Optical modulator and driving method of optical modulating element
By controlling the electrode voltage of the lithium niobate film optical modulator, the shortcomings of optical modulators in terms of miniaturization and low voltage drive are solved, realizing an optical modulator with a high extinction ratio at a voltage below half wavelength, which is suitable for data center communication.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- TDK CORP
- Filing Date
- 2021-07-13
- Publication Date
- 2026-07-10
AI Technical Summary
Existing optical modulators have shortcomings in miniaturization and low-voltage driving, especially in high-frequency applications, making it difficult to meet the communication needs within or between data centers. An extinction ratio of more than 3dB needs to be achieved at a driving voltage of less than half the wavelength.
A lithium niobate film optical modulator is used. By controlling the applied voltage between the electrodes of the optical modulation element, the applied voltage amplitude is ensured to be between 0.06×Vπ and 0.4×Vπ. The minimum and maximum values of the optical modulation element are controlled within specific ranges to achieve driving voltage below half wavelength while maintaining a high extinction ratio.
A high extinction ratio of the optical modulator was achieved at a voltage below half the wavelength, meeting the communication requirements of data centers, and enabling miniaturization and low-voltage drive of the optical modulator.
Smart Images
Figure CN115668039B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to an optical modulator and a driving method for an optical modulation element. This application claims priority based on Japanese Patent Application No. 2020-135860, 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 using lithium niobate single-crystal substrates.
[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-based 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. Furthermore, indium phosphide-based optical modulators can also operate in high-frequency bands, and their application in long-distance communications is anticipated. On the other hand, in recent years, short- and medium-distance communications within or between data centers have increased. In such applications, since a high extinction ratio is not required, there are cases where silicon-based optical modulators are used, and cases where the emitted light is directly modulated using a laser diode drive circuit without an optical modulator. While silicon-based optical modulators achieve small size and low-voltage operation (specifically, less than 5mm and less than 2.4V), they cannot handle high-frequency operation.
[0012] On the other hand, in order to apply optical modulators that can operate in the high-frequency band, such as those using lithium niobate and indium phosphide, to communications within or between data centers, miniaturization and low-voltage driving are required. Furthermore, regarding optical modulators using lithium niobate films, compared to optical modulators where an optical waveguide is formed near the surface of a lithium niobate single-crystal substrate through Ti diffusion, the driving voltage can be reduced; however, to replace silicon-based optical modulators, the driving voltage needs to be further reduced. In a typical Mach-Zehnder type optical modulator, this driving voltage corresponds to the half-wavelength voltage (half-wavelength phase modulation voltage) used to achieve a 180° phase difference in light, and its value increases with miniaturization of the optical modulator. Specifically, in Example 3, even with an interaction length of 5 mm, Vπ becomes 4.8V, which is unsuitable for use in the aforementioned data center. It is necessary to keep the driving voltage below 50% of Vπ, practically below 40%, preferably below 35%, and more preferably below 30%. Therefore, to accommodate the miniaturization and low-voltage operation of the optical modulator, it needs to operate at a driving voltage lower than the half-wavelength voltage Vπ (below 0.4Vπ). Furthermore, an extinction ratio of 3dB or higher is required.
[0013] The present invention was made in view of the above-mentioned problems, and its object is to provide an optical modulator capable of achieving an extinction ratio of 3dB or more when operating with a driving voltage of less than half a wavelength voltage, and a driving method for driving an optical modulation element with a driving voltage of less than half a wavelength voltage while ensuring an extinction ratio of 3dB or more.
[0014] Methods for solving problems
[0015] (1) The optical modulator of the first type includes: an optical modulation element having a first optical waveguide, a second optical waveguide, a first electrode for applying an electric field to the first optical waveguide, and a second electrode for applying an electric field to the second optical waveguide; and a control unit for controlling the applied voltage between the first electrode and the second electrode, wherein the control unit, when the half-wavelength voltage of the optical modulation element is Vπ, and the applied voltage amplitude, which is the amplitude of the applied voltage applied to the optical modulation element, is Vpp, such that Vpp is 0.06×Vπ≤Vpp≤0.4×Vπ, and when the minimum and maximum values of the voltage applied to the optical modulation element are Vmin and Vmax, respectively, and the null point voltage of the optical modulation element is Vn, such that Vn≤Vmin≤Vn+0.29×Vπ, or Vn-0.29×Vπ≤Vmax≤Vn.
