Thin-film lithium niobate electro-optical modulator with L-shaped traveling wave electrode structure and design method of thin-film lithium niobate electro-optical modulator

Abstract

The invention relates to the technical field of electro-optical modulators, in particular to a thin film lithium niobate electro-optical modulator of an L-shaped traveling wave electrode structure and a design method, and the method comprises the steps: scanning the structural parameters of the electro-optical modulator through an HFSS, and obtaining the microwave speed, microwave loss and S parameter of an electrode; and performing finite element method calculation on the finally optimized electro-optical modulator by using the HFSS to obtain the group velocity, the microwave loss, the radio frequency S parameter and the electro-optical response of the electro-optical modulator. The electro-optical modulator designed by the invention can effectively reduce waveguide loss, has relatively small return loss and relatively large gain through the designed traveling wave electrode structure, and shows an electro-optical bandwidth which is far greater than 60Ghz under the modulation length of 1cm; therefore, the problems that an existing electro-optical modulator is large in absorption loss and low in modulation efficiency are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of electro-optic modulators, and in particular to a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure and a design method thereof. Background Art

[0002] With the rapid development of optical communication, the performance requirements of electro-optical modulators, which convert electrical signals into optical signals, are becoming increasingly higher. Compared with internal modulators, external modulators have the advantages of stable wavelength, multiple coding formats for signals, high rate, large extinction ratio, low chirp, low modulation signal degradation, etc. Mach-Zehnder (MZ) modulators, as a type of external modulator, are widely used in optical communication systems because of their advantages in high-rate and long-distance transmission of optical signals.

[0003] However, existing electro-optic modulators have large absorption losses and low modulation efficiency, and cannot meet market demand. Summary of the invention

[0004] The purpose of the present invention is to provide a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure and a design method, aiming to solve the problems of large absorption loss and low modulation efficiency of existing electro-optic modulators.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a design method for a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure, comprising the following steps:

[0006] Using COMSOL optical simulation software, we studied the effects of the spacing between metal electrodes and the thickness of the isolation layer between the metal electrodes and the upper and lower optical waveguides on the electric field distribution and light absorption loss in the optical waveguide region, and designed the thickness of the isolation layer and the spacing between metal electrodes.

[0007] The structural parameters of the electro-optic modulator were scanned using HFSS to obtain the electrode microwave velocity, microwave loss and S parameters.

[0008] The finite element method was used to calculate the final optimized electro-optic modulator using HFSS to obtain the group velocity, microwave loss, RF S parameters and electro-optic response of the electro-optic modulator.

[0009] Among them, in "Using COMSOL optical simulation software to study the electric field distribution and light absorption loss in the optical waveguide area, and design the spacing between the metal electrodes and the thickness of the SiO2 isolation layer between the metal electrodes and the upper and lower parts of the optical waveguide", it includes:

[0010] Create a two-dimensional model of the modulation area structure of the electro-optic modulator;

[0011] According to the properties of lithium niobate material, the relative dielectric constant is set to be anisotropic and corresponds to the modeled waveguide direction. According to the lithium niobate dispersion curve, the change trend of lithium niobate is established.

[0012] Add EME solution mode and set scattering boundary conditions;

[0013] The thickness and width of the lithium niobate thin film are fixed, and the spacing between the metal electrodes is scanned from 3μm to 8μm under the condition that the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide is 0 and 800nm ​​respectively;

[0014] Verify the effect of partially raising the waveguide metal electrode to present an L-shaped structure on light absorption loss and modulation efficiency, and determine the spacing between the metal electrodes.

[0015] Among them, in the "Creation of a two-dimensional model of the modulation zone structure of the electro-optic modulator", the thickness of the silicon substrate is 500μm, the thickness of the isolation layer silicon dioxide is 4.7μm, the thickness of the thin film lithium niobate is 600nm, the ridge waveguide etching depth is 300nm, the top width of the lithium niobate waveguide is 1.5μm, the side wall inclination angle is 70 degrees, the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide is 800nm, the thickness of the metal electrode is 1.2μm, and the thickness of the upper cover isolation layer silicon dioxide is 800nm.

[0016] Among them, "Using HFSS to scan the structural parameters of the electro-optic modulator to obtain the electrode microwave speed, microwave loss and S parameters" includes:

[0017] Create a 3D model of the modulation area structure of the electro-optic modulator in HFSS;

[0018] The relative dielectric constant is set to be anisotropic according to the properties of lithium niobate material and corresponds to the modeled waveguide direction;

[0019] Set the bottom surface of the air box as a perfect conductor boundary and the other surfaces as radiation boundaries;

[0020] Two wave ports are respectively provided at both ends of the traveling wave electrode;

[0021] Set up a frequency sweep in the solution setup;

[0022] The impedance of the two wave ports is normalized to 50Ω to analyze the model, and the S parameters, microwave velocity and microwave loss of the structure in the 50Ω system are output.

