Patent ID: 12248205

DESCRIPTION OF EMBODIMENT

Hereinafter, the present embodiment will be described in detail suitably with reference to the drawings. In the drawings used in the following description, in order to make characteristics easy to understand, characteristic parts may be illustrated in an enlarged manner for the sake of convenience, and dimensional ratios or the like of each constituent element may differ from actual values thereof. Materials, dimensions, and the like exemplified in the following description are examples. The present invention is not limited thereto and can be suitably changed and performed within a range exhibiting the effects of the present invention.

First, directions will be defined. One direction on one surface of a substrate Sb will be referred to as an x direction, and a direction orthogonal to the x direction will be referred to as a y direction. For example, the x direction is a direction in which a first optical waveguide11extends. A z direction is a direction perpendicular to the substrate Sb. The z direction is a direction orthogonal to the x direction and the y direction. Hereinafter, the positive z direction may be expressed as “upward”, and the negative z direction may be expressed as “downward”. The upward and downward directions do not necessarily coincide with the direction in which the force of gravity acts.

FIG.1is a block diagram of an optical modulator200according to a first embodiment. The optical modulator200has an optical modulation element100, a drive circuit110, a DC bias application circuit120, and a DC bias control circuit130. A control unit of the optical modulator200has the drive circuit110, the DC bias application circuit120, and the DC bias control circuit130.

The optical modulation element100converts an electrical signal into an optical signal. The optical modulation element100converts input light Lin, which has been input thereto, into output light Loutin accordance with a modulation signal Sm.

The drive circuit110applies a modulation voltage Vm corresponding to the modulation signal Sm to the optical modulation element100. The DC bias application circuit120applies a DC bias voltage Vdc to the optical modulation element100. The DC bias control circuit130monitors the output light Loutand controls the DC bias voltage Vdc output from the DC bias application circuit120. An operating point Vd (which will be described below) is controlled by adjusting this DC bias voltage Vdc.

FIG.2is a plan view of an optical waveguide10of the optical modulation element100viewed in the z direction.FIG.3is a plan view of the optical modulation element100viewed in the z direction.FIG.4is a cross section cut along X1-X1′ inFIG.3. The optical modulation element100has the optical waveguide10and electrodes21,22,23, and24.

The optical modulation element100is located on the substrate Sb. The substrate Sb need only be a substrate on which an oxide film40such as a lithium niobate film can be formed as an epitaxial film, and it is preferably a sapphire single crystal substrate or a silicon single crystal substrate. A crystal orientation of the substrate Sb is not particularly limited. The lithium niobate film has properties of being easily formed as a c-axis-oriented epitaxial film with respect to the substrate Sb having various crystal orientations. Since a crystal constituting a c-axis-oriented lithium niobate film has three-fold symmetry, it is desired that the substrate Sb (base material) also have the same symmetry. In the case of a sapphire single crystal substrate, a substrate of a c-plane is preferable, and in the case of a silicon single crystal substrate, a substrate of a (111) plane is preferable.

The optical waveguide10is a light passage in which light is propagated. For example, the optical waveguide10has the first optical waveguide11, a second optical waveguide12, an input path13, an output path14, a branch portion15, and a coupling portion16. For example, the first optical waveguide11and the second optical waveguide12extend in the x direction. The first optical waveguide11and the second optical waveguide12have substantially the same length in the x direction. The branch portion15is located between the input path13, and the first optical waveguide11and the second optical waveguide12. The input path13leads to the first optical waveguide11and the second optical waveguide12with the branch portion15therebetween. The coupling portion16is located between the first optical waveguide11and the second optical waveguide12, and the output path14. The first optical waveguide11and the second optical waveguide12lead to the output path14with the coupling portion16therebetween.

The optical waveguide10includes the first optical waveguide11and the second optical waveguide12which are ridge-shaped portions protruding from a first surface40aof the oxide film40. The first surface40ais an upper surface in a part other than the ridge-shaped portions of the oxide film40. The ridge-shaped portions protrude in the z direction from the first surface40aand extend along the optical waveguide10. The shape of an X1-X1′ cross section (a cross section perpendicular to a traveling direction of light) of each ridge-shaped portion may be any shape as long as it is a shape capable of guiding light, and it may be a dome shape, a triangular shape, or a rectangular shape, for example. The width of each ridge-shaped portion16in the y direction is 0.3 μm or more and 5.0 μm or less, for example, and the height of each ridge-shaped portion16(protrusion height from the first surface40a) is 0.1 μm or more and 1.0 μm or less, for example. The ridge-shaped portions are constituted of the same material as the oxide film40.

