Thin film lithium niobate modulator with stepped three-layer electrode structure and preparation method

Abstract

The invention discloses a thin film lithium niobate modulator with a stepped three-layer electrode structure and a preparation method. The thin film lithium niobate modulator with the stepped three-layer electrode structure sequentially comprises a substrate layer, a buffer layer, a lithium niobate thin film layer, two lithium niobate ridge waveguides, a metal traveling wave electrode and a dielectric layer from bottom to top, the metal traveling wave electrodes are distributed on the two sides of the two lithium niobate ridge waveguides, adopt a ground-signal-ground push-pull mode and comprise three layers of electrodes, and the lower-layer ground-signal-ground electrode is tightly attached to a substrate area of the lithium niobate ridge waveguides and used for providing uniform initial electric field distribution; the middle-layer ground-signal-ground electrode is wider than the lower-layer ground-signal-ground electrode and is used for adjusting electric field distribution and increasing the conductor area so as to reduce microwave loss; and the upper-layer ground-signal-ground electrode is wider than the middle-layer ground-signal-ground electrode and is used for reducing impedance and enhancing the uniformity of electric field distribution. According to the invention, the modulation bandwidth can be significantly improved while the low half-wave voltage is maintained.

Description

Technical Field

[0001] The invention belongs to the field of electro-optic modulators, and in particular relates to a thin-film lithium niobate modulator with a stepped three-layer electrode structure and a preparation method thereof. Background Art

[0002] Lithium niobate is an excellent electro-optical material with a wide optical transparency window, high electro-optic coefficient, good thermal stability, and relatively low light absorption loss. When an external electric field acts on lithium niobate, its material refractive index changes, thereby achieving modulation of optical signals. At present, lithium niobate electro-optic modulators have been widely used in the field of optical communications. With the advancement of manufacturing technology, the emergence of lithium niobate on insulator technology has greatly improved the performance of modulators and promoted their application in integrated nanophotonic devices.

[0003] The traveling wave electrodes of existing thin-film lithium niobate electro-optic modulators generally adopt a coplanar waveguide structure, and are designed with thick-plated electrodes and thick buffer layers to reduce the mismatch between light waves and microwaves. Although the performance indicators of thin-film lithium niobate modulators have been significantly improved compared with the past by optimizing the electrical and optical structures, due to the combined influence of factors such as electrode structure, electro-optical effect, material properties and thermal effects, achieving high bandwidth usually requires a higher driving voltage to ensure effective modulation. When the modulation length is fixed, reducing the electrode spacing of the modulator helps to reduce the half-wave voltage and facilitate integration, but it also increases microwave losses and causes wave velocity mismatch, thereby limiting the bandwidth.

[0004] Existing electrode optimization schemes have fewer design parameters and must make a trade-off between half-wave voltage and modulation bandwidth, which results in a higher driving voltage at high frequencies, increasing energy consumption and limiting its widespread use in high-frequency and low-power applications. In addition, the heat effect caused by high power consumption not only reduces modulator performance and reliability, but also increases maintenance costs. Summary of the invention

[0005] The purpose of the present invention is to provide a thin-film lithium niobate modulator with a stepped three-layer electrode structure and a preparation method to address the trade-off limitations of voltage and bandwidth of traditional thin-film lithium niobate modulators, which can significantly improve the modulation bandwidth while maintaining a low half-wave voltage.

[0006] In order to achieve the above-mentioned object, one aspect of the present invention provides a thin-film lithium niobate modulator with a stepped three-layer electrode structure, including a substrate layer, a buffer layer, a lithium niobate thin film layer, two lithium niobate ridge waveguides, a metal traveling wave electrode and a dielectric layer;

[0007] The buffer layer is located on the substrate layer;

[0008] The lithium niobate thin film layer is located on the buffer layer;

[0009] The two lithium niobate ridge waveguides are located on the lithium niobate film layer, and the lithium niobate film is etched to form a Mach-Zehnder structure for transmitting optical signals;

[0010] The metal traveling wave electrodes are distributed on both sides of the two lithium niobate ridge waveguides, and adopt a ground-signal-ground push-pull form, including a lower ground-signal-ground electrode, a middle ground-signal-ground electrode and an upper ground-signal-ground electrode;

[0011] The dielectric layer is filled between the multilayer electrodes of the metal traveling wave electrode and between the multilayer electrodes and the two lithium niobate ridge waveguides;

[0012] The lower ground-signal-ground electrode is closely attached to the base region of the lithium niobate ridge waveguide and extends parallel to the lithium niobate ridge waveguide to provide a uniform initial electric field distribution and form an effective electric field coupling with the base region;

[0013] The middle ground-signal-ground electrode is located above the lower ground-signal-ground electrode and has a width greater than that of the lower ground-signal-ground electrode, and is used to adjust the electric field distribution and increase the conductor area to reduce microwave loss;

[0014] The upper ground-signal-ground electrode is located above the middle ground-signal-ground electrode and has a width greater than that of the middle ground-signal-ground electrode, and is used to reduce impedance and enhance uniformity of electric field distribution;

[0015] The central axis of the lower ground-signal-ground electrode is aligned with the central axes of the middle and upper ground-signal-ground electrodes to ensure symmetry and uniformity of the electric field distribution.