[0023] Among them, in "Set frequency scan in solution settings", the frequency is 1 GHz to 60 GHz, the frequency interval is 1 GHz, and the accuracy is 0.01.

[0024] In a second aspect, a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure adopts the design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure described in the first aspect.

[0025] The optical waveguide comprises a substrate, a buffer layer, an optical waveguide, a plurality of isolation layers, a plurality of metal electrodes and a cap layer, wherein the buffer layer is arranged on the top of the substrate, the optical waveguide is arranged on a side of the buffer layer away from the substrate, the plurality of metal electrodes are respectively arranged on a side of the optical waveguide away from the buffer layer, the plurality of isolation layers are arranged between the optical waveguide and the plurality of metal electrodes, and the cap layer is arranged on a side of the plurality of metal electrodes away from the optical waveguide.

[0026] Wherein, the optical waveguide is an X-shaped cut LN film and has a ridge structure.

[0027] The design method of the thin-film lithium niobate electro-optic modulator of the L-type traveling wave electrode structure of the present invention comprises the following steps: using COMSOL optical simulation software to study the influence of the spacing between metal electrodes and the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide on the electric field distribution and light absorption loss of the optical waveguide region, and designing the thickness of the isolation layer and the spacing between the metal electrodes; using HFSS to scan the structural parameters of the electro-optic modulator to obtain the electrode microwave velocity, microwave loss and S parameters; using HFSS to perform finite element method calculation on the final optimized electro-optic modulator to obtain the group velocity, microwave loss, radio frequency S parameters and electro-optic response of the electro-optic modulator. The electro-optic modulator designed by the present invention can effectively reduce the waveguide loss, and the traveling wave electrode structure designed has a small return loss and a large gain. The electro-optic modulator exhibits an electro-optic bandwidth far greater than 60Ghz at a modulation length of 1cm, thereby solving the problems of large absorption loss and low modulation efficiency of the existing electro-optic modulator. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] Figure 1 It is a cross-sectional view of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure.

[0030] Figure 2 This is a top view of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure.

[0031] Figure 3It is a graph showing the relationship between waveguide loss and modulation efficiency as the width of the metal electrode changes.

[0032] Figure 4 This is a graph showing the relationship between microwave refractive index and frequency at different thicknesses of the isolation layer between the metal electrode and the upper and lower optical waveguides.

[0033] Figure 5 This is a graph showing how microwave loss varies with frequency.

[0034] Figure 6 This is a graph showing how S parameters vary with frequency.

[0035] Figure 7 It is a graph showing the electro-optic response curve as it changes with frequency.

[0036] Figure 8 The invention is a design method of a thin film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure.

[0037] 101 - substrate, 102 - buffer layer, 103 - optical waveguide, 104 - isolation layer, 105 - metal electrode, 106 - cap layer. DETAILED DESCRIPTION

[0038] 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.

[0039] See also Figures 3 to 8 In a first aspect, the present invention provides a design method for a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure, comprising the following steps:

[0040] S1 uses COMSOL optical simulation software to study the influence of the spacing between the metal electrodes 105 and the thickness of the isolation layer 104 between the metal electrode 105 and the upper and lower portions of the optical waveguide 103 on the electric field distribution and light absorption loss in the optical waveguide 103 region, and designs the thickness of the isolation layer 104 and the spacing between the metal electrodes 105;

[0041] When the thickness of the SiO2 isolation layer 104 between the metal electrode 105 and the optical waveguide 103 is 800 nm, the waveguide loss is lower than that without the isolation layer 104, indicating that the structure can effectively reduce the waveguide loss. Figure 3As shown in the figure, the waveguide loss decreases with the increase of the electrode spacing, and the modulation efficiency increases with the increase of the electrode spacing. The larger the value of the modulation efficiency, the lower the modulation efficiency. Therefore, the modulation efficiency decreases with the increase of the electrode spacing. The lithium niobate electro-optic modulator can achieve a modulation efficiency of nearly 2.1V·cm and an electro-optic loss of 0.03dB / cm at 1mm when the electrode spacing is 5μm.

