OPTICAL MODULATOR

An optical modulator includes a plurality of optical modulation units having a Mach-Zehnder type optical waveguide consisting of two optical waveguides and a high-frequency line pair arranged along the two optical waveguides and consisting of two signal electrodes for applying a pair of differential high-frequency signals, and a plurality of high-resistance conductive films are provided between adjacent high-frequency line pairs separated from the high-frequency line pair.

TECHNICAL FIELD

The present invention relates to an optical modulator. Priority is claimed on Japanese Pat. Application No. 2020-64606, filed Mar. 31, 2020, the content of which is incorporated herein by reference.

BACKGROUND ART

Communication traffic has been remarkably increased with widespread Internet use, and optical fiber communication is increasingly significant. The optical fiber communication is a technology that converts an electric signal into an optical signal and transmits the optical signal through an optical fiber and has a wide bandwidth, a low loss, and a resistance to noise.

As a system for converting an electric signal into an optical signal, there are known a direct modulation system using a semiconductor laser and an external modulation system using an optical modulator. The direct modulation system does not require the optical modulator and is thus low in cost, but has a limitation in terms of high-speed modulation and, thus, the external modulation system is used for high-speed and long-distance applications.

As the optical modulator, a Mach-Zehnder optical modulator in which an optical waveguide is formed by Ti (titanium) diffusion in the vicinity of the surface of a lithium niobate single-crystal substrate has been put to practical use (see, e.g., Patent Literature 1). The Mach-Zehnder optical modulator uses an optical waveguide (Mach-Zehnder optical waveguide) having a Mach-Zehnder interferometer structure that separates light emitted from one light source into two beams, makes the two beams pass through different paths, and then recombines the two beams to cause interference. Although high-speed optical modulators having a modulation speed of 40 Gb/s or higher are commercially available, they have a major drawback that the entire length thereof is as long as about 10 cm.

In contrast, Patent Literatures 2 and 3 disclose a Mach-Zehnder type optical modulator using a c-axis oriented lithium niobate film. The optical modulator using the lithium niobate film has realized a significant reduction in size and a lower drive voltage as compared with the optical modulator using a lithium niobate single crystal substrate.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In an optical modulator that has been downsized, in order to reduce ripple / crosstalk and realize good characteristics, a configuration that has a differential line and does not have a ground electrode, or a configuration that does not have a ground electrode except for the end part of the differential line, has been proposed (see, Patent Literature 4). However, in order to further reduce the size, it is necessary to narrow the distance between the differential lines, but in this case, there is a problem that the crosstalk characteristics are deteriorated. A configuration that maintains good crosstalk characteristics even when the distance between the differential lines is narrowed is desired.

As a result of diligent studies, the present inventor has found that the crosstalk characteristics are improved by repeatedly arranging isolated high-resistance conductive films between two sets of adjacent differential lines, and came up with the present invention.

An object of the present invention is to provide an optical modulator having good crosstalk characteristics.

Solution to Problem

The present invention provides the following means for solving the above problems.

The optical modulator according to one aspect of the present invention includes a plurality of optical modulation units having a Mach-Zehnder type optical waveguide consisting of two optical waveguides, and a high-frequency line pair arranged along the two optical waveguides and consisting of two signal electrodes for applying a pair of differential high-frequency signals, and a plurality of high-resistance conductive films are provided between adjacent high-frequency line pairs separated from the high-frequency line pair.

In the optical modulator according to the above aspect, the plurality of high resistance conductive films may be arranged side by side along the direction in which the high frequency line pair extends.

In the optical modulator according to the above aspect, at least two or more of the plurality of high resistance conductive films may have the same shape.

In the optical modulator according to the above aspect, the conductivity of the high resistance conductive films may be 10 to 1 x 108[s / m].

Advantageous Effects of Invention

According to the present invention, it is possible to provide an optical modulator having good crosstalk characteristics.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings as appropriate. In each of the following embodiments, the same or equal parts may be designated by the same reference numerals in the drawings. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited thereto, and can be appropriately modified and carried out within the range in which the effects of the present invention. The configuration shown in one embodiment can also be applied to other embodiments.

FIG.1Ais a schematic plan view of a part of the optical modulator according to the first embodiment of the present invention, andFIG.1Bshows only optical waveguides.

