Optical modulation element and method for driving optical modulation element

Abstract

The optical modulation element (100) has a first optical waveguide path (11), a second optical waveguide path (12), a first electrode (21) that applies an electric field to the first optical waveguide path (11), and a second electrode (22) that applies an electric field to the second optical waveguide path (12), the first optical waveguide path (11) and the second optical waveguide path (12) each include a ridge portion that protrudes from a first face (40a) of a lithium niobate film, a first interaction length L1, which is a length in a lengthwise direction of a portion of the first electrode (21) that overlaps the first optical waveguide path (11), is 0.9 mm or more and 20 mm or less, and a second interaction length L2, which is a length in a lengthwise direction of a portion of the second electrode (22) that overlaps the second optical waveguide path (12), is 0.9 mm or more and 20 mm or less.

Description

Technical Field

[0001] This invention relates to an optical modulation element and a method for driving the optical modulation element. This application claims priority based on Japanese Patent Application No. 2020-135862, filed on August 11, 2020, the contents of which are incorporated herein by reference. Background Technology

[0002] With the widespread adoption of the internet and the rapid growth of communication volume, fiber optic communication has become extremely important. Fiber optic communication converts electrical signals into optical signals and transmits these signals using optical fibers, thanks to its characteristics of wide bandwidth, low loss, and strong noise immunity.

[0003] Optical modulators convert electrical signals into optical signals. For example, Patent Documents 1 and 2 describe Mach-Zehnder type optical modulators in which an optical waveguide is formed near the surface of a lithium niobate single-crystal substrate by diffusion of Ti (titanium). Furthermore, Patent Document 2 describes a technique for correcting the operating point drift of an optical modulator. The optical modulators described in Patent Documents 1 and 2 operate at high speeds of 40 Gb / s or higher and are approximately 10 cm in length.

[0004] In contrast, Patent Document 3 describes a Mach-Zehnder type optical modulator using a c-axis oriented lithium niobate film. Optical modulators using lithium niobate films are smaller and have lower driving voltages compared to optical modulators that form optical waveguides near the surface of a lithium niobate single-crystal substrate through Ti diffusion.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2004-37695

[0008] Patent Document 2: Japanese Patent No. 4164179

[0009] Patent Document 3: Japanese Patent Application Publication No. 2019-45880 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] Lithium niobate optical modulators have high extinction ratios and can operate in high-frequency bands, making them suitable for long-distance communications such as those between cities. However, in recent years, the increase in information volume has led to further demands for higher frequency bands. Furthermore, there is a simultaneous requirement for lower driving voltages. On the other hand, the increase in intra- and inter-data center communications, as well as short- and medium-distance communications between data centers, has also increased. In these applications, a high extinction ratio is not necessary, leading to the use of silicon-based optical modulators and the direct modulation of the emitted light using laser diode drive circuits without optical modulators. Modulation using silicon-based optical modulators and direct modulation cannot handle further increases in frequency bands.

[0012] Regarding optical modulators using lithium niobate films, compared to optical modulators that form optical waveguides near the surface of a lithium niobate single-crystal substrate through Ti diffusion, they can reduce the driving voltage. However, they cannot handle the low driving voltage drivers (above 2.0V and below 4.3V) used in silicon-based optical modulators.

[0013] The present invention was made in view of the above-mentioned problems, and its object is to provide an optical modulation element that can be used in the high-frequency band and uses a lithium niobate film that can be driven at low voltage, and a driving method for the optical modulation element using a lithium niobate film that can be driven at low voltage.

[0014] Methods for solving problems

[0015] (1) An optical modulation element according to one aspect of the present invention has a first optical waveguide, a second optical waveguide, a first electrode to which an electric field is applied to the first optical waveguide, and a second electrode to which an electric field is applied to the second optical waveguide. The first optical waveguide and the second optical waveguide each include a ridge portion protruding from a first surface of a lithium niobate film. The length of the portion of the first electrode that overlaps with the first optical waveguide, i.e., the first interaction length L1, is 0.9 mm or more and 20 mm or less. The length of the portion of the second electrode that overlaps with the second optical waveguide, i.e., the second interaction length L2, is 0.9 mm or more and 20 mm or less.

[0016] (2) In the optical modulation element described in (1) above, the amplitude of the applied voltage applied between the first electrode and the second electrode, i.e., the applied voltage amplitude Vpp, may be 2.0V or more and 4.3V or less, and the half-wavelength voltage Vπ may be 2.0V or more and the applied voltage amplitude Vpp.