[0042] S11 creates a two-dimensional model of the modulation area structure of the electro-optic modulator;

[0043] The thickness of the silicon substrate 101 is 500μm, the thickness of the silicon dioxide of the isolation layer 104 is 4.7μm, the thickness of the thin film lithium niobate is 600nm, the ridge waveguide etching depth is 300nm, the top width of the lithium niobate waveguide is 1.5μm, the side wall inclination angle is 70 degrees, the thickness of the isolation layer 104 between the metal electrode 105 and the optical waveguide 103 is 800nm, the thickness of the metal electrode 105 is 1.2μm, and the thickness of the silicon dioxide of the upper cover isolation layer 104 is 800nm.

[0044] Specifically, a two-dimensional model of the modulation area structure of the electro-optic modulator is created, the thickness of the silicon substrate 101 is 500μm, the thickness of the silicon dioxide of the isolation layer 104 is 4.7μm, the thickness of the thin film lithium niobate is 600nm, the ridge waveguide etching depth is 300nm, the top width of the lithium niobate waveguide is 1.5μm, the side wall inclination angle is 70 degrees, the thickness of the isolation layer 104 between the metal electrode 105 and the optical waveguide 103 is 800nm, the thickness of the metal electrode 105 is 1.2μm, and the thickness of the silicon dioxide of the upper cover isolation layer 104 is 800nm.

[0045] S12 sets the refractive index to be anisotropic according to the properties of lithium niobate material and corresponds to the modeled waveguide direction, and establishes the variation trend of lithium niobate according to the lithium niobate dispersion curve;

[0046] Specifically, material setting: according to the properties of lithium niobate material, the relative dielectric constant is set to be anisotropic and corresponds to the waveguide direction of the model; according to the lithium niobate dispersion curve, the change trend of lithium niobate is established.

[0047] S13 adds EME solution mode and sets scattering boundary conditions;

[0048] Specifically, add solution settings: add EME solution mode, and set scattering boundary conditions.

[0049] S14 fixes the thickness and width of the lithium niobate thin film, and scans the spacing between the metal electrodes 105 from 3 μm to 8 μm under the condition that the thickness of the isolation layer 104 between the metal electrode 105 and the upper and lower portions of the optical waveguide 103 is 0 nm and 800 nm respectively;

[0050] Specifically, the scanning setting is: fixing the thickness and width of the lithium niobate thin film, scanning the spacing between the metal electrodes from 3 μm to 8 μm under the condition that the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide is 0 and 800 nm respectively.

[0051] S15 verifies the influence of the waveguide metal electrode 105 partially raised to present an L-shaped structure on the light absorption loss and modulation efficiency, and determines the spacing between the metal electrodes 105 .

[0052] Specifically, the influence of partially raising the metal electrode of the waveguide to present an L-shaped structure on the light absorption loss and modulation efficiency is verified, and the spacing between the metal electrodes is determined.

[0053] S2 uses HFSS to scan the structural parameters of the electro-optic modulator to obtain the electrode microwave speed, microwave loss and S parameters;

[0054] The simulation results show that as the thickness H of the SiO2 isolation layer 104 between the metal electrode 105 and the optical waveguide 103 increases, Au The microwave refractive index decreases significantly as the refractive index of the microwave increases. The refractive index of the light group of the structure is simulated to be 2.215, and the thickness of the SiO2 isolation layer 104 between the metal electrode 105 and the optical waveguide 103 is H. Au Set to 0.8μm, at this time the refractive index of microwaves has a high degree of match with the group refractive index of light, such as Figure 4 The simulation results of microwave loss are shown in Figure 5 It can be seen that within the 60GHz range, the microwave loss is below 5dB. Figure 6 The simulation results show that within the wide simulation frequency range of 0 to 60 GHz, S 21 Parameters above -0.6dB, S 11 The parameter is below -22dB, which shows that the designed traveling wave electrode structure has smaller return loss and larger gain.

[0055] S21 Create a 3D model of the modulation area structure of the electro-optic modulator in HFSS;

[0056] Specifically, a three-dimensional model of the modulation area structure of the electro-optic modulator is created in the software, the outermost layer is a rectangular air box, and the traveling wave electrode structure of the device is placed under the air box; the electrode structure of the modulator is an L-shaped structure, the thickness of the cap silicon oxide is set to 0.8μm, the thickness HAu of the SiO2 isolation layer 104 between the metal electrode 105 and the upper and lower optical waveguide 103 is set to 0.2μm, 0.6μm, 0.8μm and 1.1μm respectively, the metal thickness dAu is set to 1μm, the width WAu of the contact part of the electrode and the lithium niobate material is set to 2μm, the electrode spacing gap is designed to be 5μm, and the signal electrode width Wsig is set to 15μm.