The optical modulator100shown inFIG.1Ahas optical modulation units30A,30B,30C,30D with Mach-Zender optical waveguides20A,20B,20C,20D composed of two optical waveguides20a,20band high frequency line pairs10A,10B,10C,10D composed of two signal electrodes10aand10barranged along the two optical waveguides20aand20bfor applying a pair of differential high-frequency signals. Further, in the optical modulator100, a plurality of high resistance conductive films40A-1to40A-5,40B-1to40B-5,40C-1to40C-5are provided, which are separated from the high frequency line pairs between the adjacent high frequency line pairs10A and10B,10B and10C,10C and10D, respectively. In the following, the Mach-Zehnder optical waveguides20A,20B,20C, and20D may be collectively referred to as the Mach-Zehnder optical waveguide20. Further, the high frequency line pairs10A,10B,10C, and10D may be collectively referred to as the high frequency line pair10. Further, the optical modulation units30A,30B,30C, and30D may be collectively referred to as the optical modulation unit 30. Further, the high resistance conductive films40A-1to40A-5,40B-1to40B-5, and40C-1to40C-5may be collectively referred to as the high resistance conductive film 40.

The optical modulator100includes a plurality of Mach-Zehnder optical waveguides20, a plurality of high-frequency line pairs10, and a plurality of high-resistance conductive films 40 arranged between adjacent high-frequency line pairs10apart from the high-frequency line pairs.

The four optical modulation units30A,30B,30C, and30D can have substantially the same structure.

Each of the Mach-Zehnder optical waveguides20A,20B,20C, and20D is an optical waveguide having a structure of a Mach-Zehnder interferometer, and the first and second optical waveguides20aand20bbranched from one optical waveguide by an optical branching portion (not shown), in which the first and second optical waveguides20aand20bare combined into one optical waveguide via an optical coupling portion (not shown). The input light is branched at the optical branching portion and travels through the first and second optical waveguides20aand20b, respectively, then combined at the optical coupling portion, and is output from the optical waveguide as modulated light.

The optical modulator of the present invention includes at least two Mach-Zehnder optical waveguides in the optical modulator, and each Mach-Zehnder optical waveguide also includes so-called nested optical waveguides in which another Mach-Zehnder optical waveguide is incorporated in a nested form in two branched optical waveguides (parallel optical waveguides, see optical waveguides shown by reference numerals20aand20b) of one Mach-Zehnder optical waveguide.

The two signal electrodes10aand10bconstituting each of the four high-frequency line pairs are arranged side by side in the two optical waveguides20aand20bin order to apply a high-frequency differential signal. In addition, only the portion of the signal electrodes10aand10bwhere the optical waveguides20aand20bextend in parallel and linearly, is shown inFIG.1A.

The optical modulator of the present invention does not have a ground electrode and has a plurality of high resistance conductive films that are isolated and floated from the surroundings.

A plurality of high-resistance conductive films arranged between adjacent high-frequency line pairs are regularly arranged side by side along the direction in which the high-frequency line pairs extend. In this embodiment, a plurality of high resistance conductive films are arranged between adjacent high frequency line pairs10A and10B,10B and10C,10C and10D. Instead of this configuration, a configuration may be provided in which a plurality of high resistance conductive films are arranged at least one between adjacent high frequency line pairs10A and10B,10B and10C,10C and10D. A plurality of high-resistance conductive films40A-1to40A-5arranged apart from each other along the extending direction of the high-frequency line pair are arranged between the high-frequency line pair10A and the high-frequency line pair10B. A plurality of high-resistance conductive films40B-1to40B-5arranged apart from each other along the extending direction of the high-frequency line pair are arranged between the high-frequency line pair10B and the high-frequency line pair10C. A plurality of high-resistance conductive films40C-1to40C-5arranged apart from each other along the extending direction of the high-frequency line pair are arranged between the high-frequency line pair10C and the high-frequency line pair10D.

The high resistance conductive films40A-1to40A-5,40B-1to40B-5, and40C-1to40C-5all have the same substantially rectangular shape.

As the material of the high resistivity conductive film 40, a material with lower conductivity than a material with high conductivity as generally used for a signal electrode is used, in other words, a material with high resistivity as compared with a material with high conductivity as generally used for a signal electrode is used. That is, a material with lower conductivity than a metal material such as Au, Cu, Ag, Pt, or a material with high electrical resistivity is used.