[0017] (3) A method for driving an optical modulation element according to one aspect of the present invention includes a first optical waveguide and a second optical waveguide, each comprising a ridge portion protruding from a first surface of a lithium niobate film, a first electrode located at a position overlapping the first optical waveguide in plan view, and a second electrode located at a position overlapping the second optical waveguide in plan view. The length of the portion of the first electrode overlapping the first optical waveguide, i.e., the first interaction length L1, is 0.9 mm or more and 20 mm or less. The length of the portion of the second electrode overlapping the second optical waveguide, i.e., the second interaction length L2, is 0.9 mm or more and 20 mm or less. The applied voltage amplitude Vpp is 2.0 V or more and 4.3 V or less.

[0018] (4) In the driving method of the optical modulation element described in (3), the half-wavelength voltage Vπ may also be the applied voltage amplitude Vpp or above.

[0019] (5) In the driving method of the optical modulation element described in (3) or (4), Vpp / Vπ may also be 0.03 or more and 0.47 or less.

[0020] Invention Effects

[0021] According to the above method, it is possible to provide an optical modulation element using a lithium niobate film capable of low-voltage driving and a driving method for the optical modulation element using a lithium niobate film capable of low-voltage driving. Attached Figure Description

[0022] Figure 1 This is a block diagram of the optical modulator of the first embodiment.

[0023] Figure 2 This is a top view of the optical waveguide of the first embodiment.

[0024] Figure 3 This is a top view of the optical modulation element according to the first embodiment.

[0025] Figure 4 This is a cross-sectional view of the optical modulation element in the first embodiment.

[0026] Figure 5 This is a graph showing the relationship between the applied voltage and the output of the optical modulator in the first embodiment.

[0027] Figure 6 This is a diagram illustrating the voltage amplitude of the optical modulator in the first embodiment.

[0028] Figure 7 This is a graph showing the relationship between the applied voltage and the extinction ratio of the optical modulator in the first embodiment.

[0029] Figure 8This is a top view of the optical modulation element in the first variation. Detailed Implementation

[0030] Hereinafter, this embodiment will be described in detail with appropriate reference to the accompanying drawings. The drawings used in the following description sometimes show enlarged portions of the features for easier understanding of the invention, and the dimensions and proportions of the constituent elements may sometimes differ from reality. The materials, dimensions, etc., illustrated in the following description are merely examples, and the present invention is not limited to these examples, but can be appropriately modified and implemented within the scope of achieving the effects of the present invention.

[0031] First, define the directions. The x-direction is defined as one direction of one face of the substrate Sb, and the y-direction is the direction orthogonal to the x-direction. The x-direction, for example, is the extension direction of the first optical waveguide 11. The z-direction is perpendicular to the substrate Sb. The z-direction is orthogonal to both the x and y directions. Hereinafter, the +z direction may sometimes be expressed as "up" and the -z direction as "down." Up and down do not necessarily coincide with the direction of gravity.

[0032] Figure 1 This is a block diagram of the optical modulator 200 according to the first embodiment. The optical modulator 200 includes an optical modulation element 100, a drive circuit 110, a DC bias application circuit 120, and a DC bias control circuit 130. The control unit of the optical modulator 200 includes the drive circuit 110, the DC bias application circuit 120, and the DC bias control circuit 130.

[0033] The optical modulation element 100 converts an electrical signal into an optical signal. The optical modulation element 100 modulates the input light L... in The modulation signal Sm is converted into output light L. out .

[0034] The driving circuit 110 applies a modulation voltage Vm corresponding to the modulation signal Sm to the optical modulation element 100. The DC bias application circuit 120 applies a DC bias voltage Vdc to the optical modulation element 100. The DC bias control circuit 130 monitors the output light L. out The DC bias voltage Vdc output from the DC bias application circuit 120 is controlled. By adjusting this DC bias voltage Vdc, the operating point Vd, described later, is controlled.

[0035] Figure 2 This is a top view of the optical waveguide 10 of the optical modulation element 100 viewed from the z-direction. Figure 3 This is a top view of the optical modulation element 100 viewed from the z-direction. Figure 4 It is along Figure 3 The X1-X1' cut-off section. The optical modulation element 100 has an optical waveguide 10 and electrodes 21, 22, 23, and 24.

[0036] The optical modulation element 100 is disposed on the substrate Sb. The substrate Sb can be any substrate capable of forming an oxide film 40, such as a lithium niobate film, as an epitaxial film; a sapphire single-crystal substrate or a silicon single-crystal substrate is preferred. The crystal orientation of the substrate Sb is not particularly limited. Furthermore, the lithium niobate film has the property of being easily formed as a c-axis oriented epitaxial film for substrates Sb with various crystal orientations. Since the crystal constituting the c-axis oriented lithium niobate film has a 3-fold symmetry, it is desirable that the substrate Sb also has the same symmetry; in the case of a sapphire single-crystal substrate, the c-plane is preferred, and in the case of a silicon single-crystal substrate, the (111)-plane substrate is preferred.