For example, the oxide film40is a c-axis-oriented lithium niobate film. For example, the oxide film40is an epitaxial film epitaxially grown on the substrate Sb. An epitaxial film indicates a single crystal film of which the crystal orientation is aligned by the substrate (base material). An epitaxial film is a film which has a single crystal orientation in the z direction and an in-plane (xy) direction and in which crystals are oriented in a manner of being aligned together in an x axis direction, a y axis direction, and a z axis direction. For example, it is possible to verify whether or not there is an epitaxial film by checking a peak intensity and a pole at an orientation position in2θ-θ X-ray diffraction. In addition, the oxide film40may be a lithium niobate film provided on a Si substrate with SiO2therebetween.

Specifically, when measurement is performed by2θ-θ X-ray diffraction, all peak intensities other than that on a target surface are equal to or less than 10% and preferably equal to or less than 5% of the maximum peak intensity of the target surface. For example, when the oxide film40is a c-axis-oriented epitaxial film, the peak intensity other than that in a (00L) plane is equal to or less than 10% and preferably equal to or less than 5% of the maximum peak intensity of the (00L) plane. Here, (00L) is generic expression of equivalent planes such as (001) and (002).

In addition, conditions for checking the peak intensity at the orientation position described above simply indicate orientations in one direction. Thus, even if the condition described above is obtained, when the crystal orientations are not aligned within a plane, the X-ray intensity at a particular angular position does not increase and no pole is seen. For example, when the oxide film40is a lithium niobate film, since LiNbO3has a crystal structure of a trigonal system, there are three poles of LiNbO3(014) in a single crystal. In the case of lithium niobate, it is known to epitaxially grow in a so-called twin crystal state in which crystals rotated about the c axis by 180° are symmetrically coupled. In this case, since two of three poles are in a symmetrically coupled state, there are six poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate of a (100) plane, since a substrate has four-fold symmetry, 12 poles (4×3) are observed. In the present disclosure, an epitaxial film also includes a lithium niobate film which has epitaxially grown in a twin crystal state.

The composition of lithium niobate is LixNbAyOz. A is an element other than Li, Nb, and O. The subscript x is 0.5 or more and 1.2 or less and preferably 0.9 or more and 1.05 or less. The subscript y is 0 or more and 0.5 or less. The subscript z is 1.5 or more and 4.0 or less and preferably 2.5 or more and 3.5 or less. Examples of the element of A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, and two or more kinds of these elements may be combined.

The film thickness of the oxide film40is 2 μm or smaller, for example. The film thickness of the oxide film40is a film thickness of a part other than the ridge-shaped portions. If the film thickness of the oxide film40is large, there is concern that crystallinity may deteriorate. In addition, the film thickness of the oxide film40is approximately 1/10 or larger than the wavelength of light used, for example. If the film thickness of the oxide film40is small, confinement of light becomes weak, and light leaks to the substrate Sb or a buffer layer30. If the film thickness of the oxide film40is small, even if an electric field is applied to the oxide film40, there is concern that change in effective refractive index of the optical waveguide10may decrease.

The electrodes21and22are electrodes for applying the modulation voltage Vm to the optical waveguide10. The electrode21is an example of a first electrode, and the electrode22is an example of a second electrode. A first end21aof the electrode21is connected to a power supply31, and a second end21bis connected to a terminal resistor32. A first end22aof the electrode22is connected to the power supply31, and a second end22bis connected to the terminal resistor32. The power supply31is a part of the drive circuit110for applying the modulation voltage Vm to the optical modulation element100.

The electrodes23and24are electrodes for applying a DC bias Vdc to the optical waveguide10. A first end23aof the electrode23and a first end24aof the power supply24are connected to a power supply33. The power supply33is a part of the DC bias application circuit120for applying the DC bias voltage Vdc to the optical modulation element100.

InFIG.3, line widths and line spacings of the electrode21and the electrode22disposed in a parallel manner are made wider than actual measurements for better visibility. For this reason, although the length of a part in which the electrode21and the first optical waveguide11overlap (interaction length) and the length of a part in which the electrode22and the second optical waveguide12overlap appear different, the lengths (interaction lengths) thereof are substantially the same. Similarly, the length of a part in which the electrode23and the first optical waveguide11overlap (interaction length) and the length of a part in which the electrode24and the second optical waveguide12overlap (interaction length) are substantially the same.