[0016] Preferably, the material of the substrate layer is silicon, the material of the buffer layer and the dielectric layer is silicon dioxide, and the material of the metal traveling wave electrode is gold or copper.

[0017] Another aspect of the present invention provides a method for preparing the above-mentioned thin-film lithium niobate modulator with a stepped three-layer electrode structure, comprising:

[0018] Selecting a silicon wafer as a substrate, and performing surface treatment on the silicon substrate by a chemical method or a plasma cleaning method to form a substrate layer;

[0019] Depositing a silicon dioxide buffer layer on the surface of the silicon substrate by chemical vapor deposition or high temperature oxidation;

[0020] The lithium niobate film is transferred onto the silicon dioxide buffer layer by using epitaxial growth technology to form a lithium niobate film layer;

[0021] The pattern of the Mach-Zehnder waveguide structure is defined on the lithium niobate film by using photolithography technology, and the lithium niobate film is etched by using reactive ion etching technology to form two lithium niobate ridge waveguides;

[0022] A metal film is deposited on both sides of the lithium niobate ridge waveguide using magnetron sputtering or electron beam evaporation technology, and the shape of the lower ground-signal-ground electrode is defined by photolithography and etching processes. A dielectric layer is deposited on both sides of the two lithium niobate ridge waveguides and in the gap between the lower ground-signal-ground electrodes using chemical vapor deposition or sputtering deposition technology; then the middle and upper ground-signal-ground electrodes are prepared and the dielectric layer is deposited in the gap between the electrodes using the same method.

[0023] The thin-film lithium niobate modulator with a stepped three-layer electrode structure and the preparation method according to the above aspects of the present invention can significantly improve the modulation bandwidth while maintaining a low half-wave voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings used in the description of the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work:

[0025] Figure 1 A schematic cross-sectional view of a thin-film lithium niobate modulator with a stepped three-layer electrode structure according to an embodiment of the present invention;

[0026] Figure 2 It is a comparison diagram of microwave loss variation with frequency in the stepped three-layer electrode structure of an embodiment of the present invention and the traditional structure. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] One embodiment of the present invention provides a thin film lithium niobate modulator with a stepped three-layer electrode structure, such as Figure 1 As shown, the thin-film lithium niobate modulator with a stepped three-layer electrode structure according to the embodiment of the present invention includes, from bottom to top:

[0029] The substrate layer B1 is made of silicon and plays a supporting and heat dissipating role;

[0030] The buffer layer B2 is located on the substrate layer B1 and is made of silicon dioxide. Its function is to suppress the scattering between the light wave and the substrate layer B1 and enhance the stability of the optical waveguide mode.

[0031] The lithium niobate thin film layer B3 is located on the buffer layer B2 and serves as the core material of the optical waveguide;

[0032] Two lithium niobate ridge waveguides B4 and B5, formed by etching lithium niobate thin film to form a Mach-Zehnder structure, are used to transmit optical signals;

[0033] Metal traveling wave electrodes are distributed on both sides of the two lithium niobate ridge waveguides B4 and B5, using ground-signal-ground push-pull form, and electric field optimization is achieved through the design of multi-layer electrodes B7-B15;

[0034] The dielectric layer B6 fills the gaps between the multilayer electrodes B7-B15 of the metal traveling wave electrode and between the multilayer electrodes B7-B15 and the two lithium niobate ridge waveguides B4 and B5. The material is a low-loss dielectric material such as silicon dioxide, which not only plays an insulating role, but also can achieve impedance matching and reduce microwave loss by optimizing the thickness.

[0035] like Figure 1 As shown, the shapes, positions, materials and mutual relationships of the electrodes of each layer of the metal traveling wave electrode of the embodiment of the present invention are as follows:

[0036] The lower ground-signal-ground electrodes B8, B7, and B9 are located on both sides closest to the lithium niobate ridge waveguides B4 and B5, and are rectangular strip electrodes extending parallel to the lithium niobate ridge waveguides B4 and B5. This layer of electrodes is close to the base region (lithium niobate film layer B3) of the lithium niobate ridge waveguides B4 and B5, and its main function is to provide a uniform initial electric field distribution and form an effective electric field coupling with the base region.