The high resistance conductive film 40 has an effect of reducing crosstalk between adjacent high frequency line pairs. The magnetic field generated in the direction perpendicular to the signal electrode generates an electromotive force in each high resistance conductive film, and an eddy current flows, but because of the high resistance, it is consumed as heat and the power that reaches the adjacent high frequency line pair is reduced. It is considered that the crosstalk is reduced by this. Therefore, the material of the high resistance conductive film 40 requires conductivity to the extent that an eddy current flows, but electrical resistance to the extent that it is consumed as heat is required.

The material of the high resistance conductive film 40 is preferably a material having a conductivity of 10 to 1 x 108[s / m], and it is more preferably a material having a conductivity of 102to 1 x 106[s / m], and it is further preferably a material having a conductivity of 1 x 103to 1 x 105[s / m]. In other words, using an electric resistivity, the material of the high resistance conductive film 40 is preferably a material having an electric resistivity of 0.1 to 1 x 10-8[Ω · m], and it is more preferably a material having an electric resistivity of 1 x 10-2to 1 x 10-6[Ω · m], and it is further preferably a material having an electric resistivity of 1 x 10-3to 1 x 10-5[Ω · m]. The film thickness of the high resistance conductive film 40 is preferably smaller than the film thickness of the signal electrode, and is preferably 1 µm or less.

As the material of the high resistance conductive film 40, for example, graphite, ITO, ZnO, CuO, NiCrTa, TaN and the like can be exemplified.

FIG.2is a schematic cross-sectional view of the optical modulator100along the A-A′ line ofFIG.1A.

The optical modulator100has a multilayer structure in which a substrate1, a waveguide layer2, a protective layer3, a buffer layer4, an insulating layer5, and a layer including signal electrodes10aand10b(hereinafter, may be referred to as an electrode layer10) are laminated in this order.

The substrate1is, for example, a sapphire substrate, and a waveguide layer2made of a lithium niobate film is formed on the surface of the substrate1. The waveguide layer2has first and second optical waveguides20aand20bcomposed of ridges. The widths of the first and second optical waveguides20aand20bcan be, for example, 1 µm.

The protective layer3is formed in a region that does not overlap with the first and second optical waveguides20aand20bin a plan view. The protective layer3covers the entire surface of the upper surface of the waveguide layer2in which the ridge is not formed. Since the side surface of the ridge is also covered with the protective layer3, it is possible to prevent the scattering loss caused by the roughness of the side surface of the ridge. The thickness of the protective layer3is substantially the same as the height of the ridge of the waveguide layer2. The material of the protective layer3is not particularly limited, but for example, silicon oxide (SiOz) can be used. It is also possible to omit the protective layer3and directly form the buffer layer4on the upper surface of the waveguide layer2.

The buffer layer4is formed on the upper surface of the ridge of the waveguide layer2in order to prevent the light propagating in the first and second optical waveguides20aand20bfrom being absorbed by the signal electrodes10aand10b. As the buffer layer4, a material having a refractive index smaller than that of the waveguide layer2, for example, silicon oxide (SiO2) or aluminum oxide (Al2O3) can be used, and the thickness thereof may be about 0.2 µm to 1 µm. In the present embodiment, the buffer layer4covers not only the upper surfaces of the first and second optical waveguides20aand20bbut also the entire surface of the base surface including the upper surface of the protective layer3. Instead of this configuration, it may be patterned so as to selectively cover only the vicinity of the upper surfaces of the first and second optical waveguides20aand20b.

The insulating layer5is provided to form a step on the lower surface of the signal electrodes10aand10b. An opening (slit) is formed in a region of the insulating layer5that overlaps with the first and second optical waveguides20aand20b, and the upper surface of the buffer layer4is exposed. By embedding a part of the electrode layer10in this opening, a step is formed on the lower surface of the signal electrodes10aand10b. The thickness T of the insulating layer5is preferably 1 µm or more. When the thickness of the insulating layer5is 1 µm or more, the effect of providing a step on the lower surfaces of the signal electrodes10aand10bcan be obtained.

The electrode layer10is provided with signal electrodes10aand10b. The signal electrode10ais provided so as to be superimposed on the ridge corresponding to the first optical waveguide20ain order to modulate the light traveling in the first optical waveguide20a, and faces the first optical waveguide20avia the buffer layer4. The signal electrode10bis provided so as to be superimposed on the ridge corresponding to the first optical waveguide20bin order to modulate the light traveling in the first optical waveguide20b, and faces the first optical waveguide20bvia the buffer layer4.

