Thin film lithium niobate phase modulator with substrate integrated coaxial line structure and preparation method of thin film lithium niobate phase modulator

Abstract

The invention relates to a thin-film lithium niobate phase modulator with a substrate integrated coaxial line structure and a preparation method, belongs to the technical field of integrated photonic devices, and solves the problems of serious low-frequency thermal drift, difficulty in impedance matching, low electro-optical overlapping efficiency, limited integration level and the like of a thin-film lithium niobate phase modulator in the prior art. The silicon-based substrate (10) is used for providing mechanical support; a lower electrode layer (20) formed of a high-conductivity metal material; a central conductor (40) is embedded in the first dielectric layer (30), and the first dielectric layer (30) is used for isolating the central conductor (40) from the lower electrode layer (20); a thin film lithium niobate layer (50) in Z-cut orientation; a ridge waveguide (60) formed in a ridge shape above the thin-film lithium niobate layer (50); the second dielectric layer (70) covers the thin film lithium niobate layer (50) and the ridge waveguide (60); and the upper electrode layer (80) is positioned on the second dielectric layer (70).

Description

Technical Field

[0001] This invention relates to the field of integrated photonic device technology, specifically to a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure and its fabrication method. Background Technology

[0002] Thin-film lithium niobate (TFLN) has become an important material for next-generation integrated photonics platforms due to its excellent electro-optic properties, low-loss waveguide characteristics, and CMOS compatibility. Most existing thin-film lithium niobate phase modulators use electrode structures such as coplanar waveguides (CPWs) or microstrips for electrical signal transmission and modulation.

[0003] However, thin-film lithium niobate phase modulators of this type still face several key bottlenecks in practical applications, including the following problems: 1) Severe low-frequency thermal drift: Due to the mismatch in thermal expansion coefficients between the electrode material and lithium niobate, the device is prone to thermal stress under ambient temperature fluctuations, affecting the electro-optic effect and causing unstable modulation phase. This leads to uncontrollable phase drift, severely impacting long-term stability, and is particularly unfavorable for applications such as fiber optic gyroscopes that require extremely high low-frequency stability; 2) Impedance matching difficulties: Traditional electrode structures often exhibit reflection and standing waves during high-frequency transmission, making impedance matching difficult. To achieve good impedance matching with the 50Ω standard interface, the bandwidth performance and signal integrity of the device are limited, reducing signal transmission efficiency; 3) Low electro-optic overlap efficiency: In structures such as coplanar waveguides, the electric field is mainly distributed in the air or cladding region between the electrodes, with limited overlap with the optical field inside the waveguide, resulting in low electro-optic modulation efficiency and increased system power consumption; 4) Limited integration: Existing structures often rely on large electrode spans and vertical structural space, which is not conducive to device size compression and the construction of highly integrated systems. These problems severely restrict the application of thin-film lithium niobate phase modulators in high-performance integrated photonic systems such as fiber optic gyroscopes and high-precision quantum systems.

[0004] Therefore, there is an urgent need in the field for an improved electrode structure that can simultaneously enhance low-frequency stability, electro-optic modulation efficiency, and high-frequency transmission performance, in order to overcome the bottlenecks of traditional thin-film lithium niobate modulators in terms of low drift, high-speed transmission, and compact integration. Summary of the Invention

[0005] In view of the above problems, this invention provides a thin-film lithium niobate (TFLN) modulator with a substrate integrated coaxial line (SICL) structure and its fabrication method. It proposes a co-design approach combining SICL embedded electrodes and TFLN ridge waveguides. By introducing a substrate integrated coaxial line structure onto the thin-film lithium niobate platform, and through synergistic optimization of electrode geometry and material distribution, not only is precise impedance matching achieved, but the electro-optic overlap factor and thermal stability are also significantly improved, fundamentally solving the aforementioned problems in the prior art. Addressing the bottlenecks of existing technologies, this invention proposes a novel modulator based on a substrate integrated coaxial line (SICL) structure. Through a combination of a three-dimensional electrode encapsulation structure and an axisymmetric air column array, it effectively improves modulation efficiency, stability, and system integration capabilities.

[0006] According to one embodiment of the present invention, a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure is provided, characterized in that it comprises layers stacked sequentially from bottom to top:

[0007] Silicon-based substrates are used to provide mechanical support;

[0008] The lower electrode layer is formed of a highly conductive metallic material;

[0009] A first dielectric layer with an embedded central conductor is used to isolate the central conductor from the lower electrode layer;

[0010] The thin-film lithium niobate layer is Z-cut oriented;

[0011] Ridge waveguide, formed in the shape of a convex ridge above a thin-film lithium niobate layer;

[0012] The second dielectric layer covers the thin-film lithium niobate layer and the ridge waveguide structure;

[0013] The upper electrode layer is located on the second dielectric layer;

[0014] The central conductor is embedded in the upper center of the first dielectric layer to be uniformly embedded within the first dielectric layer.

[0015] The lower electrode layer, the first dielectric layer, the second dielectric layer, and the upper electrode layer each have multiple through holes of the same size and corresponding positions, thereby forming an air column array that runs vertically through the assembly. The air column array is symmetrically distributed around the central conductor, forming an electromagnetic shielding structure for the coaxial outer conductor.