[0037] The middle ground-signal-ground electrodes B11, B10, and B12 are located above the lower ground-signal-ground electrodes B8, B7, and B9, and their width is slightly increased compared to the lower ground-signal-ground electrodes B8, B7, and B9. The main function of this layer of electrodes is to further adjust the electric field distribution and increase the conductor area to reduce microwave loss.

[0038] The upper ground-signal-ground electrodes B14, B13, and B15 are located at the top and have the largest width, and are used to cover a wider area to reduce impedance and enhance the uniformity of electric field strength distribution.

[0039] All three layers of electrodes are made of highly conductive metal (such as gold or copper) to ensure low resistivity and good conductivity.

[0040] According to the thin-film lithium niobate modulator of the embodiment of the present invention, the three layers of electrodes are arranged in a stepped manner, the width of each layer gradually increases, and the spacing gradually decreases, and the overall structure presents a trapezoidal shape that gradually expands from bottom to top. The central axis of the lower ground-signal-ground electrode is strictly aligned with the central axis of the middle and upper ground-signal-ground electrodes to ensure the symmetry and uniformity of the electric field distribution. The metal traveling wave electrodes on both sides of the lithium niobate ridge waveguides B4 and B5 directly act on the core area of ​​the optical waveguide through the electric field, thereby maximizing the electro-optical effect. The close coupling of the lower ground-signal-ground electrode and the optical waveguide helps to improve the overlap integral of the light wave and the electric field, and the middle and upper ground-signal-ground electrodes reduce the impact of the speed mismatch on the signal quality through progressive electric field adjustment. By adjusting the width and spacing of each layer of electrodes, a parameter combination with multiple degrees of freedom can be formed, so that the overall electrode impedance can be close to the standard 50Ω, thereby maximizing the signal transmission efficiency.

[0041] An embodiment of the present invention further provides a method for preparing a thin-film lithium niobate modulator with a stepped three-layer electrode structure, comprising the following steps:

[0042] Substrate layer preparation: Select a suitable silicon wafer as the substrate, and use chemical methods or plasma cleaning methods to treat the surface of the silicon substrate to ensure that the substrate surface is clean.

[0043] Buffer layer deposition: A silicon dioxide buffer layer is deposited on the surface of the silicon substrate by chemical vapor deposition (CVD) or high-temperature oxidation, and appropriate annealing treatment is used to improve the quality of the silicon dioxide buffer layer.

[0044] Growth of lithium niobate film: The lithium niobate film is transferred to the silicon dioxide buffer layer using epitaxial growth technology. Appropriate annealing treatment is performed to optimize the crystal quality of the lithium niobate film and ensure its optical performance.

[0045] Optical waveguide etching: Use photolithography technology to define the pattern of the Mach-Zehnder waveguide structure on the lithium niobate film, and use reactive ion etching (RIE) technology to etch the lithium niobate film to form the required waveguide structure. Polish the etched waveguide surface to ensure that the surface is flat and smooth.

[0046] Electrode preparation and dielectric layer deposition: First, use magnetron sputtering or electron beam evaporation technology to deposit metal film on both sides of the lithium niobate ridge waveguide, and define the shape of the lower electrode through photolithography and etching processes (wet or dry etching). Use chemical vapor deposition (CVD) or sputtering deposition technology to deposit the dielectric layer on both sides of the waveguide area and in the gap between the lower ground-signal-ground electrodes; then use the same method to complete the preparation of the middle ground-signal-ground electrode and the deposition of the dielectric layer between the electrode gaps; finally, complete the preparation of the upper ground-signal-ground electrode, and deposit the dielectric layer according to design requirements.

[0047] The thin-film lithium niobate modulator with a stepped three-layer electrode structure of an embodiment of the present invention is designed and verified: First, the initial model is established and the material properties are set using Ansys HFSS. The electrode shape of the initial model is a rectangular electrode structure of a coplanar waveguide of a traditional traveling wave electrode. Then, at a higher frequency (above 60 GHz), the width, height, spacing, and thickness of the covering layer of each electrode layer are parameter scanned to obtain the impedance, microwave loss, and microwave effective refractive index curves with the change of related parameters. According to the curve results, structural parameters with an impedance close to 50Ω, a small microwave loss, and a microwave effective refractive index of about 2.2 are selected to achieve the optimization goals of impedance matching, low loss, and refractive index matching. According to an embodiment of the present invention, the traveling wave electrode adopts a three-layer structure design from bottom to top, with the electrode width increasing layer by layer and the electrode spacing decreasing layer by layer.