The signal electrodes10aand10bhave a two-layer structure, and each has an upper layer portion10H formed in the electrode layer10and a lower layer portion10L embedded in an opening penetrating the insulating layer5. The width of the lower surface of each of the lower layer portions10L of the signal electrodes10aand10bis narrower than the width of the upper layer portion10H (the total width of each of the signal electrodes10aand10b). The lower layer portion10L is formed only in the vicinity of the region overlapping the first and second optical waveguides20aand20bin a plan view, and is not formed in the other regions. Therefore, the widths of the lower surface of the signal electrodes10aand10bare slightly wider than the widths of the first and second optical waveguides20aand20b, respectively. In order to concentrate the electric field on the signal electrodes10aand10b, the width of the lower surface of the signal electrodes10aand10bis preferably 1.1 to 15 times the width of the first and second optical waveguides20aand20b, respectively. It is more preferably 1.5 to 10 times.

The waveguide layer2is not particularly limited as long as it is an electro-optical material, but is preferably made of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant and is suitable as a constituent material for optical devices such as optical modulators. Hereinafter, the configuration of the present invention when the waveguide layer2is a lithium niobate film will be described in detail.

The substrate1is not particularly limited as long as it has a lower refractive index than the lithium niobate film, but a substrate capable of forming the 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. The lithium niobate film has the property of being easily formed as a c-axis oriented epitaxial film on a single crystal substrate having various crystal orientations. Since the c-axis oriented lithium niobate film has a symmetry of three times symmetry, it is desirable that the underlying single crystal substrate also has the same symmetry, and in the case of a sapphire single crystal substrate, the c-plane is preferable, an in the case of a silicon single crystal substrate, a substrate having a (111) plane is preferable.

Here, the epitaxial film is a single crystal film in which the crystal orientations are aligned by growing crystals on the underlying single crystal substrate or the single crystal film. That is, the epitaxial film is a film having a single crystal orientation in the film thickness direction and the in-plane direction, and when the in-film surface is the XY plane and the film plane direction is the Z axis, the crystals are aligned in the X-axis, Y-axis, and Z-axis directions. Whether or not it is an epitaxial film can be proved, for example, by confirming the peak intensity and the extreme point at the orientation position in 2θ-θ X-ray diffraction.

The lithium niobate film has a composition of LixNbAyOz. A denotes an element other than Li, Nb, and O. X is 0.5 or more and 1.2 or less, preferably 0.9 or more and 1.05 or less. Y is 0 or more and 0.5 or less. Z is 1.5 or more and 4.0 or less, preferably 2.5 or more and 3.5 or less. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, and the like, or may be a combination of two or more of them.

The film thickness of the lithium niobate film 40 is desirably 2 µm or less. This is because if the film thickness is thicker than this, it becomes difficult to form a high-quality film. On the other hand, if the film thickness of the lithium niobate film is too thin, the confinement of light in the lithium niobate film becomes weak, and light leaks to the substrate or the buffer layer and is guided. Even if an electric field is applied to the lithium niobate film, the change in the effective refractive index of the optical waveguides20aand20bmay be small. Therefore, it is desirable that the lithium niobate film has a film thickness of about 1/10 or more of the wavelength of the light used.

The optical modulator100can be manufactured by a known method. It can be manufactured using semiconductor processes such as epitaxial growth, photolithography, etching, vapor phase growth and metallization.

FIG.3shows simulation results of the difference in crosstalk characteristics between adjacent high-frequency line pairs depending on the presence or absence of a high-resistance conductive film.FIG.3is a graph of S41characteristics in which a signal is applied from one end side of one high frequency line pair and the signal emitted from the other end of the adjacent high frequency line pair is measured. The horizontal axis shows the signal frequency (GHz), and the vertical axis shows the crosstalk (dB) between high frequency line pairs. InFIG.3, the graph shown by (a) is the case of the present invention having high resistance conductive films, and the graph shown by (b) is the case of not having high resistance conductive films. The simulation models ofFIGS.3(a) and3(b)are shown inFIGS.4A and4B, respectively. The simulation model has a line length of 0.5 mm, and the result of the crosstalk characteristics by the simulation is converted into the line length of the signal electrode of 10 mm.