[0016] Optionally, the vias in the lower electrode layer, the first dielectric layer, the second dielectric layer, and the upper electrode layer are formed to surround the periphery of the central conductor, such that the air column formed by the vias has a common central axis with the central conductor.

[0017] Optionally, the width of the center conductor ranges from 10 to 50 μm; the thickness of the first dielectric layer ranges from 5 to 10 μm.

[0018] Optionally, the thickness of the thin-film lithium niobate layer ranges from 300 to 700 nm; the ridge width of the ridge waveguide ranges from 1 to 2 μm; and the ridge height ranges from 200 to 400 nm.

[0019] Optionally, the first dielectric layer and the second dielectric layer are made of silicon dioxide or silicon nitride.

[0020] Optionally, the lower electrode layer, the central conductor, and the upper electrode layer are made of gold or copper.

[0021] Optionally, the thicknesses of the lower electrode layer, the central conductor, and the upper electrode layer are all in the range of 1-3 μm.

[0022] According to another embodiment of the present invention, a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure is provided, comprising, from bottom to top, the following:

[0023] Silicon-based substrates are used to provide mechanical support;

[0024] The lower electrode layer is formed of a highly conductive metallic material;

[0025] The lower dielectric layer is located above the lower electrode layer;

[0026] Lower ridge waveguide and lower thin-film lithium niobate layer, the lower thin-film lithium niobate layer is Z-cut oriented;

[0027] An embedded dielectric layer containing a central conductor;

[0028] The upper thin-film lithium niobate layer and the upper ridge waveguide are arranged in a Z-cut orientation.

[0029] The upper dielectric layer covers the upper thin-film lithium niobate layer and the upper ridge waveguide;

[0030] The upper electrode layer is located on the upper dielectric layer;

[0031] The central conductor is embedded in the center of the intermediate dielectric layer to be uniformly embedded within the intermediate dielectric layer.

[0032] Among them, the lower ridge waveguide and the lower thin-film lithium niobate layer are arranged symmetrically with respect to the middle dielectric layer, as are the upper thin-film lithium niobate layer and the upper ridge waveguide.

[0033] The lower electrode layer, lower dielectric layer, middle dielectric layer, upper dielectric layer and upper electrode layer each have multiple through holes of the same size and corresponding positions, so that after assembly, they form an air column array that runs vertically through the center conductor. The air column array is symmetrically distributed around the center conductor to form an electromagnetic shielding structure for the coaxial outer conductor.

[0034] Another embodiment of the present invention provides a method for fabricating a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, comprising:

[0035] S1: Cut to obtain a silicon substrate, and then sputter and deposit metal material on the silicon substrate to form a lower electrode layer;

[0036] S2: Deposit a first dielectric layer on the lower electrode layer, and form a trench in the deposited first dielectric layer by etching, and then fill the etched trench with metal to form a central conductor;

[0037] S3: Bond the obtained structure to the thin film lithium niobate layer;

[0038] S4: Ridge waveguides were fabricated on thin-film lithium niobate layers using electron beam lithography and reactive ion etching processes;

[0039] S5: Deposit a second dielectric layer on the ridge waveguide and thin-film lithium niobate layer;

[0040] S6: An upper electrode layer is formed on the second dielectric layer, and a pattern is formed on the upper electrode layer by photolithography and etching to prepare a thin-film lithium niobate phase modulator with a substrate integrated coaxial structure.

[0041] Optionally, S2 further includes forming vias of consistent and uniform size in the lower electrode layer and the first dielectric layer by photolithography and etching; S5 further includes forming vias of consistent and uniform size in the second dielectric layer and the upper electrode layer by photolithography and etching.

[0042] Compared with the prior art, the thin-film lithium niobate phase modulator and its preparation method with substrate integrated coaxial line structure provided by the present invention have at least the following beneficial effects.

[0043] 1) Significantly reduced low-frequency phase drift: The SICL electrode embedding design effectively reduces the interference of environmental thermal disturbances on the modulation electric field. At a test frequency of 1kHz, the low-frequency phase drift Δφ < 0.1rad / V, and the drift suppression rate is more than 80% higher than that of the coplanar waveguide (CPW) structure.

[0044] 2) High electro-optic modulation efficiency: The joint design of ridge waveguide and symmetrical electrode achieves an electro-optic overlap factor Γ≥0.8. Combined with the high electro-optic coefficient of Z-cut oriented thin film lithium niobate (TFLN) material, high-efficiency modulation under low driving voltage (<3V) is achieved.

[0045] 3) Excellent impedance matching and bandwidth performance: By co-optimizing the SICL electrode structure parameters, a characteristic impedance of 50±5Ω is obtained, which is well matched with the interface of high-speed communication systems and has an operating bandwidth of >10GHz, which is significantly better than traditional microstrip or CPW structures.

[0046] 4) Compact Device Size, Facilitating Integration: This invention achieves device size compression by introducing an embedded SICL electrode structure. Specifically, a vertically stacked coaxial line structure replaces the traditional planar coplanar electrode structure. The electric field is concentrated and propagated in the dielectric layer in a quasi-transverse electromagnetic wave (quasi-TEM wave) mode, thereby significantly improving the overlap and field strength uniformity of the electro-optic field. Compared with the traditional structure, this invention can effectively reduce the electrode spacing and waveguide length while achieving the same electro-optic modulation effect, compressing the overall device length by about 30%, making it easier to achieve miniaturization of photonic chips and high-density multi-channel integrated layout.