[0048] The stepped three-layer electrode structure of the embodiment of the present invention is compared with the traditional rectangular electrode structure (the electrode spacing and width are the same as the parameters of the upper electrode, and the electrode height is the total height of the three-layer electrode). Figure 2 As shown, it was found that the microwave loss decreased by about 2dB / cm, and the modulation bandwidth increased significantly by 80GHz. Due to the relatively large spacing between the lower electrodes, the overlap integral was reduced, resulting in a slight increase in the half-wave voltage. Modeling and calculating the half-wave voltage-length product in COMSOL showed that the half-wave voltage-length product of the modulator of the embodiment of the present invention was only 0.2V·cm higher than that of the traditional structure. This verifies that the thin-film lithium niobate modulator with a stepped three-layer electrode structure significantly improves the modulation bandwidth while maintaining a low half-wave voltage.

[0049] To summarize, the thin-film lithium niobate modulator with a stepped three-layer electrode structure in the embodiment of the present invention achieves a smaller electrode spacing and a low half-wave voltage by gradually increasing the electrode width and gradually reducing the electrode spacing, while increasing the effective area of ​​the conductor, thereby reducing microwave losses. It successfully breaks through the trade-off limitation between voltage and bandwidth of traditional modulators, and can significantly improve the modulation bandwidth while maintaining a low half-wave voltage. Compared with traditional electrode structures, it significantly reduces microwave losses, is more suitable for high-frequency applications, and has good engineering application value.

[0050] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A thin film lithium niobate modulator with a stepped three-layer electrode structure, characterized in that: It includes a substrate layer, a buffer layer, a lithium niobate film layer, two lithium niobate ridge waveguides, a metal traveling wave electrode and a dielectric layer; The buffer layer is located on the substrate layer; The lithium niobate thin film layer is located on the buffer layer; The two lithium niobate ridge waveguides are located on the lithium niobate film layer, and the lithium niobate film is etched to form a Mach-Zehnder structure for transmitting optical signals; The metal traveling wave electrodes are distributed on both sides of the two lithium niobate ridge waveguides, and adopt a ground-signal-ground push-pull form, including a lower ground-signal-ground electrode, a middle ground-signal-ground electrode and an upper ground-signal-ground electrode; The dielectric layer is filled between the multilayer electrodes of the metal traveling wave electrode and between the multilayer electrodes and the two lithium niobate ridge waveguides; The lower ground-signal-ground electrode is closely attached to the base region of the lithium niobate ridge waveguide and extends parallel to the lithium niobate ridge waveguide to provide a uniform initial electric field distribution and form an effective electric field coupling with the base region; The middle ground-signal-ground electrode is located above the lower ground-signal-ground electrode and has a width greater than that of the lower ground-signal-ground electrode, and is used to adjust the electric field distribution and increase the conductor area to reduce microwave loss; The upper ground-signal-ground electrode is located above the middle ground-signal-ground electrode and has a width greater than that of the middle ground-signal-ground electrode, and is used to reduce impedance and enhance uniformity of electric field distribution; The central axis of the lower ground-signal-ground electrode is aligned with the central axes of the middle and upper ground-signal-ground electrodes to ensure symmetry and uniformity of the electric field distribution.

2. The thin film lithium niobate modulator with a stepped three-layer electrode structure as claimed in claim 1, characterized in that: The material of the substrate layer is silicon, the material of the buffer layer and the dielectric layer is silicon dioxide, and the material of the metal traveling wave electrode is gold or copper.

3. A method for preparing a thin film lithium niobate modulator with a stepped three-layer electrode structure as claimed in claim 1 or 2, characterized in that: include: Selecting a silicon wafer as a substrate, and performing surface treatment on the silicon substrate by a chemical method or a plasma cleaning method to form a substrate layer; Depositing a silicon dioxide buffer layer on the surface of the silicon substrate by chemical vapor deposition or high temperature oxidation; The lithium niobate film is transferred onto the silicon dioxide buffer layer by using epitaxial growth technology to form a lithium niobate film layer; The pattern of the Mach-Zehnder waveguide structure is defined on the lithium niobate film by using photolithography technology, and the lithium niobate film is etched by using reactive ion etching technology to form two lithium niobate ridge waveguides; A metal film is deposited on both sides of the lithium niobate ridge waveguide using magnetron sputtering or electron beam evaporation technology, and the shape of the lower ground-signal-ground electrode is defined by photolithography and etching processes. A dielectric layer is deposited on both sides of the two lithium niobate ridge waveguides and in the gap between the lower ground-signal-ground electrodes using chemical vapor deposition or sputtering deposition technology; then the middle and upper ground-signal-ground electrodes are prepared and the dielectric layer is deposited in the gap between the electrodes using the same method.

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