In addition, when the DC bias voltage Vdc overlaps the electrodes21and22, the electrodes23and24may not be provided. In addition, ground electrodes may be provided around the electrodes21,22,23, and24.

The electrodes21,22,23, and24are located on the oxide film40with the buffer layer30sandwiched therebetween. Each of the electrodes21and23can apply an electric field to the first optical waveguide11. Each of the electrodes21and23is located at a position overlapping the first optical waveguide11in a plan view in the z direction, for example. Each of the electrodes21and23is located above the first optical waveguide11. Each of the electrodes22and24can apply an electric field to the second optical waveguide12. Each of the electrodes22and24is located at a position overlapping the second optical waveguide12in a plan view in the z direction, for example. Each of the electrodes22and24is located above the second optical waveguide12.

The buffer layer30is located between the optical waveguide10and the electrodes21,22,23, and24. The buffer layer30covers and protects the ridge-shaped portions. In addition, the buffer layer30prevents light propagated through the optical waveguide10from being absorbed into the electrodes21,22,23, and24. The buffer layer30has a lower refractive index than the oxide film40. For example, the buffer layer30is made of SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, Y2O3, CaF2, In2O3, or the like, or a mixture of these.

The chip size of the optical modulation element100is 100 mm2or smaller, for example. If the chip size of the optical modulation element100is 100 mm2or smaller, it can be used as an optical modulation element for a data center.

The optical modulation element100can be produced by a known method. For example, the optical modulation element100is manufactured using a semiconductor process such as epitaxial growth, photolithography, etching, vapor phase growth, or metallization.

The optical modulation element100converts an electrical signal into an optical signal. The optical modulation element100modulates the input light Linto the output light Lout. First, modulation operation of the optical modulation element100will be described.

The input light Lininput from the input path13branches into the first optical waveguide11and the second optical waveguide12and is propagated. The phase difference between light propagated through the first optical waveguide11and light propagated through the second optical waveguide12is zero at the point of time it branches.

Next, an applied voltage is applied to a part between the electrode21and the electrode22. For example, differential signals having the same absolute values, polarities opposite to each other, and phases not deviating from each other may be respectively applied to the electrode21and the electrode22. The refractive indices of the first optical waveguide11and the second optical waveguide12change due to an electro-optic effect. For example, the refractive index of the first optical waveguide11changes by +Δn from a reference refractive index n, and the refractive index of the second optical waveguide12changes by −Δn from the reference refractive index n.

The difference between the refractive indices of the first optical waveguide11and the second optical waveguide12creates a phase difference between light propagated through the first optical waveguide11and light propagated through the second optical waveguide12. Rays of light propagated through the first optical waveguide11and the second optical waveguide12join together in the output path14and are output as the output light Lout. The output light Loutis superimposed light of light propagated through the first optical waveguide11and light propagated through the second optical waveguide12. The intensity of the output light Loutchanges in accordance with an odd multiple of the phase difference between light propagated through the first optical waveguide11and light propagated through the second optical waveguide12. For example, when the phase difference is an even multiple of π, rays of the light are mutually intensified, and when the phase difference is an odd multiple of π, rays of the light are mutually weakened. In such a procedure, the optical modulation element100modulates the input light Linto the output light Loutin accordance with an electrical signal.

Optical modulation by the optical modulation element100will be described usingFIG.5.FIG.5is a view illustrating a relationship between an applied voltage and an output of the optical modulator200according to the first embodiment. InFIG.5, the horizontal axis indicates a voltage applied to the optical modulation element100, and the vertical axis indicates a standardized output from the optical modulation element100. An output is standardized as “1” when the phase difference between light propagated through the first optical waveguide11and light propagated through the second optical waveguide12is zero.

Next, a null point voltage Vn and a half-wavelength voltage Vπ will be described. The output of the optical modulation element100is maximized when the applied voltage is zero. This is because the phase difference between light propagated through the first optical waveguide11and light propagated through the second optical waveguide12is zero when the applied voltage is zero. As the applied voltage is increased, an output from the optical modulation element100gradually decreases and becomes extremely small at a certain point. The voltage at which an output from the optical modulation element100becomes extremely small is the null point voltage Vn. A half-wavelength voltage (half-wavelength phase modulation voltage) is a voltage for making the phase difference of light 180° using a Mach-Zehnder-type optical modulator, and a voltage width in which an output from the optical modulation element100reaches the minimum from the maximum corresponding to the half-wavelength voltage Vπ. If a voltage exceeding the null point voltage Vn is applied, an output from the optical modulation element100periodically changes. An output from the optical modulation element100repeats the maximum and the minimum for each half-wavelength voltage Vπ.