[0037] Optical waveguide 10 is a path for light to propagate internally. Optical waveguide 10, for example, has a first optical waveguide 11, a second optical waveguide 12, an input path 13, an output path 14, a branch 15, and a junction 16. The first optical waveguide 11 and the second optical waveguide 12 extend, for example, in the x-direction. The x-direction lengths of the first optical waveguide 11 and the second optical waveguide 12 are approximately the same. The branch 15 is provided between the input path 13 and the first optical waveguide 11 and the second optical waveguide 12. The input path 13 is connected to the first optical waveguide 11 and the second optical waveguide 12 via the branch 15. The junction 16 is provided between the first optical waveguide 11 and the second optical waveguide 12 and the output path 14. The first optical waveguide 11 and the second optical waveguide 12 are connected to the output path 14 via the junction 16.

[0038] The optical waveguide 10 includes a first optical waveguide 11 and a second optical waveguide 12, which are ridges protruding from the first surface 40a of the oxide film 40. The first surface 40a is the upper surface of the oxide film 40 excluding the ridges. The ridges protrude from the first surface 40a in the z-direction and extend along the optical waveguide 10. The shape of the X1-X1' cross-section (the cross-section perpendicular to the direction of light travel) of the ridges is not required as long as it is a shape that can guide light; for example, it can be dome-shaped, triangular, or rectangular. The width of the ridges 16 in the y-direction is, for example, 0.3 μm to 5.0 μm, and the height of the ridges 16 (the protrusion height from the first surface 40a) is, for example, 0.1 μm to 1.0 μm. The ridges are made of the same material as the oxide film 40.

[0039] The oxide film 40 is, for example, a lithium niobate film with a c-axis orientation. The oxide film 40 is, for example, an epitaxial film grown on a Sb substrate. An epitaxial film refers to a single-crystal film with a uniform crystal orientation achieved using a substrate. An epitaxial film has a single crystal orientation in the z-direction and xy-plane, with the crystal uniformly oriented in the x, y, and z-axis directions. Whether it is an epitaxial film can be confirmed, for example, by confirming the peak intensity and poles at the orientation position using 2θ-θ X-ray diffraction. Furthermore, the oxide film 40 can also be a lithium niobate film formed on a Si substrate using SiO2.

[0040] Specifically, when performing measurements using 2θ-θ X-ray diffraction, the peak intensities outside the target plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, when the oxide film 40 is a c-axis oriented epitaxial film, the peak intensities outside the (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. Here, (00L) is a general term representing planes equivalent to (001) and (002).

[0041] Furthermore, under the condition of confirming the peak intensity at the orientation position mentioned above, it only indicates orientation in one direction. Thus, even if the above conditions are obtained, when the crystal orientation is inconsistent in the plane, the intensity of the X-ray at a specific angular position is not high, and the poles cannot be seen. For example, in the case where the oxide film 40 is a lithium niobate film, since LiNbO3 has a trigonal crystal structure, the single crystal LiNbO3 (014) has 3 poles. It is known that in the case of lithium niobate, epitaxial growth is performed in a so-called bicrystalline state where the crystals are symmetrically combined after rotating 180° around the c-axis. In this case, the 3 poles are symmetrically combined into 2, so there are 6 poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with (100) plane, the substrate becomes 4 times symmetrical, so 4x3=12 poles are observed. In addition, in the present invention, the lithium niobate film epitaxially grown in a bicrystalline state is also included in the epitaxial film.

[0042] The composition of lithium niobate is Li x NbA y O z A is 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 4.0, preferably 2.5 to 3.5. Examples of elements containing 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; combinations of two or more of these elements are also possible.

[0043] The thickness of the oxide film 40 is, for example, 2 μm or less. The thickness of the oxide film 40 refers to the thickness of the portion excluding the ridge. If the oxide film 40 is thick, there is a problem of low crystallinity. Furthermore, the thickness of the oxide film 40 is, for example, about 1 / 10 or more of the wavelength of the light used. If the oxide film 40 is thin, the light containment is weakened, and light leaks into the substrate Sb and the buffer layer 30. If the oxide film 40 is thin, there is a problem that even when an electric field is applied to the oxide film 40, the change in the effective refractive index of the optical waveguide 10 is small.

