Sunken double-layer electrode lithium niobate electro-optical modulator

Abstract

The invention relates to the technical field of electro-optical modulators, in particular to a sinking type double-layer electrode lithium niobate electro-optical modulator which comprises a substrate, an oxide layer, an optical waveguide, a transition layer and a metal electrode. The substrate is located at the bottommost part, the oxide layer is located at the top of the substrate, the optical waveguide, the transition layer and the metal electrode are all arranged above the oxide layer, the optical waveguide is an X-shaped cut LN film and adopts a traditional ridge-shaped structure, and the metal electrode is of a double-layer electrode structure and comprises a first layer of traveling wave electrode and a second layer of T-shaped structure electrode. Loss can be effectively reduced, and modulation efficiency is improved. In order to reduce the light absorption loss, the metal electrode and the optical waveguide are separated by using a 0.2 mu m silicon dioxide transition layer, the metal electrode is arranged along the Z axis of the optical waveguide, and the sunken double-layer electrode lithium niobate electro-optical modulator realizes the half-wave voltage length product of 1.52 V.cm under the light absorption loss of 0.14 dB / cm, and has important guiding significance for the design of the electro-optical modulator.

Description

Technical Field

[0001] The invention relates to the technical field of electro-optic modulators, and in particular to a sinking double-layer electrode lithium niobate electro-optic modulator. Background Art

[0002] Electro-optic modulators are an important part of optical fiber communication systems. With the rapid development of artificial intelligence, analog communications, and satellite data links, higher requirements are placed on the performance of electro-optic modulators, such as higher electro-optic bandwidth, higher modulation efficiency, and lower power consumption.

[0003] As the core device in optical communication systems, electro-optic modulators are used to load electrical signals onto light for transmission. In order to achieve high-performance electro-optic modulators, people have tried materials such as lithium niobate, silicon and gallium arsenide. Among them, lithium niobate is called "optical silicon". Due to its wide transparent window and high electro-optic coefficient, it is an ideal carrier for building electro-optic modulators.

[0004] In recent years, the development of thin-film lithium niobate has brought new opportunities for the development of electro-optic modulators. Among them, thin-film lithium niobate Mach-Zehnder modulators have a simple structure and the advantages of high bandwidth and high modulation efficiency. They have been widely studied and used in various fields. However, existing thin-film lithium niobate modulators face the problem of trade-offs between modulation efficiency, bandwidth and loss. Summary of the invention

[0005] The purpose of the present invention is to provide a sinking double-layer electrode lithium niobate electro-optic modulator, which increases the modulation efficiency and reduces the loss by the simultaneous action of the double-layer electrodes, and further improves the modulation efficiency and increases the freedom of electrode design by sinking the electrode.

[0006] To achieve the above object, the present invention provides a sinking double-layer electrode lithium niobate electro-optic modulator, comprising a substrate, an oxide layer, an optical waveguide, a transition layer and a metal electrode; the substrate is located at the bottom, the oxide layer is located at the top of the substrate, the optical waveguide, the transition layer and the metal electrode are all arranged above the oxide layer, and the transition layer is located between the optical waveguide and the metal electrode, connecting the metal electrode and the optical waveguide;

[0007] The optical waveguide is arranged in a ridge waveguide structure;

[0008] The metal electrode has a double-layer structure, including a layer of traveling wave electrode and a second layer of T-shaped electrode, and the T-shaped electrode is periodically arranged along the direction of the optical waveguide.

[0009] Wherein, the material of the substrate layer is silicon; the material of the oxide layer and the transition layer is silicon dioxide.

[0010] Wherein, the material of the optical waveguide is an X-shaped cut lithium niobate film.

[0011] Among them, a Mach-Zehnder structure is adopted, and the metal electrodes include ground electrodes on both sides and a signal electrode in the middle;

[0012] The width of the left and right ground electrodes was set to 140 μm, and the width of the signal electrodes was set to 50 μm.

[0013] Wherein, the thickness of the transition layer is 0.2um, and the thickness of the oxide layer is 5μm.