In the optical modulator101shown inFIG.4A, each high resistance conductive film has a triangular shape in a plan view, the bases of the equilateral triangles are parallel to the extending direction of the signal electrode, and the vertices of the equilateral triangles are arranged so as to be alternately opposite to each other along the extending direction of the signal electrode. The plurality of triangular high-resistance conductive films41A-1to41A-7arranged between the high-frequency line pair10A and the high-frequency line pair10B are separated from each other along the direction in which the high-frequency line pair extends, and adjacent equilateral triangles are arranged alternately and repeatedly so that their vertices are oriented in opposite directions to each other. The high-resistance conductive films41A-1,41A-3,41A-5, and41A-7are arranged so that the bases of their equilateral triangles are parallel to the extending direction of the signal electrodes10aconstituting the high-frequency line pair10B, and so that the vertices of their equilateral triangles faces the signal electrode10bside constituting the high frequency line pair10A. In contrast, the high-resistance conductive films41A-2,41A-4, and41A-6are arranged so that the bases of their equilateral triangles are parallel to the extending direction of the signal electrodes10bconstituting the high-frequency line pair10A, and so that the vertices of their equilateral triangles faces the signal electrode10aside constituting the high frequency line pair10B. Similarly, the plurality of triangular high-resistance conductive films41B-1to41B-7arranged between the high-frequency line pair10B and the high-frequency line pair10C are separated from each other along the direction in which the high-frequency line pair extends, and adjacent equilateral triangles are arranged alternately and repeatedly so that their vertices are oriented in opposite directions to each other. Similarly, the plurality of triangular high resistance conductive films41C-1to41C-7arranged between the high frequency line pair10C and the high frequency line pair10D are separated from each other along the direction in which the high frequency line pair extends, and adjacent equilateral triangles are arranged alternately and repeatedly so that their vertices are oriented in opposite directions to each other.

FromFIG.3, it can be seen that the case of having the high resistance conductive films has the effect of reducing crosstalk by about 10 dB or more in a wide high frequency region up to 60 GHz as compared with the case of not having the high resistance conductive films.

FIG.5shows simulation results of the difference in crosstalk characteristics between adjacent high-frequency line pairs for the arrangement patterns of various high-resistance conductive films.FIG.5is a graph of S41characteristics. The horizontal axis shows the signal frequency (GHz), and the vertical axis shows the crosstalk (dB) between high frequency line pairs. The graph shown inFIG.5(a)is a case without a high resistance conductive film (see,FIG.6A). The arrangement patterns of the high resistance conductive films corresponding to the simulation results ofFIGS.5(b) to5(i)are shown inFIG.6(b) to 6 (i), respectively.

FIG.6(a)is a case without a high resistance conductive film, and is shown for comparison. The arrangement pattern of the high-resistance conductive films shown inFIG.6(b)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in a row in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.6(c)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in two rows in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.6(d)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in two rows parallel to each other in the extending direction of the high-frequency line pair and are half-shifted from each other. The arrangement pattern of the high-resistance conductive films shown inFIG.6(e)is a pattern in which each of high-resistance conductive films is a continuous film having a circular shape in a plan view, and the high-resistance conductive films are arranged in two rows parallel to each other in the extending direction of the high-frequency line pair and are half-shifted from each other. The arrangement pattern of the high-resistance conductive films shown inFIG.6(f)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in three rows in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.6(g)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in four rows in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.6(h)is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view in which each of high-resistance conductive films is shorter than that shown inFIG.6(g), and the high-resistance conductive films are arranged in four rows in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.6(i)is a pattern in which each of high-resistance conductive films is a continuous film having a square ring shape in a plan view, and the high-resistance conductive films are arranged in two rows in the extending direction of the high-frequency line pair.

In the case of any of the arrangement patterns of the high resistance conductive films shown inFIGS.6(b) to6(i), the crosstalk between adjacent high frequency line pairs is reduced in a high frequency region up to 60 GHz. OfFIGS.6(b),6(c),6(f), and6(g), in which each high resistance conductive film has a rectangular continuous film pattern in a plan view, the pattern arranged in two rows had the best crosstalk characteristics. OfFIGS.6(c),6(d),6(e), and6(i), in which high resistance conductive films are arranged in two rows, the pattern in which each high resistance conductive film has a rectangular continuous film pattern in a plan view had the best crosstalk characteristics.