[0047] 5) Good process compatibility and strong feasibility for mass production: The materials and processes involved are compatible with existing CMOS, and have a good industrial manufacturing foundation.

[0048] 6) The SICL-TFLN integrated phase modulator proposed in this invention has multiple advantages such as low drift, high modulation efficiency, wide bandwidth response and integrability, and has broad application prospects and promotion value in the fields of fiber optic gyroscopes, quantum optics, high-speed optical communication and microwave photonics. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly introduced below. The features and advantages of the present invention can be more clearly understood by referring to the accompanying drawings. The accompanying drawings are schematic and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1a A longitudinal cross-sectional schematic diagram of a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure provided according to a first embodiment of the present invention.

[0051] Figure 1b A schematic cross-sectional view of a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure provided according to a first embodiment of the present invention, taken laterally along the first dielectric layer.

[0052] Figure 2 In an embodiment of the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure provided for applying the first embodiment of the present invention, a two-dimensional optical mode field cross-section of the SICL-TFLN integrated structure phase modulator is shown.

[0053] Figure 3 In an embodiment of the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure provided for applying the first embodiment of the present invention, a two-dimensional electric field cross-section of the SICL-TFLN integrated structure phase modulator is shown.

[0054] Figure 4 A schematic diagram of a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure provided according to a second embodiment of the present invention.

[0055] Explanation of reference numerals in the attached figures:

[0056] 10-Silicon substrate;

[0057] 20-Lower electrode layer;

[0058] 30 - First dielectric layer;

[0059] 40 - Center conductor;

[0060] 50-thin film lithium niobate layer;

[0061] 60-ridge waveguide;

[0062] 70 - Second dielectric layer;

[0063] 80 - Upper electrode layer;

[0064] 90-air column;

[0065] 100-Silicon-based substrate;

[0066] 200-lower electrode layer;

[0067] 300-under dielectric layer;

[0068] 310 - Medium dielectric layer;

[0069] 400 - Center conductor;

[0070] 500-thin lithium niobate layer;

[0071] 510- Thin film lithium niobate layer;

[0072] 600-lower ridge waveguide;

[0073] 610-Upper Ridge Waveguide;

[0074] 700 - Upper dielectric layer;

[0075] 800 - Upper electrode layer;

[0076] 900-air column. Detailed Implementation

[0077] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0078] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0079] The following describes in detail, with reference to the accompanying drawings, a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure and its fabrication method according to an embodiment of the present invention.

[0080] like Figure 1a and Figure 1b As shown, a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure is provided according to a first embodiment of the present invention. As a thin-film lithium niobate phase modulator, it includes, from bottom to top, the following layers stacked sequentially: a silicon substrate 10 for providing mechanical support; a lower electrode layer 20 formed of a highly conductive metal material; a first dielectric layer 30 with an embedded central conductor 40 for isolating the central conductor 40 from the lower electrode layer 20; a thin-film lithium niobate layer 50 with a Z-cut orientation; a ridge waveguide 60 forming a convex ridge shape above the thin-film lithium niobate layer 50; a second dielectric layer 70 covering the structure of the thin-film lithium niobate layer 50 and the ridge waveguide 60; and an upper electrode layer 80 located on the second dielectric layer. The central conductor 40 is embedded in the upper center of the first dielectric layer 30 to be uniformly embedded within the first dielectric layer 30 and isolated from the lower electrode layer 20 below. The upper surface of the central conductor 40 can contact the upper thin-film lithium niobate layer 50. The lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80 may have the same external dimensions, for example, they may be rectangular. The thin-film lithium niobate layer 50 may be rectangular (i.e., rectangular), and its dimensions are smaller in width than the widths of the lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80, while its length may be the same as the lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80.

[0081] like Figure 1aAs shown, the central conductor 40, together with the lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80, constitute a substrate integrated coaxial line (SICL) electrode structure, constructing an embedded three-dimensional coaxial electrode layout. The substrate integrated coaxial line electrode structure is an embedded design, achieved through the following method: an embedded substrate integrated coaxial line electrode structure is constructed on the silicon substrate 10, specifically consisting of the lower electrode layer 20, the first dielectric layer 30, the central conductor 40, the second dielectric layer 70, and the upper electrode layer 80 forming a coaxial three-dimensional conductive path. Microfabrication processes such as deposition, etching, filling, and bonding are employed. The upper electrode layer 80 and the lower electrode layer 20 surround the central conductor 40 from above and below to form a closed electromagnetic shielding path, forming a symmetrical structure of inner and outer conductors, constituting a ring-shaped coaxial outer conductor. The outer conductor (i.e., the upper electrode layer 80 and the lower electrode layer 20) forms a complete surrounding shielding path to the inner conductor (i.e., the central conductor 40), realizing the transmission of the quasi-transverse electromagnetic wave (TEM) mode of the electric field, improving the uniformity and stability of the electric field distribution, significantly improving the high-frequency bandwidth and impedance matching capability, and effectively suppressing reflection and loss.

