved portion 10f ₂ of the linear portion 10e ₃ toward from one end to the other traveling in a first same direction as the straight portion 10e _1.

  In the optical modulator, a long element length is a big problem in practical use, but as shown in the figure, the element length can be greatly shortened by configuring the optical waveguide to be folded, and a remarkable effect can be obtained. In particular, an optical waveguide formed of a lithium niobate film is characterized in that the loss is small even if the radius of curvature is reduced to, for example, about 50 μm, and is suitable for the present embodiment.

  The plurality of bonding wires 9 are preferably discretely arranged over the entire length of the optical waveguide. As the number of bonding wires 9 increases, the ground function of the ground electrode can be enhanced, but it is desirable to efficiently enhance the number of bonding wires as small as possible. For that purpose, as described above, the number of bonding wires 9 should be set so that the lowest frequency of dip appearing in the EO frequency response characteristic of the optical modulator is higher than the frequency which decreases 3 dB (half value) from the reference value. preferable.

A pair of optical waveguides 10A wired in a substantially S-shaped folded twice as in this embodiment, in 10B, the bonding wire 9 is provided on at least a second cross the straight portion 10e ₂ position of the central Is preferred. By doing this, the unbalance of the in-plane potential of the ground electrode can be greatly improved, and the broadening of the light band can be achieved. In the case of the meander pattern in which the S-shaped pattern is continuous by folding back three times or more, all the straight line portions sandwiched between the two straight line portions on both sides are the placement targets of the bonding wire 9.

  FIGS. 5 (a) and 5 (b) are schematic cross-sectional views showing the configuration of the optical modulator according to the fourth and fifth embodiments of the present invention.

The optical modulators 400A and 400B shown in FIGS. 5A and 5B are so-called dual drive types, and the first and second optical waveguides 10A and 10B are provided with the first and the second optical waveguides via buffer layers 4. The second signal electrodes 7A and 7B have symmetrical left and right electrode structures. The first and second signal electrodes 7A and 7B respectively have upper layer portions 7A _H and 7B _H and lower layer portions 7A _L and 7B _L, and the upper layer portions 7A _H and 7B _H have widths of the lower layer portions 7A _L and 7B. _It is wider than the width of _L. The inter-electrode region G _1A between the first signal electrode 7A and the first ground electrode 8A is equal to the inter-electrode region G _1B between the second signal electrode 7B and the second ground electrode 8B. According to the present embodiment, the magnitudes of the electric fields respectively applied to the pair of optical waveguides can be made as equal as possible when the voltages having the same size and opposite sign are applied to the pair of signal electrodes Wavelength chirp of light can be prevented.

  The optical modulator 400A shown in FIG. 5A has an electrode structure in which the first and second ground electrodes 8A and 8B are disposed so as to sandwich the first and second signal electrodes 7A and 7B. A ground electrode is not provided between the two signal electrodes 7A and 7B. In addition, since bonding wire 9 electrically connecting first ground electrode 8A and second ground electrode 8B is provided across first and second signal electrodes 7A and 7B, the ground function of the ground electrode is provided. To enhance the optical band bandwidth.

In addition to the first and second ground electrodes 8A and 8B, the optical modulator 400B shown in FIG. 5B has an inter-electrode region G ₀ between the first signal electrode 7A and the second signal electrode 7B. It further includes a third ground electrode 8C disposed therein. As bonding wires, a bonding wire 9A electrically connecting the first ground electrode 8A and the third ground electrode 8C across the first signal electrode 7A, and a second connection across the second signal electrode 7B And a bonding wire 9B electrically connecting the third ground electrode 8C to the third ground electrode 8C. The ends of the bonding wires 9A and 9B connected to the upper surface of the third ground electrode 8C are crimped at positions mutually offset with respect to the traveling direction of the optical waveguide without being directly connected to each other. In this case, it is preferable that the bonding wires 9A and the bonding wires 9B be alternately arranged along the traveling direction of the optical waveguide. Even with such a configuration, it is possible to enhance the ground function of the ground electrode and achieve broadening of the light band.

  Although the preferred embodiments of the present invention have been described above, 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. It is needless to say that they are included in the scope.

  For example, although the bonding wire 9 is used as a connection conductor for electrically connecting between the ground electrodes on both sides across the signal electrode in the above embodiment, the present invention is not limited to such a configuration, for example, The whole or a part of the bonding wire 9 may be replaced with a thin film bridge or the like. That is, in the present invention, it is also possible to connect two ground electrodes using other connection conductors other than the bonding wire 9.