[0014] Among them, the effect of the electric field on the modulation area is enhanced by sinking the electrode, and the degree of freedom of the electrode is improved.

[0015] The present invention provides a sinking double-layer electrode lithium niobate electro-optic modulator, comprising a substrate, an oxide layer, an optical waveguide, a transition layer and a metal electrode; the substrate is located at the bottom, the oxide layer is located at the top of the substrate, the optical waveguide, the transition layer and the metal electrode are all arranged above the oxide layer, the optical waveguide is an X-shaped cut LN film, adopts a traditional ridge structure, and the metal electrode is a double-layer electrode structure, including a layer of traveling wave electrode and a second layer of T-shaped structure electrode, which can effectively reduce the loss and improve the modulation efficiency, and enhance the effect of the electric field on the modulation area by completely sinking the electrode, thereby improving the degree of freedom of electrode design. In order to reduce the light absorption loss, a 0.2um silicon dioxide transition layer is used to separate the metal electrode and the optical waveguide, and the metal electrode is arranged along the Z axis of the optical waveguide. The sinking double-layer electrode lithium niobate electro-optic modulator achieves a half-wave voltage-length product of 1.52V·cm at a light absorption loss of 0.14dB / cm, which has important guiding significance for the design of electro-optic modulators. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 It is a top view of a sunken double-layer electrode lithium niobate electro-optic modulator of the present invention.

[0018] Figure 2 It is a schematic diagram of the cross-sectional structure of a sinking double-layer electrode lithium niobate electro-optic modulator of the present invention.

[0019] Figure 3 It is a comparison diagram of the light absorption loss of a structure of a sunken double-layer electrode lithium niobate electro-optic modulator of the present invention and a non-sunk structure.

[0020] Figure 4 It is a comparison diagram of the half-wave voltage-length product of a structure of a sunken double-layer electrode lithium niobate electro-optic modulator of the present invention and a non-sunk structure.

[0021] Figure 5 It is a schematic diagram of the light spot of the ridge waveguide during single-mode transmission in a specific embodiment of the present invention.

[0022] Figure 6 The diagram is a relationship diagram between the upper electrode spacing and the half-wave voltage-length product of a sunken double-layer electrode lithium niobate electro-optic modulator of the present invention.

[0023] Figure 7 The diagram is a relationship diagram between the upper electrode spacing and light absorption loss of a sunken double-layer electrode lithium niobate electro-optic modulator of the present invention.

[0024] Figure 8 The invention discloses a sinking type double-layer electrode lithium niobate electro-optic modulator and an S11 reflection coefficient and an electro-optic coefficient.

[0025] 1-substrate, 2-oxide layer, 3-optical waveguide, 4-transition layer, 5-metal electrode. DETAILED DESCRIPTION

[0026] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0027] See also Figure 1 and Figure 2 The present invention provides a sunken double-layer electrode lithium niobate electro-optical modulator, comprising a substrate 1, an oxide layer 2, an optical waveguide 3, a transition layer 4 and a metal electrode 5, wherein the substrate 1 is located at the bottom layer, the oxide layer 2 is located at the top of the substrate 1, and the metal electrode 5 and the optical waveguide 3 are located at the top of the oxide layer 2 and are separated by the transition layer 4.

[0028] Specifically, the optical waveguide 3 is an X-shaped cut LN film, and adopts a ridge waveguide structure;

[0029] The metal electrode 5 is a double-layer electrode structure, the first layer is a coplanar waveguide (traveling wave) electrode, and the second layer is a T-type electrode. The two layers of electrodes play a modulatory role at the same time. The upper and lower layers of electrodes work together to effectively reduce losses and improve modulation efficiency. The use of T-type electrodes can control the uniform distribution of current, increase the effective conductor area, and reduce the ohmic loss of the conductor. In order to improve the modulation efficiency and reduce the half-wave voltage, the usual method is to reduce the electrode spacing, but reducing the electrode spacing will increase the light absorption loss. According to electromagnetic theory, sinking the electrode so that the waveguide is as close as possible to the middle of the two electrodes can make the waveguide be at a position with strong electric field, thereby improving the modulation efficiency.