[0082] See Figure 1b In this embodiment, the lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80 each have multiple through-holes formed on the periphery of the central conductor 40. The through-holes formed on each layer are of the same size and corresponding in position, thereby forming a vertically penetrating array of air columns 90 after assembly. Optionally, uniformly arranged through-holes can be formed on the outer side of the two long sides of the rectangles of the lower electrode layer 20, the first dielectric layer 30, the second dielectric layer 70, and the upper electrode layer 80, thereby arranging the multiple air columns 90 into two rows of arrays with uniform spacing between adjacent air columns 90. The array of air columns 90 is symmetrical about the central conductor 40 and has a common central axis with the central conductor 40. The vertically penetrating cavity structure formed by the multiple peripheral air columns 90 constitutes a coaxial outer shielding layer surrounding the central conductor 40, providing a good electromagnetic shielding and mode confinement environment for the SICL electrode structure, ensuring the stability and purity of signal transmission, and further improving the reliability and performance stability of the device in complex environments. The location of the through hole, i.e. the location of the air column 90, is on the outside of the long side of the rectangular thin film lithium niobate layer 50.

[0083] This substrate-integrated coaxial electrode structure enables electric field transmission in a quasi-transverse electromagnetic mode. Compared with planar electrode structures of traditional coplanar waveguides or microstrip lines, the electric field is more concentrated and uniform, significantly improving electro-optical overlap efficiency and enhancing electro-optical modulation effects. Based on the thermo-mechanical-electrical multi-physics collaborative design of the substrate-integrated coaxial electrode structure, thermal stress and low-frequency phase drift caused by ambient temperature fluctuations are effectively suppressed. Specifically, in this embodiment, the central conductor 40 is embedded in the center of the first dielectric layer 30, thus being uniformly embedded within the first dielectric layer 30, forming a vertically embedded coaxial inner conductor structure. This significantly reduces thermal stress caused by ambient temperature fluctuations and reduces stress accumulation caused by the mismatch in thermal expansion coefficients between the lower electrode layer 20, the upper electrode layer 80, and the thin-film lithium niobate layer 50. The central conductor 40 can be formed by creating an elongated groove in the first dielectric layer 30 and filling the groove with metal material. The thin-film lithium niobate phase modulator with substrate-integrated coaxial structure provided by this method can control the low-frequency phase drift to below 0.1 rad / V at a test frequency of 1 kHz, which is more than 80% better than the traditional structure.

[0084] By vertically stacking a substrate-integrated coaxial electrode structure including a lower electrode layer 20 and an upper electrode layer 80, the vertical dimension of the assembled modulator is significantly reduced while achieving the same electro-optic modulation effect. Compared to traditional planar layouts, the thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure in this embodiment can reduce the modulator length by approximately 30%, which is beneficial for chip miniaturization and higher-density photonic device integration, significantly improving space utilization and integration. Furthermore, the substrate-integrated coaxial electrode structure guides uniform heat diffusion in the vertical direction, avoiding localized heat accumulation. Simultaneously, the dielectric layer's encapsulation structure of the electrodes buffers temperature changes, significantly reducing thermal stress and interface stress accumulation. In addition, insulating dielectric materials with low thermal conductivity and coefficients of thermal expansion well matched to lithium niobate (such as SiO2 or Si3N4) are used as the first dielectric layer 30 and the second dielectric layer 70, achieving optimized thermal conduction path design for three-dimensional packaging.

[0085] Continue to refer to Figure 1a and 1b In this embodiment, the center conductor 40 can be configured as an elongated strip along the longitudinal direction of the modulator, with a width ranging from 10 to 50 μm; and the thickness of the first dielectric layer 30 can be configured to range from 5 to 10 μm. The width of the center conductor 40 refers to its width dimension within the horizontal cross-section of the modulator (i.e., the horizontal direction perpendicular to the vertical stacking direction). The thickness of the first dielectric layer 30 is... Figure 1aThe vertical dimension. The shape of the central conductor 40 is a rectangular or approximately rectangular flat structure. This flat design is conducive to the concentration and uniform distribution of the electromagnetic field, further improving the overlap between the electric field and the optical field in the waveguide, thereby enhancing the electro-optic modulation efficiency and optimizing the high-frequency transmission performance.

[0086] In this embodiment, the conductor materials of the lower electrode layer 20, the central conductor 40, and the upper electrode layer 80 can be gold (Au) or copper (Cu) with high conductivity, and their conductivity is not less than 4 × 10⁻⁶. 7 The thickness of each layer in the lower electrode layer 20, the central conductor 40, and the upper electrode layer 80 ranges from 1 to 3 μm. This material and thickness design helps to further reduce the resistive and heat losses of the electrodes, improving the transmission efficiency and thermal stability of electrical signals. Through coordinated optimization of the dimensional parameters of the central conductor 40 and the first dielectric layer 30, as well as the conductor materials and thicknesses of the lower electrode layer 20, the central conductor 40, and the upper electrode layer 80, precise impedance matching of the electrode structure is achieved, resolving impedance mismatch issues. Specifically, by optimizing the width of the central conductor 40 and the thickness of the first dielectric layer 30 in the above manner, and combining this with the selection of conductor materials and thicknesses, the characteristic impedance is ensured to be precisely matched within the range of 50 ± 5 Ω. This effectively reduces reflection and standing wave phenomena in high-frequency signal transmission, significantly improves device bandwidth and signal integrity, and meets the standard interface requirements of high-speed communication systems.