[0044] Electrodes 21 and 22 are electrodes for applying a modulation voltage Vm to the optical waveguide 10. Electrode 21 is an example of a first electrode, and electrode 22 is an example of a second electrode. The first end 21a of electrode 21 is connected to a power supply 31, and the second end 21b is connected to a terminating resistor 32. The first end 22a of electrode 22 is connected to a power supply 31, and the second end 22b is connected to a terminating resistor 32. Power supply 31 is part of a drive circuit 110 that applies the modulation voltage Vm to the optical modulation element 100.

[0045] Electrodes 23 and 24 are electrodes for applying a DC bias Vdc to the optical waveguide 10. The first terminal 23a of electrode 23 and the first terminal 24a of power supply 24 are connected to power supply 33. Power supply 33 is part of a DC bias application circuit 120 that applies the DC bias voltage Vdc to the optical modulation element 100.

[0046] exist Figure 3 In the diagram, for ease of viewing, the linewidth and spacing of the side-by-side electrodes 21 and 22 are wider than they actually are. Therefore, the lengths (interaction lengths) of the portions where electrode 21 overlaps with the first optical waveguide 11 and the lengths of the portions where electrode 22 overlaps with the second optical waveguide 12 appear different, but their actual lengths (interaction lengths) are approximately the same. Similarly, the lengths (interaction lengths) of the portions where electrode 23 overlaps with the first optical waveguide 11 and the lengths (interaction lengths) of the portions where electrode 24 overlaps with the second optical waveguide 12 are approximately the same.

[0047] Furthermore, if electrodes 21 and 22 are subjected to an overlapping DC bias voltage Vdc, electrodes 23 and 24 may not be required. Alternatively, grounding electrodes may be provided around electrodes 21, 22, 23, and 24.

[0048] Electrodes 21, 22, 23, and 24, sandwiching a buffer layer 30, are located on the oxide film 40. Electrodes 21 and 23 apply an electric field to the first optical waveguide 11. Electrodes 21 and 23 are respectively positioned, for example, overlapping with the first optical waveguide 11 in a top view along the z-direction. Electrodes 21 and 23 are respectively positioned above the first optical waveguide 11. Electrodes 22 and 24 are capable of applying an electric field to the second optical waveguide 12. Electrodes 22 and 24 are respectively positioned, for example, overlapping with the second optical waveguide 12 in a top view along the z-direction. Electrodes 22 and 24 are respectively positioned above the second optical waveguide 12.

[0049] A buffer layer 30 is disposed between the optical waveguide 10 and the electrodes 21, 22, 23, and 24. The buffer layer 30 covers and protects the ridge portion. Furthermore, the buffer layer 30 prevents light transmitted through the optical waveguide 10 from being absorbed by the electrodes 21, 22, 23, and 24. The refractive index of the buffer layer 30 is lower than that of the oxide film 40. The buffer layer 30 is, for example, SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, Y2O3, CaF2, In2O3, or mixtures thereof.

[0050] The chip size of the optical modulation element 100 is 100mm. 2 For example, 100mm 2 The following applies if the chip size of the optical modulation element 100 is 100mm. 2 The following can be used as an optical modulation element for data centers.

[0051] The optical modulation element 100 can be fabricated using known methods. For example, the optical modulation element 100 can be fabricated using semiconductor processes such as epitaxial growth, photolithography, etching, vapor phase growth, and metal sputtering.

[0052] The optical modulation element 100 converts an electrical signal into an optical signal. The optical modulation element 100 modulates the input light L... in Modulated output light L out First, let's explain the modulation operation of the optical modulation element 100.

[0053] Input light L from input path 13 in The light is branched to the first optical waveguide 11 and the second optical waveguide 12 for transmission. The phase difference between the light transmitted in the first optical waveguide 11 and the light transmitted in the second optical waveguide 12 is zero at the time of the branch.

[0054] Next, a voltage is applied between electrodes 21 and 22. Alternatively, differential signals with the same absolute value, opposite signs, and no phase shift can be applied to electrodes 21 and 22 respectively. The refractive indices of the first optical waveguide 11 and the second optical waveguide 12 vary according to electro-optical effects. For example, the refractive index of the first optical waveguide 11 varies from a reference refractive index n and +Δn, and the refractive index of the second optical waveguide 12 varies from a reference refractive index n and -Δn.