[0030] In order to illustrate the benefits of sinking electrodes in improving modulation efficiency, in this embodiment, the electrodes are sunk so that the waveguide is as close as possible to the middle of the two electrodes. This can place the waveguide at a position with higher electric field strength, thereby improving the modulation efficiency. Furthermore, sinking the electrodes can retain a higher degree of freedom for electrode design. Figure 3 , 4 The modulation efficiency and light absorption loss of the modulator when the electrode is not sunken and when the electrode is completely sunken are compared. Figure 3 , 4 As shown in the figure, the half-wave voltage-length product of the modulator is more advantageous when fully sunk than when not sunk, and the light absorption loss is also kept at a lower value. This has important guiding significance for the design of electro-optic modulators.

[0031] For further information, see Figures 5 to 8 , the present invention is further described by optimizing the design:

[0032] Using COMSOL optical simulation software, the half-wave voltage-length product and light absorption loss of the electro-optic modulator at different electrode spacings are studied.

[0033] The environment is set up as follows:

[0034] Modeling process: In the software, a two-dimensional model of the cross-sectional structure of the modulation area of ​​the electro-optic modulator is created according to the design, including the optical waveguide, metal electrode, transition layer and oxide layer. The lithium niobate waveguide is a ridge structure with an upper surface width of 1.5μm, a side wall inclination of 66°, and an etching depth of 0.3um; the metal electrode is a double-layer electrode structure with a first layer thickness of 0.5μm and a second layer thickness of 0.8μm, a SiO2 transition layer thickness of 0.2um, and a SiO2 oxide layer thickness of 5μm.

[0035] Material settings: According to the properties of the x-cut lithium niobate material, the relative dielectric constant and refractive index are set anisotropically in the waveguide direction of the model. The diagonal elements of the refractive index matrix are set to (2.1376, 2.2111, 2.2111), and the diagonal elements of the relative dielectric constant matrix are set to (27.9, 44.3, 44.3).

[0036] Study setup: Add an electrostatic field solution project, set the modeled electrode parts as signal electrodes and ground electrodes, and apply corresponding voltages.

[0037] Add solution settings: solve the effective mode refractive index of the waveguide and the corresponding electric field distribution in the optical waveguide area.

[0038] Parameter scanning setting: while fixing other parameters, scan the upper electrode spacing W2 from 2 μm to 4 μm to obtain the calculation results under different electrode spacings.

[0039] Results: Figure 5 As shown in the figure, it is verified that the fundamental mode of the waveguide is the TE mode, the feasibility of the structure is determined, and the electrode spacing between the signal electrode and the ground electrode is optimized by parameter scanning.

[0040] Calculation formula: Refer to the calculation formula of half-wave voltage-length product to calculate the half-wave voltage-length product under different electrode spacings. The calculation results of light absorption loss and half-wave voltage-length product are as follows: Figure 6 , Figure 7 shown.

[0041]

[0042] In formula (1), ne is the optical refractive index of lithium niobate crystal in the z direction, r33 is the electro-optic coefficient of lithium niobate crystal in the z direction, ez(x, z) is the electric field intensity of TE mode, and ez(x, z) is the electric field along the z axis.

[0043] Using HFSS software, the electro-optic coefficient and microwave loss of the electro-optic modulator at different frequencies are studied to determine the maximum operating bandwidth of the electro-optic modulator.

[0044] Here are the steps:

[0045] Modeling: Create a three-dimensional model of the electro-optic modulator in the software according to the design, including a substrate, an oxide layer, an optical waveguide, a metal electrode and a transition layer; the oxide layer is located on the top of the substrate, the optical waveguide is located on the top of the buffer layer, the transition layer is located between the optical waveguide and the metal electrode, the metal electrode is located on the top of the oxide layer, and the metal electrode has a double-layer structure, the double-layer structure includes a layer of traveling wave electrode and a layer of T-shaped structure electrode, the T-shaped structure is periodically arranged along the direction of the optical waveguide, and the transition layer connects the metal electrode and the optical waveguide. Finally, add a rectangular air box to the outermost layer and place the electro-optic modulator at the bottom of the air box.