The half-wavelength voltage Vπ of the optical modulation element100changes depending on the constitution of the optical modulation element100. For example, the half-wavelength voltage Vπ changes depending on the length of the electrode21on the first optical waveguide11, the length of the electrode22on the second optical waveguide12, and the like. Here, the length of the first electrode21and the length of the second electrode22are lengths in a propagation direction of light. In the case ofFIG.3, it is a length of a part of the electrode21overlapping the first optical waveguide11or a length of a part of the electrode22overlapping the second optical waveguide12. This length is referred to as an interaction length. If the interaction length is long, the half-wavelength voltage Vπ decreases, and if the interaction length is short, the half-wavelength voltage Vπ increases.

In the optical modulation element100, a first interaction length L1that is a length of a part of the first electrode21overlapping the first optical waveguide11in a longitudinal direction is 0.9 mm or more and 20 mm or less. When the first interaction length L1is shorter than 0.9 mm, an extinction ratio of 3 dB or larger required for a data center cannot be obtained by low-voltage driving of 2.0 V or more and 4.3 V or less. For this reason, the first interaction length L1is 0.9 mm or longer.

When the first interaction length L1is longer than 20 mm, attenuation in a high-frequency band of 60 GHz or higher is significant. For this reason, the first interaction length L1is 20 mm or shorter. Similarly, a second interaction length L2that is a length of a part of the second electrode22overlapping the second optical waveguide12in the longitudinal direction is 0.9 mm or more and 20 mm or less.

The first electrode21and the second electrode22are formed such that the first interaction length L1and the second interaction length L2become substantially the same. InFIG.3, although the first interaction length L1and the second interaction length L2appear different, the first electrode21and the second electrode22actually have narrow line widths of the electrodes, and the gap between the first electrode21and the second electrode22is also narrow. Therefore, the interaction lengths of the two electrodes become substantially the same.

The modulation voltage Vm corresponding to a modulation signal is applied to the electrodes21and22for applying a modulation voltage of the optical modulation element100. A voltage applied to the electrodes23and24for applying a DC bias voltage, namely, the DC bias voltage Vdc output from the DC bias application circuit120is controlled by the DC bias control circuit130. The DC bias control circuit130adjusts the operating point Vd of the optical modulation element100by controlling the DC bias voltage Vdc. The operating point Vd is a voltage at the center of the amplitude of a modulation voltage.

The DC bias application circuit120controls an operating point voltage Vd of the optical modulation element100. The operating point voltage Vd is a midpoint between a minimum value (Vmin) and a maximum value (Vmax) of an applied voltage. Further, the difference between the minimum value (Vmin) and the maximum value (Vmax) of an applied voltage is an applied voltage width Vpp.

The operating point voltage Vd may fluctuate due to a temperature or the like of a usage environment. When the operating point voltage Vd fluctuates while being used, the operating point voltage Vd is corrected by the DC bias control circuit130in accordance with the set applied voltage width Vpp such that it is included within a range in which the operating point voltage has an extinction ratio of 3 dB or larger. For example, the DC bias control circuit130corrects fluctuation of the operating point voltage Vd on the basis of branch light Lbwhich has branched from the output light Lout.

In addition, the drive circuit110controls the applied voltage width Vpp applied to the optical modulation element100. The applied voltage width Vpp applied to the optical modulation element is a range of 2.0 V or more and 4.3 V or less. If the interaction length is 0.9 mm or more and 20.0 mm or less, the extinction ratio can become 3 dB or larger in the applied voltage width Vpp of 2.0 V or more and 4.3 V or less. The drive circuit110inputs an electrical signal converted into an optical signal to the optical modulation element100. For example, the drive circuit110includes a power supply, a driver, and the like.

FIG.6is an explanatory view of the applied voltage width Vpp of the optical modulator200according to the first embodiment.FIG.6is a view illustrated by adding description of the applied voltage width Vpp toFIG.5.