[0016] (2) In the optical modulator described above, the first optical waveguide and the second optical waveguide may each include a ridge protruding from the first surface of the lithium niobate film.
[0017] (3) The second method of driving the optical modulation element is a method of driving an optical modulation element having a first optical waveguide and a second optical waveguide, a first electrode located at a position overlapping with the first optical waveguide in top view, and a second electrode located at a position overlapping with the second optical waveguide in top view. When the half-wavelength voltage of the optical modulation element is Vπ, and the amplitude of the applied voltage applied to the optical modulation element is Vpp, Vpp is 0.06×Vπ≤Vpp≤0.4×Vπ. When the minimum and maximum values of the voltage applied to the optical modulation element are Vmin and Vmax, respectively, and the null point voltage of the optical modulation element is Vn, Vn≤Vmin≤Vn+0.29×Vπ, or Vn-0.29×Vπ≤Vmax≤Vn.
[0018] (4) In the driving method of the optical modulation element in the above manner, the first optical waveguide and the second optical waveguide may also include ridges protruding from the first surface of the lithium niobate film.
[0019] Invention Effects
[0020] The above-described method of driving optical modulators and optical modulation elements can operate with a driving voltage lower than half the wavelength and achieve an extinction ratio of more than 3dB. Attached Figure Description
[0021] Figure 1 This is a block diagram of the optical modulator of the first embodiment.
[0022] Figure 2 This is a top view of the optical waveguide of the first embodiment.
[0023] Figure 3 This is a top view of the optical modulation element according to the first embodiment.
[0024] Figure 4 This is a cross-sectional view of the optical modulation element in the first embodiment.
[0025] Figure 5 This is a graph showing the relationship between the applied voltage and the output of the optical modulator in the first embodiment.
[0026] Figure 6 This is a diagram illustrating the voltage amplitude R1 of the optical modulator in the first embodiment.
[0027] 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.
[0028] Figure 8 This is a diagram illustrating the voltage amplitude R2 of the optical modulator in the first embodiment.
[0029] Figure 9 This 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. In addition, 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. Let the amplitude of the applied voltage of the modulation signal be Vpp. 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 in the y-direction is, for example, 0.3 μm to 5.0 μm and the height of the ridges (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 of 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 4 x 3 = 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 (interaction 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. Electrode 21 is positioned above the first optical waveguide 11. Electrodes 22 and 24 can apply 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 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. Examples of materials that can be used for the buffer layer 30 include SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, Y2O3, CaF2, In2O3, or mixtures thereof.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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 out The 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. 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 .
[0055] 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.
[0056] The DC bias control circuit 130, when the minimum applied voltage Vmin is greater than the null point voltage Vn (described later), becomes... Figure 5 As shown, the minimum voltage Vmin of the optical modulation element 100 is controlled within the voltage amplitude R1 to control the operating point voltage Vd. The operating point voltage Vd is the midpoint between the minimum voltage Vmin and the maximum voltage Vmax of the applied voltage. The voltage amplitude R1 is defined by the half-wavelength voltage Vπ and the null point voltage Vn.
[0057] The voltage amplitude R1 is in the range of above Vn and below Vn + 0.291Vπ. When the minimum applied voltage Vpp, Vmin, is above the null point voltage Vn, it becomes... Figure 6 As shown, the minimum voltage Vmin is designed in a manner that satisfies the following equation (1).
[0058] Vn≤Vmin≤Vn+0.29Vπ (1)
[0059] Furthermore, when the maximum applied voltage Vmax is below the null point voltage Vn, it becomes... Figure 8 As shown, the maximum voltage Vmax is designed to satisfy the following equation (2).
[0060] Vn-0.29Vπ≤Vmax≤Vn (2)
[0061] 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.
[0062] 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.
[0063] The half-wavelength voltage Vπ of the optical modulation element 100 varies according to the structure of the optical modulation element 100. The half-wavelength voltage Vπ varies according to the length of the electrode 21 on the first optical waveguide 11 and the length of the electrode 22 on the second optical waveguide 12. The length is the length 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.