[0046] Set up materials: According to the properties of the x-cut lithium niobate material, the relative dielectric constant and refractive index are set anisotropically in the waveguide direction of the model. The diagonal elements of the refractive index matrix are set to (2.1376, 2.2111, 2.2111), and the diagonal elements of the relative dielectric constant matrix are set to (27.9, 44.3, 44.3) respectively.

[0047] Set boundary conditions: Set the bottom surface of the air box as a perfect conductor boundary and the other surfaces as radiation boundaries.

[0048] Set the excitation port: set two wave ports at the input and output ends of the traveling wave electrode respectively, and place the optical waveguide in the middle of the wave ports;

[0049] Solution settings: Set the frequency sweep in the solution settings to sweep from 1 GHz to 60 GHz with a frequency interval of 1 GHz.

[0050] Results: The port impedance is normalized to 50Ω. At this time, the port can be regarded as a standard 50Ω external system. In this case, the model is analyzed and the S parameters, Z0, and microwave loss of the structure in the 50Ω system can be output.

[0051] Formula calculation: Refer to the calculation formula of the modulator electro-optical response to calculate the working bandwidth of the modulator. The calculation results of the S11 reflection coefficient and electro-optical coefficient of the modulator are as follows: Figure 8 shown.

[0052]

[0053] In this embodiment, the transition layer of SiO2 with a thickness of 0.2 um is used to separate the metal electrode and the optical waveguide to reduce the loss. The metal electrode and the optical waveguide are arranged along the Y axis.

[0054] The modulation efficiency, loss and electro-optic bandwidth of this structure are simulated by software. Compared with the traditional electro-optic modulator, a smaller half-wave voltage-length product can be obtained under the same light absorption loss. This electro-optic modulator can achieve a half-wave voltage-length product of 1.52V·cm at an optical loss of 0.4dB / cm, and has a high electro-optic bandwidth of more than 60GHz, with excellent performance.

[0055] What is disclosed above is only one or more preferred embodiments of the present invention, which certainly cannot be used to limit the scope of rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of implementing the above embodiments and making equivalent changes according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A double-layer electrode lithium niobate electro-optic modulator with a sunken structure, characterized in that: The optical waveguide comprises a substrate, an oxide layer, an optical waveguide, a transition layer and a metal electrode; the substrate is located at the bottom, the oxide layer is located at the top of the substrate, the optical waveguide, the transition layer and the metal electrode are all arranged above the oxide layer, and the transition layer is located between the optical waveguide and the metal electrode to connect the metal electrode and the optical waveguide; The optical waveguide is arranged in a ridge waveguide structure; The metal electrode has a double-layer structure, including a layer of traveling wave electrode and a second layer of T-shaped electrode, and the T-shaped electrode is periodically arranged along the direction of the optical waveguide.

2. The sinking double-layer electrode lithium niobate electro-optic modulator according to claim 1, characterized in that: The material of the substrate layer is silicon; the material of the oxide layer and the transition layer is silicon dioxide.

3. The sinking double-layer electrode lithium niobate electro-optic modulator according to claim 1, characterized in that: The material of the optical waveguide is an X-shaped cut lithium niobate film.

4. The sinking double-layer electrode lithium niobate electro-optic modulator according to claim 1, characterized in that: Using Mach-Zehnder structure, the metal electrodes include ground electrodes on both sides and a signal electrode in the middle; The width of the left and right ground electrodes was set to 140 μm, and the width of the signal electrodes was set to 50 μm.

5. The sinking double-layer electrode lithium niobate electro-optic modulator according to claim 1, characterized in that: The thickness of the transition layer is 0.2 μm, and the thickness of the oxide layer is 5 μm.

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