The applied voltage width Vpp becomes a range of a voltage utilized when the optical modulation element100is operated. The applied voltage width Vpp is applied to the optical modulation element100with the operating point voltage Vd as the midpoint. An output from the optical modulation element100changes in a range corresponding to the minimum value (Vmin) of an applied voltage and the maximum value (Vmax) of the applied voltage. The half-wavelength voltage Vπ is equal to or higher than the applied voltage width Vpp. The operating point voltage Vd is set such that the minimum value (Vmin) of a voltage applied to the optical modulation element100becomes equal to or larger than the null point voltage Vn, but it may be set such that the maximum value (Vmax) becomes equal to or smaller than the null point voltage Vn.

For example, a modulation signal at a high-frequency voltage is controlled by the drive circuit110. The band of a modulation element is 60 GHz or higher. If the frequency band of the modulation element is 60 GHz or higher, it is easy to cope with high-speed modulation.

FIG.7is a view illustrating a relationship between an applied voltage and an extinction ratio of the optical modulator200according to the first embodiment. InFIG.7, the horizontal axis indicates a voltage applied to the optical modulation element100, and the vertical axis indicates a ratio between the output light Loutin an applied voltage and the output light Loutin the null point voltage Vn. The extinction ratio is a ratio of the maximum value and the minimum value of the output light Loutwithin a range of an applied voltage.

As described above, the optical modulation element100and the optical modulator200according to the first embodiment can be driven at a low voltage and can be used in a high-frequency band.

Thus far, the optical modulation element100and the optical modulator200according to the first embodiment have been described as an example. However, the present invention is not limited to the first embodiment, and various modifications can be made.

For example, the first interaction length L1and the second interaction length L2may be set to 18.6 mm or shorter. When the first interaction length L1and the second interaction length L2are set to 18.6 mm or shorter, response characteristics are improved even in a high-frequency band of 70 GHz or higher. Moreover, when they are set to 16.9 mm or shorter, response characteristics are improved even in a high-frequency band of 80 GHz or higher. When they are set to 14.4 mm or shorter, response characteristics are improved even in a higher frequency band.

In addition, in the optical modulator of the first embodiment, the operating point voltage Vd has been controlled, but the minimum value (Vmin) or the maximum value (Vmax) of a voltage applied to the optical modulation element100may be controlled. When the minimum value (Vmin) is controlled, the minimum value (Vmin) is controlled to be equal to or larger than the null point voltage Vn. Meanwhile, when the maximum value (Vmax) is controlled, the maximum value (Vmax) is controlled to be equal to or smaller than the null point voltage Vn.

It is preferable that Vpp/Vx be 0.03 or more and 0.47 or less. In this range, it is possible to have an extinction ratio of 3 dB or larger and a frequency band of the modulation element of 60 GHz or higher.

The chip size of the optical modulation element100may be set to 100 mm2or smaller and more preferably 50 mm2or smaller. If the chip size of the optical modulation element100is reduced, it can also be used in existing transceivers for a data center.

In addition,FIG.8is a plan view of an optical modulation element101according to a first modification in a plan view in the z direction. The optical modulation element101has an optical waveguide50and electrodes61,62,63, and64.

The optical waveguide50has a first optical waveguide51, a second optical waveguide52, an input path53, an output path54, a branch portion55, and a coupling portion56. The optical waveguide50differs from the optical waveguide10in that the first optical waveguide51and the second optical waveguide52are curved in the middle thereof. The optical waveguide50is otherwise similar to the optical waveguide10.

The electrodes61and62are electrodes for applying the modulation voltage Vm to the optical waveguide50. The electrode61is an example of the first electrode, and the electrode62is an example of the second electrode. A first end61aof the electrode61is connected to the power supply31, and a second end61bis connected to the terminal resistor32. A first end62aof the electrode62is connected to the power supply31, and a second end62bis connected to the terminal resistor32. The electrodes63and64are electrodes for applying the DC bias Vdc to the optical waveguide50. A first end63aof the electrode63and a first end64aof the power supply64are connected to the power supply33.

InFIG.8, since the line widths and the line spacings of the electrode61and the electrode62disposed in a parallel manner are made wider, although the length of a part in which the electrode61and the first optical waveguide51overlap and the length of a part in which the electrode62and the second optical waveguide52overlap are illustrated such that they are different, the lengths thereof are substantially the same. Similarly, the length of a part in which the electrode63and the first optical waveguide51overlap and the length of a part in which the electrode64and the second optical waveguide52overlap are substantially the same.