<Discussion of existence of bonding wire>
Insertion loss characteristics of the traveling wave electrode directly linked to the EO frequency response characteristics of the optical modulator in order to consider the influence of the bonding wire connecting the pair of ground electrodes across the signal electrodes on the high frequency characteristics of the optical modulator (S21 Characteristics) were evaluated. In the evaluation test, as shown in FIG. 6A, an electrode structure in which an optical waveguide is not formed on a sapphire substrate but an insulating layer 5 made of a silicon oxide film and an electrode layer 6 forming a coplanar line are sequentially formed is used. It was. In this electrode structure for evaluation, the electrode spacing W _G2 = 8.5 μm, the electrode spacing W _G3 = 60 μm, and the width W ₁ of the signal electrode 7 = 34 μm. Further, as shown in FIG. 6 (b), the traveling wave electrode has an S-shaped wiring pattern in plan view, and in the first embodiment, in particular, 29 bondings across the signal electrode 7 between the pair of ground electrodes 8A and 8B. A wire 9 was provided. Specifically, nine bonding wires 9 are equally allocated to the linear portions 10e ₁ , 10e ₂ and 10e ₃ of the traveling wave electrodes, and one bonding wire 9 is provided at the middle point of the _two curved portions 10f ₁ and 10f _2. I assigned each book. As the bonding wire 9, for example, a metal wire with a diameter of 10 to 100 μm can be used, and here, a gold wire with a diameter of 23 μm was used. On the other hand, such a bonding wire 9 was not provided in the comparative example. Then, a high frequency signal was applied to one end of the signal electrode 7, and the S21 characteristic when the high frequency signal was extracted from the other end of the signal electrode 7 was determined. The results are shown in FIG.

  As shown in FIG. 7, the S21 characteristics gradually decrease with the increase of the frequency, and a relatively large number of relatively large dips are generated in the S21 characteristics of the comparative example in which the bonding wire 9 is not provided. In the S21 characteristics of Example 1 in which D. was provided, almost no dip occurred, and good results were obtained. In the comparative example, a slight sign of dip appears at about 2.5 GHz, the dip appears clearly at about 7.5 GHz which is the third harmonic, and the dip becomes larger as the frequency becomes higher at higher harmonics.

<Discussion of the lowest dip frequency>
The influence of the number of bonding wires on the lowest dip frequency appearing in the S21 characteristics of the traveling wave electrode was evaluated. FIG. 8 is a view showing the relationship between the evaluation model of the traveling wave electrode and the lowest frequency of the dip appearing in the S21 characteristic, and FIG. 8 (a) is a schematic plan view of the evaluation model, and FIG. 8 (b) is an evaluation. It is a graph of the S21 characteristic of a model.

As shown in FIG. 8A, when the length L of the coplanar line is 22.8 mm and the number N of bonding wires is 5, the S21 characteristic becomes as shown in FIG. F _dip1 became about 8.1 GHz. Also, assuming that the effective refractive index n of the traveling wave electrode is n = 2.24, the effective division length L _diveff of the coplanar line is L × n / (N + 1) = 0.12. The effective division length L _diveff of the coplanar line is an average value of the lengths of a plurality of coplanar line portions divided by providing bonding wires, and the lengths of the respective coplanar line portions are two adjacent _ones . The distance along the coplanar line between the wires or the distance along the coplanar line between the wire and the input or output.

Subsequently, an approximate straight line is obtained from a plurality of plots of the lowest dip frequency F _dip1 when the length L of the coplanar line and the number N of bonding wires are changed as variable parameters, and the effective division length L _diveff and dip The relationship with the lowest frequency F _{dip1 of} Depending on the length L of the coplanar line and the number N of bonding wires, the bonding wires may be provided at equal intervals or at different intervals.

FIG. 9 is a graph showing the relationship between the number of bonding wires and the lowest dip frequency of the S21 characteristic, the horizontal axis _being the reciprocal 1 / _Ldiveff [1 / mm] of the effective division length _Ldiveff of the coplanar line, The vertical axis indicates the dip's lowest frequency F _dip1 [GHz].

As shown in FIG. 9, the relational expression between the reciprocal of the effective division length L _diveff and the lowest dip frequency F _{dip1 differs} between the case where the optical waveguide is linear and the case where it is S-shaped, and the case where it is S-shaped Y = 65. 84x, and y = 150x in the case of the linear type. Using this relational expression, it is possible to determine the number of bonding wires such that the lowest frequency of the dip appearing in the S21 characteristic is higher than the frequency which is reduced by 3 dB (half value) from the reference value.

Here, the graph of FIG. 9 obtained here is the full width W _{1 of} the signal electrode 7, the distance W _G2 between the signal electrode 7 and the second ground electrode 8B, and the distance between the signal electrode 7 and the first ground electrode 8A. _Varying the value of the interval W _G3 hardly changed and was not influenced by the electrode width and the electrode spacing. However, when the values of the interval _WG2 and the interval _WG3 are different, the dip improving effect is particularly large.

<Consideration of the position of the bonding wire to the S-shaped optical waveguide>
In order to consider the influence of the position of the bonding wire 9 on the EO frequency response characteristic of the optical modulator, the S21 characteristic of the S-shaped coplanar electrode was evaluated.

As shown in FIG. 10A, in the traveling wave electrode structure of the second embodiment, a bonding wire 9 is provided on the central linear portion 10e ₂ of the linear portions 10e ₁ , 10e ₂ and 10e ₃ of the traveling wave electrodes. Instead, nine bonding wires 9 (total 18 wires) were provided at equal intervals on the straight portions 10 e ₁ and 10 e ₃ on both sides. As a result, as shown in FIG. 11A, the S21 characteristic of Example 2 had a very large loss at high frequencies.