[0064] Optical modulation elements using lithium niobate thin films can more efficiently apply an electric field to the optical waveguide compared to those using bulk lithium niobate, thus reducing the half-wavelength voltage Vπ. However, for assembling transceivers such as those used in data centers, further miniaturization of the optical modulation element 100 is required, necessitating a shorter interaction length. Furthermore, shortening the interaction length is necessary to broaden the modulation frequency band of the optical modulation element 100. On the other hand, shortening the interaction length increases the half-wavelength voltage Vπ, thus requiring the driving voltage (applied voltage amplitude Vpp) to be below 0.4Vπ.
[0065] 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 for this variation, ensuring that the minimum and maximum applied voltage amplitudes Vpp fall within the range of voltage amplitudes R1 and R2. The DC bias control circuit 130, for example, is based on the output light L... out Branching light L b Correct changes to action points.
[0066] Furthermore, the drive circuit 110 also controls the applied voltage amplitude Vpp applied to the optical modulation element 100. The drive circuit 110 controls the high-frequency voltage applied to the optical modulation element 100. 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.
[0067] The driving circuit 110 controls the applied voltage amplitude Vpp applied to the optical modulation element 100 within the range of 0.06×Vπ≤Vpp≤0.4×Vπ.
[0068] 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.
[0069] The applied voltage amplitude Vpp represents the range of voltages used when the optical modulation element 100 is operated. The applied voltage amplitude Vpp is expressed as Vmax-Vmin, where the minimum and maximum values of the applied voltage are set to the minimum voltage Vmin and the maximum voltage Vmax, respectively. A voltage within a specified range is applied to the optical modulation element 100 with the operating point voltage Vd as the midpoint. A high-frequency voltage with an applied voltage amplitude Vpp is applied to the optical modulation element 100. The output from the optical modulation element 100 varies correspondingly to the ranges of Vmax and Vmin.
[0070] For example, when the minimum voltage Vmin is set to Vn, and the applied voltage amplitude Vpp is set to half-wavelength voltage Vπ, a voltage ranging from Vn to Vn+Vπ is typically applied to the optical modulation element 100. The output from the optical modulation element 100 is minimum when the applied voltage is Vn and maximum when the applied voltage is Vn+Vπ. By varying the applied voltage between Vn and Vn+Vπ, the range of variation in the output of the optical modulation element 100 becomes maximum. On the other hand, the driving voltage required to drive the optical modulation element 100 increases.
[0071] 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 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.
[0072] like Figure 5 and Figure 7 As shown, when the minimum voltage Vmin is set within the voltage amplitude R1, the optical modulation element 100 outputs light L... out It operates in areas with relatively low light intensity, but can increase the relative extinction ratio relative to the applied voltage amplitude Vpp.
[0073] Here, as Figure 7 As shown, the output light L of the optical modulation element 100 out The extinction ratio varies greatly in regions with very low light intensity (near the null point voltage Vn). Optical modulators used in data centers require an extinction ratio of at least 3 dB, and more preferably at least 6 dB. By setting the minimum voltage Vmin within the voltage amplitude R1, regions with large extinction ratio variations can be effectively utilized, enabling the achievement of a high extinction ratio with a low drive voltage (below 0.4 Vπ).
[0074] Furthermore, although the drive voltage increases when the extinction ratio is maximized in the optical modulation element 100 using lithium niobate film, by setting the minimum voltage Vmin within R1, the applied voltage amplitude Vpp is kept in the range of 0.06×Vπ≤Vpp≤0.4×Vπ, which ensures an extinction ratio of 3dB or more that can be used in data centers while enabling low-voltage drive (below 0.4Vπ).