The effect of reducing crosstalk in the arrangement pattern of the high resistance conductive films shown inFIG.4(a)was larger than that of the arrangement patterns of the high resistance conductive films shown inFIGS.6(b) to6(i).

FIG.7shows simulation results of the difference in crosstalk characteristics between adjacent high-frequency line pairs for various arrangement patterns of high-resistance conductive films with a rectangular continuous film in a plan view.FIG.7is a graph of S41characteristics. The horizontal axis shows the signal frequency (GHz), and the vertical axis shows the crosstalk (dB) between high frequency line pairs. The graph shown inFIG.7(a)is a case without a high resistance conductive film (see,FIG.8(a)). The arrangement patterns of the high resistance conductive films corresponding to the simulation results ofFIGS.7(b) to7(f)are shown inFIGS.8(b) to8(f), respectively.

FIG.8(a)is a case without a high resistance conductive film, and is shown for comparison. The arrangement pattern of the high-resistance conductive films shown inFIG.8(b)is the same as arrangement pattern shown inFIG.6(b), and is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in a row in the extending direction of the high-frequency line pair. The arrangement pattern of the high-resistance conductive films shown inFIG.8(c)is the same as arrangement pattern shown inFIG.6(c), and is a pattern in which each of high-resistance conductive films is a continuous film having a rectangular shape in a plan view, and the high-resistance conductive films are arranged in two rows in the extending direction of the high-frequency line pair. The arrangement pattern of the high resistance conductive films shown inFIG.8(d)is a pattern in which each high resistance conductive film is a continuous film having a rectangular shape in a plan view narrower than that ofFIG.8(c). Further, the high resistance conductive films are arranged in two rows in parallel in the extending direction of the high-frequency line pair, and the distance between the rows is larger than that inFIG.8(c)due to the narrow width. The arrangement pattern of the high resistance conductive films shown inFIG.8(e) is a pattern in which each of high-resistance conductive films is narrower than that ofFIG.8(c), to the same extent as in that ofFIG.8(d), and is a continuous film having a rectangular shape in a plan view. Further, the pattern is a pattern in which the high resistance conductive films are arranged in two rows parallel to the extending direction of the high-frequency line pair, close to one high-frequency line pair, and have a large distance from the other high-frequency line pair. The arrangement pattern of the high resistance conductive films shown inFIG.8(f)is a pattern in which each of high-resistance conductive films is narrower than that ofFIG.8(c), to the same extent as in that ofFIG.8(d), and is a continuous film having a rectangular shape in a plan view, and the high resistance conductive films arranged in two rows parallel to the extending direction of the high-frequency line pair. One row of the high resistance conductive films is closer to one high-frequency line pair and the other row of the high resistance conductive films is closer to the other high-frequency line pair. Further, in the pattern, the distance between the two rows of high-resistance conductive films is larger than the distance between the row of high-resistance conductive films and the high-frequency line pair, and there is a large space in the central portion.

The arrangement pattern of high resistance conductive film shown inFIG.8(c), which is a pattern in which the high-resistance conductive films are arranged most densely, among the arrangement patterns having two rows of high-resistance conductive films shown inFIGS.8(c) to8(f), has the best crosstalk characteristics in the entire high frequency range up to 60 GHz.

FIG.9(a)shows another arrangement pattern of high resistance conductive films having good crosstalk characteristics. The arrangement pattern of the high resistance conductive films shown inFIG.9(a)includes a plurality of high resistance conductive films having a rectangular shape in a plan view and a triangular shape in a plan view. Further, this arrangement pattern consists of a first row consisting of a substantially rectangular shape arranged along the extending direction of the high frequency line pair, and two rows consisting of substantially triangular shapes arranged so as to sandwich the first row.

FIG.9(b)shows an arrangement pattern in which the crosstalk characteristics are not improved as compared with the arrangement pattern having no high resistance conductive films. The reason why the crosstalk characteristics are not improved in the arrangement pattern of the high resistance conductive films is that it has a continuous film.

FIG.10shows the results of investigating the conductivity dependence of the crosstalk characteristics (S41characteristics). The graph indicated by reference numeral (a) inFIG.10is an average taken at all frequency points, and the graph indicated by reference numeral (b) is an average taken at 30 GHz to 60 GHz. FromFIG.10, it can be seen that there is a region where the crosstalk characteristics are improved when the conductivity is 4 x 107[s / m] or less. When the frequency range is narrowed down to 30 GHz to 60 GHz, the effect of improving the crosstalk characteristics is more remarkable, and the improvement effect is about 10 dB.