[0087] In this embodiment, the materials of the first dielectric layer 30 and the second dielectric layer 70 can be silicon dioxide or silicon nitride.

[0088] The Z-cut orientation of the thin-film lithium niobate layer 50 specifically involves aligning the optical axis (crystal axis) of the thin-film lithium niobate crystal perpendicular to the film surface, thereby aligning the electric field with the direction of the maximum electro-optic coefficient of the thin-film lithium niobate crystal. The thickness of the thin-film lithium niobate layer 50 can range from 300 to 700 nm. The electro-optic coefficient r of the thin-film lithium niobate layer 50... 33 ≥30pm / V. (e.g.) Figure 1a and 1bAs shown, the thin-film lithium niobate layer 50 can be an elongated structure located between the air column arrays 90 on both sides, thereby allowing the air columns 90 to avoid the thin-film lithium niobate layer 50 and prevent the formation of through holes on the thin-film lithium niobate layer 50. The portions on both sides of the thin-film lithium niobate layer 50 that are narrower than the first dielectric layer 30 and the second dielectric layer 70 above and below can be filled with dielectric layer material. In addition, to further improve the anti-interference capability and signal integrity of the device, in this embodiment, the air column array 90 is not only used as a through hole in the mechanical structure, but also constitutes a functional coaxial shielding layer. This air column structure exhibits the following two key characteristics in electromagnetic simulation: on the one hand, its relative permittivity is approximately 1, which has a good shielding effect on peripheral high-frequency signals; on the other hand, this structure forms an equipotential boundary condition around the central conductor, enhancing the axial symmetry and vertical concentration of the electric field in the modulation region. This design not only suppresses lateral electric field leakage, but also effectively controls the longitudinal electric field distribution, reducing the risk of mode dispersion, which is one of the important supporting mechanisms for the low drift characteristics of this invention.

[0089] The ridge waveguide 60, located on the thin-film lithium niobate layer 50, enhances the confinement of the optical field and the electro-optic coupling efficiency through precise structural design. The ridge waveguide 60 can be formed on the thin-film lithium niobate layer 50 using electron beam lithography and reactive ion etching (RIE) processes. Specifically, the ridge width of the ridge waveguide 60 ranges from 1 to 2 μm, and the ridge height ranges from 200 to 400 nm. Optionally, the ridge width of the ridge waveguide 60 can be set to 1.5 μm, and the ridge height can be set to 300 nm. The thin-film lithium niobate layer 50 adopts a Z-cut orientation, combined with the precise geometric design of the ridge waveguide 60 structure (e.g., ridge width of 1.5 μm and ridge height of 300 nm), further improving the ability of the ridge waveguide 60 to confine the optical field. Meanwhile, the electric field distribution formed by the substrate-integrated coaxial electrode structure is symmetrical and concentrated. The combination of the two results in a high degree of spatial overlap between the optical field and the electric field, thereby achieving a lower driving voltage (less than 3 V) and a higher electro-optic modulation efficiency (electro-optic overlap factor Γ≥0.8), thus improving the modulation efficiency and reducing the driving voltage.

[0090] The distribution overlap factor between the ridge waveguide 60 and the modulation electric field applied to the lower electrode layer 20 and the upper electrode layer 80 ranges from Γ≥0.8. This high electro-optic overlap factor is achieved by optimizing parameters such as the cross-sectional dimensions (ridge width and ridge height) of the ridge waveguide 60, the thickness of the thin-film lithium niobate layer 50, the vertical distance between the lower electrode layer 20 and the upper electrode layer 80 and the ridge waveguide 60, and the thicknesses of the first dielectric layer 30 and the second dielectric layer 70.

[0091] A ridge waveguide 60 is integrated on the thin-film lithium niobate layer 50 and symmetrically positioned directly above the central conductor 40, such that the central axis of the ridge waveguide 60 is vertically aligned with the central axis of the central conductor 40. The overall structure has mirror symmetry about the longitudinal central axis of the modulator. This symmetrical layout ensures that the modulation electric field forms a highly uniform and concentrated vertical electric field distribution in the waveguide region.

[0092] Furthermore, the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure provided in the first embodiment also features low driving voltage (<3V) and high operating bandwidth (>10GHz). The low driving voltage is achieved through the synergistic symmetrical layout of the ridge waveguide 60 and the substrate-integrated coaxial line electrode structure. This structure concentrates the modulation electric field highly in the core region of the ridge waveguide 60, significantly overlapping with the optical mode field, thereby significantly improving electro-optic modulation efficiency and reducing the required voltage. The high operating bandwidth stems from the quasi-TEM mode transmission characteristics in the substrate-integrated coaxial line electrode structure. Its coaxial conductor layout effectively avoids mode dispersion and reflection interference, while precisely controlling the characteristic impedance to 50±5Ω ensures reflection-free matching with external RF systems, significantly improving signal integrity and high-speed transmission capabilities.