[0075] As described above, the optical modulator 200 of the first embodiment can perform low-voltage driving (0.4 × Vπ or less) while ensuring an extinction ratio of 3 dB or more. Furthermore, it is required that the driving voltage be 0.4 × Vπ or less, more preferably 0.35 × Vπ or less, further preferably 0.3 × Vπ or less, and most preferably 0.25 × Vπ or less. Moreover, it is preferable to set the operating point such that the extinction ratio shown in Table 1 is 6 dB or more when the driving voltage is 0.4 × Vπ or less. Table 1 is a table showing the relationship between the minimum voltage Vmin, the applied voltage amplitude Vpp, and the extinction ratio. Both the minimum voltage Vmin and the applied voltage amplitude Vpp are values normalized to half-wavelength voltage. The first column in Table 1 represents the minimum voltage (Vmin / Vπ), and the first row represents Vpp / Vπ. The value of each cell in Table 1 represents the extinction ratio (dB) when the minimum voltage of the row belonging to that cell is combined with the Vpp of the column belonging to that cell. For example, with a minimum voltage (Vmin / Vπ) of 1.00 and a Vpp / Vπ of 0.25, the extinction ratio is 16.7 dB.
[0076] For example, when the driving voltage is 0.4 × Vπ, it is preferable to set the operating point Vd such that the minimum applied voltage amplitude Vpp, Vmin, is within the range of Vn to Vn + 0.29 Vπ. Furthermore, when the driving voltage is 0.35 × Vπ, it is preferable to set the operating point Vd such that the minimum applied voltage amplitude Vpp, Vmin, is within the range of Vn to Vn + 0.27 Vπ. Furthermore, when the driving voltage is 0.3 × Vπ, it is preferable to set the operating point Vd such that the minimum applied voltage amplitude Vpp, Vmin, is within the range of Vn to Vn + 0.24 Vπ. Furthermore, when the driving voltage is 0.25 × Vπ, it is preferable to set the operating point Vd such that the minimum applied voltage amplitude Vpp, Vmin, is within the range of Vn to Vn + 0.21 Vπ.
[0077] [Table 1]
[0078] Minimum voltage Vpp = 0.25 Vpp = 0.30 Vpp = 0.35 Vpp = 0.40 1.00 16.7 18.2 19.4 20.4 1.01 16.7 18.1 19.3 20.3 1.02 16.2 17.5 18.7 19.6 1.03 15.3 16.6 17.7 18.6 1.04 14.4 15.7 16.7 17.6 1.05 13.5 14.7 15.7 16.6 1.06 12.7 13.8 14.8 15.6 1.07 11.9 13.0 14.0 14.8 1.08 11.2 12.3 13.2 14.0 1.09 10.6 11.6 12.5 13.3 1.10 10.0 11.0 11.9 12.6 1.11 9.5 10.5 11.3 12.0 1.12 9.0 10.0 10.8 11.5 1.13 8.6 9.5 10.3 10.9 1.14 8.2 9.1 9.8 10.5 1.15 7.8 8.7 9.4 10.0 1.16 7.5 8.3 9.0 9.6 1.17 7.2 8.0 8.7 9.2 1.18 6.9 7.7 8.3 8.9 1.19 6.6 7.4 8.0 8.6 1.20 6.3 7.1 7.7 8.2 1.21 6.1 6.8 7.4 7.9 1.22 5.9 6.6 7.2 7.7 1.23 5.7 6.3 6.9 7.4 1.24 5.5 6.1 6.7 7.1 1.25 5.3 5.9 6.4 6.9 1.26 5.1 5.7 6.2 6.7 1.27 4.9 5.5 6.0 6.4 1.28 4.7 5.3 5.8 6.2 1.29 4.6 5.1 5.6 6.0
[0079] The above is an example illustrating the optical modulator 200 of the first embodiment; however, the present invention is not limited to the first embodiment and various modifications are possible.
[0080] For example, the applied voltage amplitude Vpp can be set to 0.06×Vπ≤Vpp≤0.35×Vπ. In this case, the extinction ratio of the optical modulator 200 can be greater than 3dB with a smaller applied voltage.
[0081] Furthermore, for example, the applied voltage amplitude Vpp can be set to 0.06×Vπ≤Vpp≤0.3×Vπ. In this case, the extinction ratio of the optical modulator 200 can be 3dB or more with a smaller applied voltage compared to the case where the applied voltage amplitude Vpp is 0.06×Vπ≤Vpp≤0.35×Vπ. It is further preferred that the applied voltage amplitude Vpp is 0.06×Vπ≤Vpp≤0.25×Vπ.