10 as shown in (b), the traveling wave electrode structure of Example 3, only the upper central straight portion 10e ₂ on provided nine bonding wires 9 at regular intervals, on both sides of the straight portions _10e 1, 10e ₃ above No bonding wire 9 was provided. As a result, as shown in FIG. 11B, in the S21 characteristic of Example 3, although a large number of dips were generated, the loss at a high frequency was small, and a good high frequency characteristic was obtained.

DESCRIPTION OF SYMBOLS 1 substrate 2 waveguide layer 2 r ridge portion 3 protective layer 4 buffer layer 5 insulating layer 6 electrode layer 7, 7 A, 7 B signal electrode 7 _H , 7 A _H , 7 B _H signal electrode upper layer portion 7 _L , 7 A _L , 7 B _L signal Lower layer portion 7i of electrode One end 7o of signal electrode Other end 8A of signal electrode First ground electrode 8B Second ground electrode 8B _H Second ground electrode upper layer 8B _L Second ground electrode lower layer 8C Third Installation electrodes 9, 9A, 9B Bonding wire (connection conductor)
10 Mach-Zehnder optical waveguides 10A first optical waveguide 10B second optical waveguide 10c branching portion 10d multiplexing section 10e ₁ first straight portion 10e ₂ second linear portion 10e ₃ third linear portion 10f ₁ first bending portion 10f ₂ the second bending portion 10i input optical waveguide 10o output optical waveguide 12 terminating resistor 100,200,300,400A, 400B light modulator _{G 0, G 1A,} the _{G 1B,} G _2, G ₃ inter-electrode region _{S 11} signal electrodes lower surface Si input light So modulated light upper portion of the lower surface S ₂₂ second ground electrode in the lower portion of the lower surface S ₂₁ second ground electrode of the upper portion of the lower surface S ₁₂ signal electrodes of the lower portion

Claims (10)

A substrate,
A waveguide layer made of an electro-optical material film formed on the substrate and including first and second optical waveguides adjacent to each other;
A buffer layer covering at least the upper surfaces of the first and second optical waveguides;
A first signal electrode facing the first optical waveguide via the buffer layer;
First and second ground electrodes provided so as to sandwich the first signal electrode;
An optical modulator comprising: at least one connection conductor electrically connecting the first ground electrode and the second ground electrode across the first signal electrode.   The light modulator according to claim 1, wherein the connection conductor is a bonding wire.   The optical modulator according to claim 2, wherein the number of bonding wires is set such that the lowest frequency of dip appearing in the Electro-Optical frequency response characteristic is higher than the frequency at which the dip is 3 dB down from the reference value. The first ground electrode is provided on the opposite side of the second optical waveguide as viewed from the first optical waveguide,
The second ground electrode faces the second optical waveguide through the buffer layer,
The distance between the first signal electrode and the first ground electrode is wider than the distance between the first signal electrode and the second ground electrode. Light modulator. The first signal electrode has a first lower layer portion facing the first optical waveguide through the buffer layer, and a first upper layer portion provided above the first lower layer portion. And
The second ground electrode has a second lower layer portion facing the second optical waveguide through the buffer layer, and a second upper layer portion provided above the second lower layer portion. And
The light modulator according to claim 4, wherein a width of a lower surface of the second lower layer portion is narrower than a width of the second upper layer portion.   The light modulator according to claim 5, wherein a width of a lower surface of the first lower layer portion is narrower than a width of the first upper layer portion. An electrode layer provided above the buffer layer and including the first signal electrode and the first and second ground electrodes;
An insulating layer provided between the buffer layer and the electrode layer;
The insulating layer has first and second openings located above the first and second optical waveguides,
The first upper layer portion is formed on the electrode layer, and the first lower layer portion is embedded in the first opening,
The light modulator according to claim 5, wherein the second upper layer portion is formed in the electrode layer, and the second lower layer portion is embedded in the second opening. The first and second optical waveguides have at least one straight portion and at least one curved portion.
The lower surface of the first lower layer portion of the first signal electrode faces the straight portion of the first optical waveguide via the buffer layer,
The lower surface of the second lower layer portion of the second ground electrode faces the straight portion of the second optical waveguide via the buffer layer. Light modulator as described. The first and second optical waveguides have a first curve connecting the first to third straight portions parallel to each other, and the other end of the first straight portion and one end of the second straight portion. A substantially S-shaped waveguide including a portion and a second curved portion connecting the other end of the second linear portion and one end of the third linear portion,
The light according to any one of claims 1 to 7, wherein the connecting conductor is provided at a position straddling the first signal electrode covering at least the second linear portion of the first optical waveguide. Modulator. It further comprises a second signal electrode facing the second optical waveguide through the buffer layer,
The second ground electrode is provided on the opposite side of the first optical waveguide as viewed from the second optical waveguide,
The first and second ground electrodes are provided to sandwich the first and second signal electrodes,
The connection conductor electrically connects the first ground electrode and the second ground electrode across the first and second signal electrodes. The light modulator described in.