[0055] The difference in refractive index between the first optical waveguide 11 and the second optical waveguide 12 creates a phase difference between the light transmitted in the first optical waveguide 11 and the light transmitted in the second optical waveguide 12. The light transmitted in the first optical waveguide 11 and the second optical waveguide 12 merges in the output path 14, becoming the output light L. out Output. Output light L out It is the light obtained by aligning the light transmitted in the first optical waveguide 11 with the light transmitted in the second optical waveguide 12. Output light L outThe intensity varies according to the phase difference between the light transmitted in the first optical waveguide 11 and the light transmitted in the second optical waveguide 12, which is an odd multiple of π. For example, when the phase difference is an even multiple of π, the light enhances each other, and when it is an odd multiple of π, the light weakens each other. Following this process, the optical modulation element 100 modulates the input light L according to the electrical signal. in Modulated output light L out .

[0056] use Figure 5 This describes the optical modulation performed using the optical modulation element 100. Figure 5 This is a graph showing the relationship between the applied voltage and the output of the optical modulator 200 in the first embodiment. Figure 5 The horizontal axis represents the voltage applied to the optical modulation element 100, and the vertical axis represents the normalized value of the output from the optical modulation element 100. The output is normalized to "1" when the phase difference between the light transmitted in the first optical waveguide 11 and the light transmitted in the second optical waveguide 12 is zero.

[0057] Next, the null-point voltage Vn and the half-wavelength voltage Vπ will be explained. The output of the optical modulation element 100 is maximum when the applied voltage is zero. This is because, when the applied voltage is zero, the phase difference between the light transmitted in the first optical waveguide 11 and the light transmitted in the second optical waveguide 12 is zero. The output of the optical modulation element 100 gradually decreases as the applied voltage is gradually increased, reaching a minimum at a certain point. The voltage at which the output of the optical modulation element 100 reaches a minimum is the null-point voltage Vn. The half-wavelength voltage (half-wavelength phase modulation voltage) is the voltage used to make the phase difference of the light 180° in a Mach-Zehnder type optical modulator. The voltage amplitude from maximum to minimum of the output of the optical modulation element 100 corresponds to the half-wavelength voltage Vπ. When a voltage exceeding the null-point voltage Vn is applied, the output of the optical modulation element 100 changes periodically. The output of the optical modulation element 100 repeatedly reaches a maximum and a minimum every half-wavelength voltage Vπ.

[0058] The half-wavelength voltage Vπ of the optical modulation element 100 varies according to the structure of the optical modulation element 100. For example, the half-wavelength voltage Vπ varies according to the length of the electrode 21 on the first optical waveguide 11, the length of the electrode 22 on the second optical waveguide 12, etc. Here, the lengths of the first electrode 21 and the second electrode 22 are the lengths of the light in the transmission direction. Figure 3 In the case of an interaction length, the length of the portion of electrode 21 that overlaps with the first optical waveguide 11 or the length of the portion of electrode 22 that overlaps with the second optical waveguide 12 is called the interaction length. When the interaction length is long, the half-wavelength voltage Vπ decreases, and when the interaction length is short, the half-wavelength voltage Vπ increases.

[0059] In the optical modulation element 100, the first interaction length L1 is 0.9 mm or more and 20 mm or less. The first interaction length L1 is the length of the portion of the first electrode 21 that overlaps with the first optical waveguide 11 along its length direction. When the first interaction length L1 is less than 0.9 mm, it is impossible to achieve the extinction ratio of 3 dB or more required for a data center by driving with a low voltage of 2.0 V to 4.3 V. Therefore, the first interaction length L1 is 0.9 mm or more.

[0060] When the first interaction length L1 exceeds 20 mm, the attenuation in the high-frequency band above 60 GHz is significant. Therefore, the first interaction length L1 is less than 20 mm. Similarly, the second interaction length L2 is between 0.9 mm and 20 mm, and the second interaction length L2 is the length of the portion of the second electrode 22 that overlaps with the second optical waveguide 12 in the length direction.

[0061] The first electrode 21 and the second electrode 22 are formed in such a manner that the first interaction length L1 and the second interaction length L2 are approximately the same. Furthermore, in Figure 3 In the diagram, it appears that the first interaction length L1 and the second interaction length L2 are different. However, in reality, the first electrode 21 and the second electrode 22 have narrow linewidths and a narrow gap between them, so the interaction lengths of the two electrodes are approximately the same.

[0062] A modulation voltage Vm corresponding to the modulation signal is applied to electrodes 21 and 22 of the optical modulation element 100 for applying the modulation voltage. The voltage applied to electrodes 23 and 24 for applying the DC bias voltage, i.e., the DC bias voltage Vdc output from the DC bias application circuit 120, is controlled by the DC bias control circuit 130. The DC bias control circuit 130 adjusts the operating point Vd of the optical modulation element 100 by controlling the DC bias voltage Vdc. The operating point Vd is the voltage at the center of the modulation voltage amplitude.