FIG.11shows a schematic plan view of an example of the whole including a part of the optical modulator according to the first embodiment shown inFIGS.1and2(optical modulators having four optical modulators30A to30D and a plurality of high resistance conductive films 40 between them). InFIG.11, in order to show the arrangement relationship between the Mach-Zehnder optical waveguide and the high-frequency line pair, the Mach-Zehnder optical waveguide located in the layer below the high-frequency line pair is shown by a dotted line. Further,FIG.12shows a schematic plan view of only the optical waveguides of the optical modulator of the present invention shown inFIG.11.

The optical modulator100A shown inFIG.11includes an optical waveguide unit120including four Mach Zender optical waveguides20having a straight portion and a curved portion, and a high-frequency line110including high-frequency line pairs10consisting two signal electrodes10aand10bfor applying a pair of differential high frequency signals, and a plurality of high-resistance conductive films140arranged apart from the high-frequency line pair between adjacent high-frequency line pairs. InFIG.11, the plurality of high resistance conductive films140are drawn as continuous films for convenience of illustration, but as illustrated inFIGS.6,8and9, each of them are composed of a plurality of isolated high resistance conductive films.

The optical modulator100A shown inFIG.11is configured such that the cross-sectional structure of the straight line portion (for example, the cross-sectional structure along the B-B' line ofFIG.11) corresponds to the cross-sectional structure shown inFIG.2.

The optical waveguide unit120includes an input optical waveguide120iinput by input light Si, branched optical waveguides120iiand120ijbranched from the input optical waveguide120i, branched optical waveguides120iiiand120iij,120ijiand120ijjbranched from the branched optical waveguides120iiand120ij, respectively, Mach Zender optical waveguides20A,20B,20C,20D branched from the branched optical waveguides120iii,120iij,120iji, and120ijj, respectively, and Mach Zender optical waveguides120o1,120o2,120o3and120o4where the light traveling through the20A,20B,20C and20D is combined, and the combined and modulated light travels. The light traveling through the combined optical waveguides120o1,120o2,120o3and120o4is output from the combined wave optical waveguides120o1,120o2,120o3and120o4as modulated light So1, So2, So3and So4, respectively.

Each of the Mach-Zehnder optical waveguides20A,20B,20C, and20D constituting the Mach-Zehnder optical waveguide20has a straight portion and a curved portion, and is a substantially S-shaped optical waveguide as a whole.

The Mach-Zehnder optical waveguide20A includes straight portions20As1,20As2,20As3and curved portions20Ac1,20Ac2, and is connected in the order of straight portions20As1, curved portions20Ac1, straight portions20As2, curved portions20Ac2, and straight portions20As3. The Mach-Zehnder optical waveguide20B includes straight portions20Bs1,20Bs2,20Bs3and curved portions20Bc1,20Bc2, and is connected in the order of straight portions20Bs1, curved portions20Bc1, straight portions20Bs2, curved portions20Bc2, and straight portions20Bs3. The Mach-Zehnder optical waveguide20C includes straight portions20Cs1,20Cs2,20Cs3and curved portions20Cc1,20Cc2, and is connected in the order of straight portions20Cs1, curved portions20Cc1, straight portions20Cs2, curved portions20Cc2, and straight portions20Cs3. The Mach-Zehnder optical waveguide20D includes straight portions20Ds1,20Ds2,20Ds3and curved portions20Dc1,20Dc2, and is connected in the order of straight portions20Ds1, curved portions20Dc1, straight portions20Ds2, curved portions20Dc2, and straight portions20Ds3.

In optical modulators, the long element length is often a problem for miniaturization. By folding the optical waveguide like the optical modulator100A, its element length can be significantly shortened and its size can be reduced. In particular, the optical waveguides formed of the lithium niobate film are suitable for the present embodiment because it has a feature that the loss is small even if the radius of curvature is reduced to, for example, about 50 µm.

Each of the four high-frequency line pairs10A,10B,10C, and10D constituting the two signal electrodes10aand10bhas straight portions and curved portions corresponding to the plan-view shape of the Mach Zender optical waveguide.