[0093] The first embodiment provides a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, which solves the problems of insufficient low-frequency stability, difficulty in impedance matching, low electro-optical overlap efficiency, excessive size and sensitivity to electromagnetic interference. It realizes a novel thin-film lithium niobate phase modulator with multiple advantages such as low driving voltage, high modulation efficiency, wide bandwidth response, low drift, high integration and high stability, and is suitable for fields such as fiber optic gyroscopes, quantum optics, high-speed optical communication and microwave photonics.

[0094] Example 1

[0095] To better understand the technical solution of the present invention, Example 1 of applying the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure provided by the first embodiment of the present invention will be described.

[0096] Example 1 is a schematic example of a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, the specific structural configuration of which is as follows. The thin-film lithium niobate phase modulator of Example 1 with a substrate-integrated coaxial line structure includes, from bottom to top: a silicon substrate, a lower electrode layer, a first dielectric layer, a central conductor, a thin-film lithium niobate layer, a ridge waveguide, a second dielectric layer, and an upper electrode layer. Specifically, the width of the central conductor is 30 μm; the thickness of the first dielectric layer is 8 μm (verified by finite element simulation software, the SICL characteristic impedance is approximately 49.8 Ω); the thickness of the thin-film lithium niobate layer is 500 nm (Z-cut), and the Z-cut oriented thin-film lithium niobate has a large electro-optic coefficient r. 33This design is beneficial for improving electro-optic modulation efficiency. The ridge waveguide structure has a ridge width of 1.5 μm and a ridge height of 300 nm. This size design can effectively confine the optical field and improve the overlap between the optical and electric fields. The upper and lower electrode layers are made of gold (Au) with a thickness of 2 μm. Gold has good conductivity and chemical stability, which can ensure the high conductivity and long-term reliability of the electrodes. The electro-optic coefficient of the thin-film lithium niobate is r. 33 ≥30pm / V; The materials of the first and second dielectric layers are silicon dioxide (SiO2). Silicon dioxide has good insulation properties and optical transparency, making it suitable as a dielectric layer isolation electrode and for transmitting optical signals.

[0097] Figure 2 A two-dimensional optical mode field cross-section of the SICL-TFLN integrated structure phase modulator, as shown in an embodiment of the substrate-integrated coaxial line structure of the present invention. Figure 2 As shown, in the TFLN phase modulator based on the SICL electrode structure of Embodiment 1, the optical field is concentrated in the core region of the ridge waveguide, which facilitates maximum electro-optic coupling with the vertical electric field direction.

[0098] Figure 3 A two-dimensional electric field cross-section of the SICL-TFLN integrated phase modulator is shown in an embodiment of the substrate-integrated coaxial line structure of the present invention. Figure 3 As shown, in the TFLN phase modulator based on the SICL electrode structure of Embodiment 1, symmetrical and vertical electric field lines are formed in the modulation region, which is beneficial to achieve high overlap between the waveguide region and the electric field and improve the electro-optic modulation efficiency.

[0099] like Figure 4As shown, a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure is provided according to a second embodiment of the present invention, comprising, from bottom to top, the following layers stacked sequentially: a silicon substrate 100 for providing mechanical support; a lower electrode layer 200 formed of a highly conductive metal material; a lower dielectric layer 300; a lower ridge waveguide 600 and a lower thin-film lithium niobate layer 500, the lower thin-film lithium niobate layer 500 being Z-cut oriented; a middle dielectric layer 310 with an embedded center conductor 400; an upper thin-film lithium niobate layer 510 and an upper ridge waveguide 610, the upper thin-film lithium niobate layer 510 being Z-cut oriented; an upper dielectric layer 700 covering the upper thin-film lithium niobate layer 510 and the upper ridge waveguide 610 structure; and an upper electrode layer 800 located on the upper dielectric layer 700. The lower ridge waveguide 600 and the lower thin-film lithium niobate layer 500 are arranged symmetrically with respect to the middle dielectric layer 310, mirroring the upper ridge waveguide 610 and the upper thin-film lithium niobate layer 510. This arrangement is beneficial for suppressing optical noise. The center conductor 400 is embedded in the center of the middle dielectric layer 310 to uniformly encapsulate it. The lower electrode layer 200, lower dielectric layer 300, middle dielectric layer 310, upper dielectric layer 700, and upper electrode layer 800 can have consistent external dimensions, for example, a rectangular shape. The lower thin-film lithium niobate layer 500 and the upper thin-film lithium niobate layer 510 can be rectangular in shape. Their dimensions are smaller in width than the lower electrode layer 200, lower dielectric layer 300, middle dielectric layer 310, upper dielectric layer 700 and upper electrode layer 800, and their lengths can be the same as the lower electrode layer 200, lower dielectric layer 300, middle dielectric layer 310, upper dielectric layer 700 and upper electrode layer 800.

[0100] In this second embodiment, the lower ridge waveguide 600 is located below the lower thin-film lithium niobate layer 500, and the upper ridge waveguide 610 is located above the upper thin-film lithium niobate layer 510. These two sets of thin-film lithium niobate layer-ridge waveguide structures are arranged in a mirror-symmetrical configuration relative to the central conductor 400, forming a dual-path modulation channel. Each modulation channel is located within a symmetrical electric field region, ensuring electro-optic modulation consistency and significantly improving system stability. This symmetrical structure of the two sets of thin-film lithium niobate layer-ridge waveguides supports dual-path output signals and can be connected to differential interferometry or dual-channel interferometry systems. It possesses excellent common-mode noise suppression capabilities, such as suppressing the effects of environmental thermal disturbances and power supply interference. The dual-path modulation channel structure can achieve a signal-to-noise ratio improvement of over 20 dB, making it suitable for low-noise applications such as high-precision fiber optic gyroscopes and quantum interferometry.