[0057] S22 sets the relative dielectric constant to be anisotropic according to the properties of lithium niobate material and corresponds to the modeled waveguide direction;

[0058] Specifically, special attention should be paid to the setting of lithium niobate material. According to the properties of lithium niobate material, the relative dielectric constant is set to be anisotropic and corresponds to the waveguide direction of the modeling;

[0059] S23 sets the bottom surface of the air box as a perfect conductor boundary and the other surfaces as radiation boundaries;

[0060] Specifically, boundary conditions are set: the bottom surface of the air box is set as a perfect conductor boundary, and the other surfaces are set as radiation boundaries.

[0061] S24 sets two wave ports at both ends of the traveling wave electrode;

[0062] Specifically, an excitation port is set: two wave ports are set at both ends of the traveling wave electrode.

[0063] S25 sets the frequency sweep in the solution settings;

[0064] The frequency is from 1 GHz to 60 GHz, the frequency interval is 1 GHz, and the accuracy is 0.01.

[0065] Specifically, a frequency scan is set in the solution settings, scanning from 1 GHz to 60 GHz, with a frequency interval of 1 GHz, a center frequency of 30 GHz, and a simulation accuracy of 0.01.

[0066] S26 normalizes the impedance of the two wave ports to 50Ω to analyze the model, and outputs the S parameters, microwave speed and microwave loss of the structure under the 50Ω system.

[0067] Specifically, 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 finally the S parameters, microwave speed and microwave loss of the structure in the 50Ω system can be output.

[0068] S3 uses HFSS to perform finite element method calculations on the final optimized electro-optic modulator to obtain the group velocity, microwave loss, RF S parameters and electro-optic response of the electro-optic modulator.

[0069] Specifically, based on optical and electrical design considerations, the above optimization parameters are designed as follows: the electrode structure of the modulator is an L-shaped structure, the thickness of the cap silicon oxide is set to 0.8μm, the thickness HAu of the SiO2 isolation layer 104 between the metal electrode 105 and the upper and lower optical waveguide 103 is set to 0.8μm, the metal thickness dAu is set to 1μm, the width WAu of the contact part between the electrode and the lithium niobate material is set to 2μm, the electrode spacing gap is designed to be 5μm, and the signal electrode width Wsig is set to 15μm. Figure 2 As shown, the bending radius of the bending part is set to 70μm, the length of the gradually widening part is set to 50μm, the signal electrode width Ws of the input and output areas is set to 65μm, the electrode spacing Wsg is set to 24μm, and the length of the modulation area is set to 1cm.

[0070] In order to meet engineering requirements, the modulator needs to meet three conditions: 1. Impedance matching; 2. Microwave speed matching; 3. Minimize RF loss as much as possible. Once the above conditions are determined, the bandwidth can be calculated using the following formula.

[0071]

[0072] Where α is the transmission line loss, b is the deviation expression between light wave and microwave, L is the total length of the modulator, Z in The input impedance is generally 50Ω, V PD is the output voltage of the photodetector, V in is the RF signal input; It is a speed match item.

[0073] like Figure 7 As shown, the thin-film lithium niobate MZ electro-optic modulator with an L-type traveling wave electrode structure exhibits an electro-optic bandwidth far greater than 60 GHz at a modulation length of 1 cm, which has important guiding significance for the design of electro-optic modulators.

[0074] See also Figure 1-Figure 2 In the second aspect, a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure adopts the design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure described in the first aspect.

[0075] The optical waveguide 103 includes a substrate 101, a buffer layer 102, an optical waveguide 103, a plurality of isolation layers 104, a plurality of metal electrodes 105 and a cap layer 106, wherein the buffer layer 102 is arranged on the top of the substrate 101, the optical waveguide 103 is arranged on a side of the buffer layer 102 away from the substrate 101, the plurality of metal electrodes 105 are respectively arranged on a side of the optical waveguide 103 away from the buffer layer 102, the plurality of isolation layers 104 are all arranged between the optical waveguide 103 and the plurality of metal electrodes 105, and the cap layer 106 is arranged on a side of the plurality of metal electrodes 105 away from the optical waveguide 103.

[0076] The optical waveguide 103 is an X-shaped cut LN film and has a ridge structure.

[0077] In this embodiment, the buffer layer 102 is located on the top of the substrate 101, the optical waveguide 103 is located on the top of the buffer layer 102, the isolation layer 104 isolates the optical waveguide 103 from the metal electrode 105, the metal electrode 105 is located around the isolation layer 104, and the cap layer 106 completely covers the metal electrode 105 and the optical waveguide 103. The optical waveguide 103 is an X-cut LN film and has a ridge structure. Figure 2 As shown, this design adds input and output matching structures based on the microwave electrode.