[0082] When Vpp is 0.4 × Vπ or less, it is preferable to set the operating point voltage Vd such that the minimum voltage Vmin of Vpp is Vn ≤ Vmin ≤ Vn + 0.29 × Vπ. When Vpp is 0.35 × Vπ or less, it is preferable to set the operating point voltage Vd such that the minimum voltage Vmin is Vn ≤ Vmin ≤ Vn + 0.27 × Vπ. When Vpp is 0.3 × Vπ or less, it is preferable to set the operating point voltage Vd such that the minimum voltage Vmin of Vpp is Vn ≤ Vmin ≤ Vn + 0.24 × Vπ. When Vpp is 0.25 × Vπ or less, it is preferable to set the operating point voltage Vd such that the minimum voltage Vmin of Vpp is Vn ≤ Vmin ≤ Vn + 0.21 × Vπ.
[0083] exist Figure 6 In this context, let the minimum voltage Vmin of the applied voltage amplitude Vpp be the range of the voltage amplitude R1. However, it can also be done as follows: Figure 8 This makes the maximum voltage Vmax of the applied voltage amplitude Vpp fall within the range of voltage amplitude R2. In this case, voltage amplitude R2 is defined by the half-wavelength voltage Vπ and the null point voltage Vn. Specifically, voltage amplitude R2 is in the range of Vn-0.29×Vπ to Vn. That is, it is designed in a way that makes the maximum voltage Vmax satisfy the above equation (2).
[0084] Vn-0.29×Vπ≤Vmax≤Vn (2)
[0085] By setting Vmax to a range of Vn-0.29×Vπ or less than Vn, an extinction ratio of 3dB or more can be achieved by applying an voltage with an applied voltage amplitude Vpp in the range of 0.06×Vπ≤Vpp≤0.4×Vπ. Furthermore, it is preferable to set the operating point such that the extinction ratio is 6dB or more when the driving voltage is 0.4×Vπ or less.
[0086] For example, when the voltage amplitude R2 is set on the side smaller than the null point voltage Vn, and the driving voltage is 0.4 × Vπ, it is preferable to set the operating point Vd such that the maximum voltage Vmax of the applied voltage amplitude Vpp is within the range of Vn to Vn-0.29Vπ. Furthermore, when the driving voltage is 0.35 × Vπ, it is preferable to set the operating point Vd such that the maximum voltage Vmax of the applied voltage amplitude Vpp is within the range of Vn to Vn-0.27Vπ. Furthermore, when the driving voltage is 0.3 × Vπ, it is preferable to set the operating point Vd such that the maximum voltage Vmax of the applied voltage amplitude Vpp is within the range of Vn to Vn-0.24 × Vπ. Furthermore, when the driving voltage is 0.25 × Vπ, it is preferable to set the operating point Vd such that the maximum voltage Vmax of the applied voltage amplitude Vpp is within the range of Vn to Vn-0.21Vπ.
[0087] For example, the applied voltage amplitude Vpp can be set to 0.06×Vπ≤Vpp≤0.35×Vπ. In this case, the extinction ratio of the optical modulator 200 can be greater than 3dB with a smaller applied voltage.
[0088] Furthermore, for example, the applied voltage amplitude Vpp can be set to 0.06×Vπ≤Vpp≤0.3×Vπ. In this case, the extinction ratio of the optical modulator 200 can be 3dB or more with a smaller applied voltage compared to the case where the applied voltage amplitude Vpp is 0.06×Vπ≤Vpp≤0.35×Vπ. More preferably, the applied voltage amplitude Vpp is 0.06×Vπ≤Vpp≤0.25×Vπ.
[0089] When Vpp is 0.4 × Vπ or less, it is preferable to set the operating point voltage Vd such that the maximum voltage Vmax of Vpp is Vn - 0.29 × Vπ ≤ Vmaxb ≤ Vn. When Vpp is 0.35 × Vπ or less, it is preferable to set the operating point voltage Vd such that the maximum voltage Vmax of Vpp is Vn - 0.27 × Vπ ≤ Vmax ≤ Vn. When Vpp is 0.3 × Vπ or less, it is preferable to set the operating point voltage Vd such that the maximum voltage Vmax of Vpp is Vn - 0.24 × Vπ ≤ Vmax ≤ Vn. When Vpp is 0.25 × Vπ or less, it is preferable to set the operating point voltage Vd such that the maximum voltage Vmax of Vpp is Vn - 0.21 × Vπ ≤ Vmax ≤ Vn.