[0101] In this second embodiment, the three-layer dielectric structure, comprising a lower dielectric layer 300, a middle dielectric layer 310, and an upper dielectric layer 700, together ensures the structural integrity, field distribution uniformity, and symmetrical modulation characteristics of the upper and lower channels of the device.

[0102] In this embodiment, the central conductor 400, together with the lower electrode layer 200, lower dielectric layer 300, middle dielectric layer 310, upper dielectric layer 700, and upper electrode layer 800, constitute a substrate integrated coaxial line (SICL) electrode structure. Through the substrate integrated coaxial line (SICL) electrode structure, an embedded three-dimensional coaxial electrode layout is constructed. The lower electrode layer 200, lower dielectric layer 300, middle dielectric layer 310, upper dielectric layer 700, and upper electrode layer 800 have multiple through-holes formed around the central conductor 400, and the through-holes in each layer are of the same size and corresponding in position, thereby forming multiple vertically penetrating air columns 900 in the circumferential direction after assembly.

[0103] The other designs of the components in the thin-film lithium niobate phase modulator with substrate-integrated coaxial line structure provided in the second embodiment, including layout, size, materials, etc., are basically the same as those in the first embodiment, and will not be described again here.

[0104] The second embodiment provides a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure. Through a symmetrical electric field distribution, the double-ridged waveguide can synchronously modulate two optical signals, theoretically increasing the electro-optic overlap factor to Γ. dual ≈1.6 (single waveguide Γ≥0.8); the provided dual-channel output signal can form a differential detection system, effectively suppressing common-mode noise (such as thermal drift) and improving the signal-to-noise ratio by more than 20dB.

[0105] According to a third embodiment of the present invention, a method for fabricating a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure is provided, which is used to fabricate the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure of the first embodiment above, and specifically includes the following steps.

[0106] S1: A silicon substrate is cut to the required size, and a metal material, such as gold or copper, is sputtered and deposited on the silicon substrate to form the lower electrode layer. The sputtering deposition process can obtain a uniform and dense metal film, ensuring good performance of the lower electrode.

[0107] S2: A first dielectric layer is deposited on the lower electrode layer, and a trench of the desired structure is formed in the deposited first dielectric layer by etching. Then, metal is filled into the etched trenches to form a central conductor. This deposition step can be performed using plasma-enhanced chemical vapor deposition (PECVD), which produces a dielectric layer with high purity and uniformity. Reactive ion etching (RIE) can be used, offering high etching precision and accurate trench dimensions. Metal filling can be achieved through electroplating or vapor deposition. This step also includes forming uniformly sized and distributed vias in the lower electrode layer and the first dielectric layer using photolithography and etching techniques, such as RIE or ICP (inductively coupled plasma etching). These vias are used to form a through-hole array of air columns after assembly.

[0108] S3: The obtained structure (i.e., the silicon substrate with the first dielectric layer, lower electrode layer, and central conductor prepared) is bonded to the thin-film lithium niobate (TFLN) layer. Plasma-activated bonding can be used to ensure a strong bond between the structure on the silicon substrate and the thin-film lithium niobate layer, and good interface properties.

[0109] S4: Ridge waveguides are fabricated on thin-film lithium niobate layers using electron beam lithography and reactive ion etching (RIE) processes. Electron beam lithography enables high-precision pattern transfer, while RIE allows for precise control of etching depth and sidewall perpendicularity, ensuring the dimensional accuracy of the ridge waveguide.

[0110] S5: Deposit a second dielectric layer on the ridge waveguide and the thin-film lithium niobate layer. This deposition step can also be performed using the PECVD method.

[0111] S6: An upper electrode layer is formed on the second dielectric layer, and then the desired pattern is formed on the upper electrode layer by photolithography and etching. This yields a thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure. The upper electrode layer can be formed using sputtering or evaporation processes. This step also includes forming vias of consistent size and uniformly distributed in the second dielectric layer and the upper electrode layer using photolithography and etching techniques such as RIE or ICP (Inductively Coupled Plasma Etching). The vias formed in the lower electrode layer, the first dielectric layer, the second dielectric layer, and the upper electrode are of consistent size and corresponding in position, collectively forming a vertically penetrating array of air columns.

[0112] In the above manufacturing steps, the required materials, shapes, dimensions, and other parameters of each part in the thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure can be set and processed in the corresponding steps.

[0113] The fabrication process of this third embodiment is compatible with CMOS processes: the material systems used, including Si, SiO2, TFLN, Au / Cu, etc.; and the process steps, including PECVD, sputtering, electroplating, RIE, bonding, etc., are all compatible with existing CMOS platforms, which is conducive to large-scale integration and commercial deployment.