[0078] The above disclosure is only a preferred embodiment of the thin-film lithium niobate electro-optic modulator and design method of the L-type traveling wave electrode structure of the present invention. Of course, this 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 the above embodiments and equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A design method for a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure, characterized in that: The following steps are involved: Using COMSOL optical simulation software, we studied the effects of the spacing between metal electrodes and the thickness of the isolation layer between the metal electrodes and the upper and lower optical waveguides on the electric field distribution and light absorption loss in the optical waveguide region, and designed the thickness of the isolation layer and the spacing between metal electrodes. The structural parameters of the electro-optic modulator were scanned using HFSS to obtain the electrode microwave velocity, microwave loss and S parameters. The finite element method was used to calculate the final optimized electro-optic modulator using HFSS to obtain the group velocity, microwave loss, RF S parameters and electro-optic response of the electro-optic modulator.

2. The design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure as claimed in claim 1, characterized in that: In "Using COMSOL optical simulation software to study the electric field distribution and light absorption loss in the optical waveguide area, and to design the spacing between the metal electrodes and the thickness of the SiO2 isolation layer between the metal electrodes and the upper and lower parts of the optical waveguide", the following are included: Create a two-dimensional model of the modulation area structure of the electro-optic modulator; According to the properties of lithium niobate material, the relative dielectric constant is set to be anisotropic and corresponds to the modeled waveguide direction. According to the lithium niobate dispersion curve, the change trend of lithium niobate is established. Add EME solution mode and set scattering boundary conditions; The thickness and width of the lithium niobate thin film are fixed, and the spacing between the metal electrodes is scanned from 3μm to 8μm under the condition that the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide is 0 and 800nm ​​respectively; Verify the effect of partially raising the waveguide metal electrode to present an L-shaped structure on light absorption loss and modulation efficiency, and determine the spacing between the metal electrodes.

3. The design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure as claimed in claim 2, characterized in that: In the "Creation of a two-dimensional model of the modulation zone structure of the electro-optic modulator", the thickness of the silicon substrate is 500μm, the thickness of the isolation layer silicon dioxide is 4.7μm, the thickness of the thin film lithium niobate is 600nm, the ridge waveguide etching depth is 300nm, the top width of the lithium niobate waveguide is 1.5μm, the side wall inclination angle is 70 degrees, the thickness of the isolation layer between the metal electrode and the upper and lower optical waveguide is 800nm, the thickness of the metal electrode is 1.2μm, and the thickness of the upper cover isolation layer silicon dioxide is 800nm.

4. The design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure as claimed in claim 3, characterized in that: In "Using HFSS to scan the structural parameters of the electro-optic modulator to obtain the electrode microwave speed, microwave loss and S parameters", it includes: Create a 3D model of the modulation area structure of the electro-optic modulator in HFSS; The relative dielectric constant is set to be anisotropic according to the properties of lithium niobate material and corresponds to the modeled waveguide direction; Set the bottom surface of the air box as a perfect conductor boundary and the other surfaces as radiation boundaries; Two wave ports are respectively provided at both ends of the traveling wave electrode; Set up a frequency sweep in the solution setup; The impedance of the two wave ports is normalized to 50Ω to analyze the model, and the S parameters, microwave velocity and microwave loss of the structure in the 50Ω system are output.

5. The design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure as claimed in claim 4, characterized in that: In "Set frequency sweep in solution setup", the frequency is 1 GHz to 60 GHz, the frequency interval is 1 GHz, and the accuracy is 0.

01.

6. A thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure, adopting the design method of a thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure according to any one of claims 1 to 5, characterized in that: The optical waveguide comprises a substrate, a buffer layer, an optical waveguide, a plurality of isolation layers, a plurality of metal electrodes and a cap layer, wherein the buffer layer is arranged on the top of the substrate, the optical waveguide is arranged on a side of the buffer layer away from the substrate, the plurality of metal electrodes are respectively arranged on a side of the optical waveguide away from the buffer layer, the plurality of isolation layers are arranged between the optical waveguide and the plurality of metal electrodes, and the cap layer is arranged on a side of the plurality of metal electrodes away from the optical waveguide.

7. The thin-film lithium niobate electro-optic modulator with an L-type traveling wave electrode structure as claimed in claim 6, characterized in that: The optical waveguide is an X-shaped cut LN film and has a ridge structure.

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