[0063] The DC bias application circuit 120 controls the operating point voltage Vd of the optical modulation element 100. The operating point voltage Vd is the midpoint between the minimum (Vmin) and maximum (Vmax) of the applied voltage. Furthermore, the difference between the minimum (Vmin) and maximum (Vmax) of the applied voltage is the applied voltage amplitude Vpp.

[0064] The operating point voltage Vd can vary due to factors such as ambient temperature. When the operating point voltage Vd changes during use, the DC bias control circuit 130 corrects it by adjusting the set applied voltage amplitude Vpp to ensure that the operating point voltage Vd falls within the range where the extinction ratio is 3dB or higher.out Branching light L b Correct the variation of the operating point voltage Vd.

[0065] Furthermore, the drive circuit 110 controls the applied voltage amplitude Vpp applied to the optical modulation element 100. The applied voltage amplitude Vpp applied to the optical modulation element is in the range of 2.0V or more and 4.3V or less. If the interaction length is 0.9mm or more and 20.0mm or less, the applied voltage amplitude Vpp can be between 2.0V or more and 4.3V or less, resulting in an extinction ratio of 3dB or more. The drive circuit 110 inputs an electrical signal converted into an optical signal to the optical modulation element 100. The drive circuit 110 includes, for example, a power supply and a driver.

[0066] Figure 6 This is a diagram illustrating the applied voltage amplitude Vpp of the optical modulator 200 in the first embodiment. Figure 6 Is Figure 5 A diagram illustrating the applied voltage amplitude Vpp has been added.

[0067] The applied voltage amplitude Vpp is the range of voltages used when the optical modulation element 100 is operated. In the optical modulation element 100, the applied voltage amplitude Vpp is applied with the operating point voltage Vd as the midpoint. The output from the optical modulation element 100 varies within a range corresponding to the minimum value (Vmin) and the maximum value (Vmax) of the applied voltage. The half-wavelength voltage Vπ is the applied voltage amplitude Vpp or higher. Furthermore, the operating point voltage Vd is set such that the minimum value (Vmin) of the voltage applied to the optical modulation element 100 is greater than or equal to the operating point voltage Vn, but it can also be set such that the maximum value (Vmax) is less than or equal to the operating point voltage Vn.

[0068] The high-frequency voltage modulation signal is controlled, for example, by the drive circuit 110. The frequency band of the modulation element is above 60 GHz. If the frequency band of the modulation element is above 60 GHz, high-speed modulation can be easily handled.

[0069] Figure 7 This is a graph showing the relationship between the applied voltage and the extinction ratio of the optical modulator 200 in the first embodiment. Figure 7 The horizontal axis represents the voltage applied to the optical modulation element 100, and the vertical axis represents the output light L of the applied voltage. out The output light L of the point voltage Vn is related to the null point voltage. out The extinction ratio is the ratio of the output light intensity L within the applied voltage range. out The ratio of the maximum value to the minimum value.

[0070] As described above, the optical modulation element 100 and optical modulator 200 of the first embodiment can be driven at low voltage and used in the high-frequency band.

[0071] The above is an example illustrating the optical modulation element 100 and optical modulator 200 of the first embodiment. However, the present invention is not limited to the first embodiment and various modifications are possible.

[0072] For example, the first interaction length L1 and the second interaction length L2 can also be 18.6 mm or less. By making the first interaction length L1 and the second interaction length L2 18.6 mm or less, the response characteristics in the high-frequency band above 70 GHz are also improved. Furthermore, by making them 16.9 mm or less, the response characteristics in the high-frequency band above 80 GHz are also improved. By making them 14.4 mm or less, the response characteristics in even higher frequency bands are also improved.

[0073] Furthermore, in the optical modulator of the first embodiment, the operating point voltage Vd is controlled, but the minimum value (Vmin) or maximum value (Vmax) of the voltage applied to the optical modulation element 100 can also be controlled. In addition, when controlling the minimum value (Vmin), control is performed such that the minimum value (Vmin) is above the null point voltage Vn. On the other hand, when controlling the maximum value (Vmax), control is performed such that the maximum value (Vmax) is below the null point voltage Vn.

[0074] Ideally, Vpp / Vπ should be 0.03 or higher and 0.47 or lower. Within this range, an extinction ratio of 3 dB or higher and a modulation element bandwidth of 60 GHz or higher can be achieved.

[0075] The chip size of the optical modulation element 100 is 100mm. 2 The following is more preferably 50mm 2 The following applies. If the chip size of the optical modulation element 100 is reduced, it can also be used in existing data center transceivers.