[0090] Furthermore, in the above-described manner, the control unit controls the operating point voltage Vd of the optical modulation element 100, but it can also control the minimum voltage Vmin or the maximum voltage Vmax.
[0091] like Figure 9 As shown, the first optical waveguide 51 and the second optical waveguide 52 of the optical modulation element 101 in this embodiment can also be bent. Figure 9 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.
[0092] 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.
[0093] 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.
[0094] exist Figure 9 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.
[0095] 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.
[0096] 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.
[0097] Explanation of reference numerals in the attached figures
[0098] 10, 50 optical waveguides
[0099] 11, 51 First Optical Waveguide
[0100] 12, 52 Second optical waveguide
[0101] 13, 53 Input Path
[0102] Output paths 14 and 54
[0103] Branches 15 and 55
[0104] 16, 56 Joint
[0105] Electrodes 21, 22, 23, 24, 61, 62, 63, 64
[0106] 30 Buffer Layer
[0107] 40 Oxide film
[0108] 40a Side 1
[0109] 100, 101 optical modulation elements
[0110] 110 drive circuit
[0111] 120 DC bias application circuit
[0112] 130 DC bias control circuit
[0113] 200 optical modulator
[0114] L in Input light
[0115] L out Output light
[0116] L b Branching light
[0117] Vmin minimum voltage
[0118] Vmax (maximum voltage)
[0119] Vd Operating point voltage
[0120] Vn null point voltage
[0121] Vπ half-wavelength voltage
[0122] Vpp represents the applied voltage amplitude.
Claims
1. An optical modulator, characterized in that, include: An optical modulation element having 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. and A control unit that controls the applied voltage between the first electrode and the second electrode. The control unit, Let the half-wavelength voltage of the optical modulation element be Vπ, and the amplitude of the applied voltage, which is the amplitude of the applied voltage applied to the optical modulation element, be Vpp. This ensures that Vpp is 0.06×Vπ≤Vpp≤0.4×Vπ. Given that the minimum and maximum values of the voltage applied to the optical modulation element by the facility are Vmin and Vmax, respectively, and assuming that the null point voltage of the optical modulation element is Vn, Make Vn≤Vmin≤Vn+0.29×Vπ, or Vn - 0.29 × Vπ ≤ Vmax ≤ Vn, The applied voltage amplitude is the difference between the maximum and minimum values of the applied voltage applied between the first electrode and the second electrode, and the null point voltage is the voltage from the output of the optical modulation element that becomes extremely small.
2. The optical modulator as described in claim 1, characterized in that: The first optical waveguide and the second optical waveguide each include a ridge protruding from the first surface of the lithium niobate film.
3. A method for driving an optical modulation element, the optical modulation element having a first optical waveguide and a second optical waveguide, a first electrode located at a position overlapping the first optical waveguide in top view, and a second electrode located at a position overlapping the second optical waveguide in top view, the method for driving the optical modulation element being characterized in that: Let the half-wavelength voltage of the optical modulation element be Vπ, and the amplitude of the applied voltage, which is the amplitude of the applied voltage applied to the optical modulation element, be Vpp. This ensures that Vpp is 0.06×Vπ≤Vpp≤0.4×Vπ. Given that the minimum and maximum values of the voltage applied to the optical modulation element by the facility are Vmin and Vmax, respectively, and assuming that the null point voltage of the optical modulation element is Vn, Make Vn≤Vmin≤Vn+0.29×Vπ, or Vn - 0.29 × Vπ ≤ Vmax ≤ Vn, The applied voltage amplitude is the difference between the maximum and minimum values of the applied voltage applied between the first electrode and the second electrode, and the null point voltage is the voltage from the output of the optical modulation element that becomes extremely small.
4. The driving method for the optical modulation element as described in claim 3, characterized in that: The first optical waveguide and the second optical waveguide each include a ridge protruding from the first surface of the lithium niobate film.