The high frequency line pair10A has straight portions10As1,10As2and a curved portion10Ac, and includes a portion formed by connecting the straight portion10As1, the curved portion10Ac, and the straight portion10As2in this order. The straight portions10As1,10As2and the curved portion10Acof the high-frequency line pair10A are arranged above the straight portions20As2and20As3and the curved portion20Ac2of the Mach-Zehnder optical waveguide20A. The high frequency line pair10B has straight portions10Bs1,10Bs2and a curved portion10Bc, and includes a portion formed by connecting the straight portion10Bs1, the curved portion10Bc, and the straight portion10Bs2in this order. The straight portions10Bs1,10Bs2and the curved portion10Bcof the high-frequency line pair10B are arranged above the straight portions20Bs2and20Bs3and the curved portion20Bc2of the Mach-Zehnder optical waveguide20B. The high frequency line pair10C has straight portions10Cs1,10Cs2and a curved portion10Cc, and includes a portion formed by connecting the straight portion10Cs1, the curved portion10Cc, and the straight portion10Cs2in this order. The straight portions10Cs1,10Cs2and the curved portion10Ccof the high-frequency line pair10C are arranged above the straight portions20Cs2and20Cs3and the curved portion20Cc2of the Mach-Zehnder optical waveguide20C. The high frequency line pair10D has straight portions10Ds1,10Ds2and a curved portion10Dc, and includes a portion formed by connecting the straight portion10Ds1, the curved portion10Dc, and the straight portion10Ds2in this order. The straight portions10Ds1,10Ds2and the curved portion10Dcof the high-frequency line pair10D are arranged above the straight portions20Ds2and20Ds3and the curved portion20Dc2of the Mach-Zehnder optical waveguide20D.

The high frequency line pairs10A,10B,10C and10D are connected to the terminating resistors11A,11B,11C and11D, respectively.

The plurality of high-resistance conductive films40A,40B, and40C arranged between adjacent high-frequency line pairs all have a straight portion and a curved portion corresponding to the plan-view shape of the high-frequency line pair.

The plurality of high resistance conductive films40A have straight portions40As1,40As2and a curved portion40Ac, and are connected in the order of the straight portion40As1, the curved portion40Ac, and the straight portion40As2. The straight portions40As1,40As2and the curved portion40Acof the plurality of high-resistance conductive films40A are arranged between the straight portion10As1, the curved portion10Ac, and the linear portion10As2of the high frequency line pair10A, and the straight portion10Bs1, the curved portion10Bc, and the straight portion10Bs2of the high frequency line pair10B. The plurality of high resistance conductive films40B have straight portions40Bs1,40Bs2and a curved portion40Bc, and are connected in the order of the straight portion40Bs1, the curved portion40Bc, and the straight portion40Bs2. The straight portions40Bs1,40Bs2and the curved portion40Bcof the plurality of high-resistance conductive films40B are arranged between the straight portion10Bs1, the curved portion10Bc, and the linear portion10Bs2of the high frequency line pair10B, and the straight portion10Cs1, the curved portion10Cc, and the straight portion10Cs2of the high frequency line pair10C. The plurality of high resistance conductive films40C have straight portions40Cs1,40Cs2and a curved portion40Cc, and are connected in the order of the straight portion40Cs1, the curved portion40Cc, and the straight portion40Cs2. The straight portions40Cs1,40Cs2and the curved portion40Ccof the plurality of high-resistance conductive films40C are arranged between the straight portion10Cs1, the curved portion10Cc, and the linear portion10Cs2of the high frequency line pair10C, and the straight portion10Ds1, the curved portion10Dc, and the straight portion10Ds2of the high frequency line pair10D.

In the present embodiment, the plurality of high resistance conductive films40A,40B, and40C all have a straight portion and a curved portion corresponding to the plan view shape of the high frequency line pair, but the present invention is not limited to this. For example, the plurality of high resistance conductive films may have a configuration having only a straight portion. A plurality of high resistance conductive films are arranged apart from each other. When a plurality of high resistance conductive films are arranged linearly, unlike the case where the plurality of high resistance conductive films are arranged in a curve, the design is easily designed to suppress the crosstalk characteristics.

Further, when the high frequency line pair has a plurality of straight portions or a plurality of curved portions, the configuration may have a plurality of high resistance conductive films only between any of the plurality of straight portions of the high frequency line pair, or may have a plurality of high resistance conductive films only between any of the plurality of curved portions of the high frequency line pair.

REFERENCE SIGNS LIST

DRAWING