[0076] in addition, Figure 8 This is a top view of the first deformed optical modulation element 101 viewed from the z-direction. The optical modulation element 101 has an optical waveguide 50 and electrodes 61, 62, 63, and 64.

[0077] Optical waveguide 50 has a first optical waveguide 51, a second optical waveguide 52, an input path 53, an output path 54, a branch 55, and a junction 56. Optical waveguide 50 differs from optical waveguide 10 in that it bends midway between the first optical waveguide 51 and the second optical waveguide 52. Other aspects of optical waveguide 50 are the same as those of optical waveguide 10.

[0078] Electrodes 61 and 62 are electrodes for applying a modulation voltage Vm to the optical waveguide 50. Electrode 61 is an example of the first electrode, and electrode 62 is an example of the second electrode. Terminal 61a of electrode 61 is connected to power supply 31, and terminal 61b of electrode 61 is connected to terminating resistor 32. Terminal 62a of electrode 62 is connected to power supply 31, and terminal 62b of electrode 62 is connected to terminating resistor 32. Electrodes 63 and 64 are electrodes for applying a DC bias Vdc to the optical waveguide 50. Terminal 63a of electrode 63 and terminal 64a of power supply 64 are connected to power supply 33.

[0079] exist Figure 8 In the illustration, due to the widened linewidth and spacing of the side-by-side electrodes 61 and 62, the lengths of the overlapping portions of electrode 61 and the first optical waveguide 51 and electrode 62 and the second optical waveguide 52 appear different, although their lengths are approximately the same. Similarly, the lengths of the overlapping portions of electrode 63 and the first optical waveguide 51 and electrode 64 and the second optical waveguide 52 are approximately the same.

[0080] The fact that electrodes 61 and 62 bend along the first optical waveguide 51 and the second optical waveguide 52 differs from that of electrodes 21 and 22. Other aspects of electrodes 61, 62, 63, and 64 are the same as those of electrodes 21, 22, 23, and 24, respectively.

[0081] The optical modulation element 101 has a small element size in the x-direction due to the bending of the first optical waveguide 51 and the second optical waveguide 52. For example, the optical modulation element 101 can achieve a size of 100 mm. 2 The following is preferred: 50mm 2 The following are the component dimensions. Optical modulators for data centers are required to be miniaturized. By bending the optical waveguide 50, the optical modulation element 101 can be accommodated even in a small area corresponding to existing optical modulators for data centers.

[0082]

Example

[0083] The following are examples of embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0084] Furthermore, it is obvious that anyone skilled in the art can conceive of various modifications or alterations within the scope of the ideas described in the claims, and understand that these modifications and alterations are of course also within the scope of the present invention.

[0085] (Example)

[0086] Figure 3 and Figure 4The structure was actually prototyped according to the following process. The substrate was made of sapphire. A 1.5 μm thick lithium niobate film was fabricated on the surface of the substrate using sputtering. Then, a 0.8 μm thick LaAlO3 buffer layer was deposited on the lithium niobate film using vapor deposition. Ridges were formed using a photoresist mask and dry etching with Ar plasma. The ridge width was 2.5 μm and the ridge height was 0.4 μm. Finally, the first and second electrodes were formed through optical and gold plating processes. The relative permittivity of LaAlO3 is 13.

[0087] The modulation characteristics of the obtained optical modulator were evaluated using light at a wavelength of 1310 nm. In Tables 1 and 2, the applied voltage amplitudes of 2 V and 4.3 V represent the half-wavelength voltage Vπ (Vpi) (V), the applied voltage amplitude Vpp (V), the maximum extinction ratio ER (ERmax) (dB), and the usable high-frequency domain RF (GHz) with varying interaction lengths L. Here, the first interaction length L1 and the second interaction length L2 are the same value, denoted by the interaction length L in Table 1.

[0088] [Table 1]

[0089] L(mm) Vpi(V) Vpp(V) Vpp / Vpi ERmax (dB) RF (GHz) 0.6 101 2.0 0.02 2.4 ＞80 0.9 66.4 2.0 0.03 3.5 ＞80 1.1 56.4 2.0 0.04 4.1 ＞80 1.4 46.6 2.0 0.04 4.9 ＞80 1.9 33.3 2.0 0.06 6.6 ＞80 5.9 11.4 2.0 0.18 14.0 ＞80 8.5 8.3 2.0 0.24 16.5 ＞80 14.4 5.5 2.0 0.36 19.7 ＞80 16.9 4.9 2.0 0.41 20.6 80 18.6 4.7 2.0 0.43 20.9 70 20.0 4.3 2.0 0.47 21.5 60 22.0 4.1 2.0 0.49 21.9 50

[0090] [Table 2]