The electrode61and the electrode62differ from the electrode21and the electrode22in that they are curved along the first optical waveguide51and the second optical waveguide52. Each of the electrodes61,62,63, and64is otherwise similar to each of the electrodes21,22,23, and24.

In the optical modulation element101, since the first optical waveguide51and the second optical waveguide52are curved, the element size in the x direction is small. For example, the optical modulation element101can be realized to have an element size of 100 mm2or smaller and preferably 50 mm2or smaller. An optical modulator for a data center is required to be miniaturized. Since the optical waveguide50is curved, the optical modulation element101can also be accommodated in a small-sized region corresponding to an existing optical modulator for a data center.

EXAMPLES

Hereinafter, Examples of the present disclosure will be exemplified, but the present disclosure is not limited to the following Examples.

It is obvious that those skilled in the art can conceive of various modification examples or revision examples within the scope of the idea described in the claims, and it is understood that these naturally belong to the technical scope of the present disclosure.

EXAMPLES

The structures inFIGS.3and4were actually made as a trial in the following procedure. Sapphire was used as the material of the substrates. A lithium niobate film having a film thickness of 1.5 μm was produced on surfaces of the substrates by a sputtering method. Next, a buffer layer having a film thickness of 0.8 μm and made of a material LaAlO3was formed on the lithium niobate film by a vapor deposition method. The ridge-shaped portions were formed by forming a mask using a resist and performing dry etching processing using Ar plasma. The ridge widths of the ridge-shaped portions were set to 2.5 μm, and the ridge heights were set to 0.4 μm. Last, the first electrode and the second electrode were formed by a photolithography step and a gold plating step. The relative dielectric constant of LaAlO3was 13.

Regarding the obtained optical modulators, modulation characteristics were evaluated using light having a wavelength of 1,310 nm. Tables 1 and 2 show the half-wavelength voltages Vπ (Vpi) (V), the applied voltage widths Vpp (V), the maximum values of the extinction ratio ER (ERmax) (dB), and the available high-frequency ranges RF (GHz) when the interaction length L was varied while having the applied voltage width set to 2 V and 4.3 V. Here, the first interaction length L1and the second interaction length L2were the same values and was expressed as the interaction length L in Table 1.

TABLE 1LVpiVppVpp/ERmaxRF(mm)(V)(V)Vpi(dB)(GHz)0.61012.00.022.4>800.966.42.00.033.5>801.156.42.00.044.1>801.446.62.00.044.9>801.933.32.00.066.6>805.911.42.00.1814.0>808.58.32.00.2416.5>8014.45.52.00.3619.7>8016.94.92.00.4120.68018.64.72.00.4320.97020.04.32.00.4721.56022.04.12.00.4921.950

TABLE 2LVpiVppVpp/ERmaxRF(mm)(V)(V)Vpi(dB)(GHz)0.32414.30.022.2>800.41434.30.033.5>800.61014.30.044.9>800.966.44.30.067.1>801.156.44.30.088.0>801.446.64.30.099.2>801.933.34.30.1311.7>805.911.44.30.3820.0>808.58.34.30.5222.3>8014.45.54.30.7824.5>8016.94.94.30.8824.88018.64.74.30.9124.97020.04.34.31.0025.06022.04.14.31.0525.050

As shown in Tables 1 and 2, if the interaction length L is in a range of 0.4 mm to 20 mm while having Vpp set to 4.3 V, it has been confirmed that the optical modulator can be used at 60 GHz or higher and the extinction ratio of 3 dB or larger can be obtained. Moreover, if the interaction length L is in a range of 0.9 mm or more and 20 mm or less while having Vpp set to 2.0 V, it has been confirmed that the optical modulator can be used at 60 GHz or higher and the extinction ratio of 3 dB or larger can be obtained.

REFERENCE SIGNS LIST

10,50Optical waveguide11,51First optical waveguide12,52Second optical waveguide13,53Input path14,54Output path15,55Branch portion16,56Coupling portion21,22,23,24,61,62,63,64Electrode30Buffer layer40Oxide film40aFirst surface100,101Optical modulation element110Drive circuit120DC bias application circuit130DC bias control circuit200Optical modulatorLinInput lightLoutOutput lightLbBranch lightVd Operating point voltageVn Null point voltageVπ Half-wavelength voltageVpp Applied voltage width