[0114] Alternatively, the steps of the above method can be adjusted, for example, by adding steps to form a lower dielectric layer, a lower ridge waveguide, and a lower thin-film lithium niobate layer, to prepare the thin-film lithium niobate phase modulator with a substrate-integrated coaxial structure according to the second embodiment.

[0115] All of the above-mentioned optional technical solutions can be combined in any way to form optional embodiments of the present invention, and will not be described in detail here.

[0116] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, characterized in that, Including those arranged in layers from bottom to top: A silicon substrate (10) is used to provide mechanical support; The lower electrode layer (20) is formed of a highly conductive metallic material; A first dielectric layer (30) with an embedded center conductor (40) is used to isolate the center conductor (40) from the lower electrode layer (20); The thin-film lithium niobate layer (50) is Z-cut oriented; Ridge waveguide (60) is formed in the shape of a convex ridge above a thin film lithium niobate layer (50); A second dielectric layer (70) covers the thin-film lithium niobate layer (50) and the ridge waveguide (60) structure; The upper electrode layer (80) is located on the second dielectric layer (70); The center conductor (40) is embedded in the upper center of the first dielectric layer (30) to be uniformly embedded in the first dielectric layer (30); Among them, multiple through holes of the same size and corresponding positions are formed in the lower electrode layer (20), the first dielectric layer (30), the second dielectric layer (70) and the upper electrode layer (80), so that after assembly, an air column (90) array is formed that runs through the top and bottom. The air column (90) array is symmetrically distributed around the central conductor (40) to form an electromagnetic shielding structure of the coaxial outer conductor.

2. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The vias in the lower electrode layer (20), the first dielectric layer (30), the second dielectric layer (70) and the upper electrode layer (80) are formed to surround the center conductor (40), such that the air column (90) formed by the vias has a common central axis with the center conductor (40).

3. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The width of the center conductor (40) ranges from 10 to 50 μm; The thickness of the first dielectric layer (30) ranges from 5 to 10 μm.

4. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The thickness of the thin-film lithium niobate layer (50) ranges from 300 to 700 nm; The ridge width of the ridge waveguide (60) ranges from 1 to 2 μm; the ridge height ranges from 200 to 400 nm.

5. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The first dielectric layer (30) and the second dielectric layer (70) are made of silicon dioxide or silicon nitride.

6. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The lower electrode layer (20), the central conductor (40), and the upper electrode layer (80) are made of gold or copper.

7. The thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 1, characterized in that, The thicknesses of the lower electrode layer (20), the central conductor (40), and the upper electrode layer (80) are all in the range of 1-3 μm.

8. A thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, characterized in that, Including those arranged in layers from bottom to top: A silicon-based substrate (100) is used to provide mechanical support; The lower electrode layer (200) is formed of a highly conductive metallic material; The lower dielectric layer (300) is located on the lower electrode layer (200); The lower ridge waveguide (600) and the lower thin-film lithium niobate layer (500) are Z-cut oriented. A middle dielectric layer (310) with an embedded central conductor (400); The upper thin-film lithium niobate layer (510) and the upper ridge waveguide (610) are Z-cut oriented. An upper dielectric layer (700) covers an upper thin-film lithium niobate layer (510) and an upper ridge waveguide (610); The upper electrode layer (800) is located on the upper dielectric layer (700); The center conductor (400) is embedded in the center of the intermediate dielectric layer (310) to be uniformly embedded in the intermediate dielectric layer (310); Among them, the lower ridge waveguide (600) and the lower thin-film lithium niobate layer (500) are arranged symmetrically with respect to the middle dielectric layer (310) with respect to the upper thin-film lithium niobate layer (510) and the upper ridge waveguide (610). Among them, multiple through holes of the same size and corresponding positions are formed in the lower electrode layer (200), lower dielectric layer (300), middle dielectric layer (310), upper dielectric layer (700) and upper electrode layer (800), so that after assembly, an array of air columns (900) is formed that runs vertically through the center conductor (400). The array of air columns (900) is symmetrically distributed around the center conductor (400) to form an electromagnetic shielding structure of the coaxial outer conductor.

9. A method for fabricating a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure, characterized in that, include: S1: Cut to obtain a silicon substrate, and then sputter and deposit metal material on the silicon substrate to form a lower electrode layer; S2: Deposit a first dielectric layer on the lower electrode layer, and form a trench in the deposited first dielectric layer by etching, and then fill the etched trench with metal to form a central conductor; S3: Bond the obtained structure to the thin film lithium niobate layer; S4: Ridge waveguides were fabricated on thin-film lithium niobate layers using electron beam lithography and reactive ion etching processes; S5: Deposit a second dielectric layer on the ridge waveguide and thin-film lithium niobate layer; S6: An upper electrode layer is formed on the second dielectric layer, and a pattern is formed on the upper electrode layer by photolithography and etching to prepare a thin-film lithium niobate phase modulator with a substrate integrated coaxial structure.

10. The method for fabricating a thin-film lithium niobate phase modulator with a substrate-integrated coaxial line structure according to claim 9, characterized in that, S2 also includes forming through holes of consistent size and uniform distribution in the lower electrode layer and the first dielectric layer by photolithography and etching. S5 also includes through-holes of consistent size and uniform distribution formed in the second dielectric layer and the upper electrode layer by photolithography and etching.

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