[0091] L(mm) Vpi(V) Vpp(V) Vpp / Vpi ERmax (dB) RF (GHz) 0.3 241 4.3 0.02 2.2 ＞80 0.4 143 4.3 0.03 3.5 ＞80 0.6 101 4.3 0.04 4.9 ＞80 0.9 66.4 4.3 0.06 7.1 ＞80 1.1 56.4 4.3 0.08 8.0 ＞80 1.4 46.6 4.3 0.09 9.2 ＞80 1.9 33.3 4.3 0.13 11.7 ＞80 5.9 11.4 4.3 0.38 20.0 ＞80 8.5 8.3 4.3 0.52 22.3 ＞80 14.4 5.5 4.3 0.78 24.5 ＞80 16.9 4.9 4.3 0.88 24.8 80 18.6 4.7 4.3 0.91 24.9 70 20.0 4.3 4.3 1.00 25.0 60 22.0 4.1 4.3 1.05 25.0 50

[0092] As shown in Tables 1 and 2, it can be confirmed that when Vpp is 4.3V, as long as the interaction length L is in the range of 0.4mm to 20mm, it can be used at frequencies above 60GHz, and the extinction ratio can be above 3dB. Furthermore, it can be confirmed that when Vpp is 2.0V, as long as the interaction length L is in the range of 0.9mm to 20mm, it can be used at frequencies above 60GHz, and the extinction ratio can be above 3dB.

[0093] Explanation of reference numerals in the attached figures

[0094] 10, 50 optical waveguides

[0095] 11, 51 First Optical Waveguide

[0096] 12, 52 Second optical waveguide

[0097] 13, 53 Input Path

[0098] Output paths 14 and 54

[0099] Branches 15 and 55

[0100] 16, 56 Joint

[0101] Electrodes 21, 22, 23, 24, 61, 62, 63, and 64

[0102] 30 Buffer Layer

[0103] 40 Oxide film

[0104] 40a Side 1

[0105] 100, 101 optical modulation elements

[0106] 110 drive circuit

[0107] 120 DC bias application circuit

[0108] 130 DC bias control circuit

[0109] 200 optical modulator

[0110] L in Input light

[0111] L out Output light

[0112] L b Branching light

[0113] Vd Operating point voltage

[0114] Vn null point voltage

[0115] Vπ half-wavelength voltage

[0116] Vpp represents the applied voltage amplitude.

Claims

1. An optical modulation element, characterized in that: It has a first optical waveguide, a second optical waveguide, a first electrode to which an electric field is applied, and a second electrode to which an electric field is applied, the second optical waveguide. The first optical waveguide and the second optical waveguide each include a ridge-shaped portion protruding from the first surface of the lithium niobate film. The first interaction length L1 is greater than 0.9 mm and less than 20 mm, wherein the first interaction length L1 is the length of the portion of the first electrode that overlaps with the first optical waveguide in the length direction. The second interaction length L2 is greater than 0.9 mm and less than 20 mm, wherein the second interaction length L2 is the length of the portion of the second electrode that overlaps with the second optical waveguide in the length direction. The applied voltage amplitude Vpp is above 2.0V and below 4.3V, wherein the applied voltage amplitude Vpp is the difference between the maximum and minimum values ​​of the applied voltage applied between the first electrode and the second electrode. The applied voltage is set such that the applied voltage amplitude Vpp / half-wavelength voltage Vπ is greater than 0.03 and less than 0.

47. The frequency band of the optical modulation element is above 60 GHz.

2. A method for driving an optical modulation element, the optical modulation element having a first optical waveguide and a second optical waveguide, each comprising a ridge protruding from a first surface of a lithium niobate film, a first electrode located at a position overlapping the first optical waveguide in plan view, and a second electrode located at a position overlapping the second optical waveguide in plan view, wherein a first interaction length L1 is 0.9 mm or more and 20 mm or less, wherein, The first interaction length L1 is the length of the portion of the first electrode that overlaps with the first optical waveguide in the length direction, and the second interaction length L2 is 0.9 mm or more and 20 mm or less, wherein the second interaction length L2 is the length of the portion of the second electrode that overlaps with the second optical waveguide in the length direction. The driving method of the optical modulation element is characterized in that: The applied voltage amplitude Vpp is above 2.0V and below 4.3V, wherein the applied voltage amplitude Vpp is the difference between the maximum and minimum values ​​of the applied voltage applied between the first electrode and the second electrode. The applied voltage is set such that the applied voltage amplitude Vpp / half-wavelength voltage Vπ is greater than 0.03 and less than 0.

47. The frequency band of the optical modulation element is above 60 GHz.

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