Lithium niobate end face coupler
By combining a multilayer silicon nitride dielectric layer vertical transfer module with a lithium niobate waveguide structure, the problem of size mismatch between lithium niobate waveguide and optical fiber mode is solved, achieving efficient and low-loss optical fiber coupling, adapting to large mode field optical fibers, and improving the robustness and process compatibility of the system.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INST OF SEMICONDUCTORS - CHINESE ACAD OF SCI
- Filing Date
- 2026-02-24
- Publication Date
- 2026-05-01
AI Technical Summary
Existing lithium niobate waveguides suffer from mode size mismatches with optical fibers or other on-chip photonic platforms, resulting in low end-face coupling efficiency, high substrate absorption loss, and difficulty in achieving large mode field fiber matching.
A multilayer silicon nitride dielectric layer vertical transfer module is combined with a lithium niobate waveguide structure. Through layer-by-layer coupling, the optical signal is gradually coupled from the silicon nitride dielectric layer near the substrate to the silicon nitride dielectric layer far away from the substrate, which increases the distance between the mode field and the substrate and reduces the substrate absorption loss. The thickness and refractive index of the multilayer silicon nitride dielectric layer can be flexibly adjusted to adapt to different mode field sizes.
It significantly improves fiber coupling efficiency, reduces substrate absorption loss, achieves broad adaptability and high robustness to different mode fields, and enhances process compatibility.
Smart Images
Figure CN121956243A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of optical signal processing technology, and in particular to a lithium niobate end-face coupler. Background Technology
[0002] In recent years, the rapid development of large-scale data centers, artificial intelligence computing, and high-performance information processing systems has led to a continuous increase in demand for high-speed, low-power, and high-reliability chip-level interconnects. Traditional electronic interconnects are gradually approaching their physical limits in terms of bandwidth density, energy consumption, and transmission distance, making it difficult to support the future growth trend of information systems. In contrast, optical interconnects, with their ultra-high bandwidth, low transmission loss, and excellent electromagnetic interference resistance, are considered an important technological direction for breaking through interconnect bottlenecks. Photonic integrated devices have therefore become the core foundation of the next generation of high-performance interconnect systems. Among many material systems, thin-film lithium niobate on insulator (LNOI) has attracted much attention due to its excellent electro-optic modulation performance and wide operating bandwidth, and has become an important platform for building high-speed, low-power modulators and on-chip optical signal processing devices.
[0003] However, LNOI waveguides typically exhibit strong confinement and small mode field characteristics, leading to significant mode size mismatches with optical fibers or other on-chip photonic platforms, thus limiting end-face coupling efficiency. Furthermore, lithium niobate has a high refractive index, causing the optical mode field to propagate close to the substrate, making it susceptible to substrate absorption and introducing additional losses. Existing end-face coupling schemes often employ single-layer dielectric transitions or tapered structures to improve coupling performance, but these generally suffer from insufficient substrate absorption suppression, limited mode field enhancement, small process tolerances, and difficulty in achieving fiber matching for large mode fields. Therefore, constructing a lithium niobate end-face coupling structure that can effectively enhance mode field height, reduce substrate loss, and simultaneously possess a wide adaptability range and high robustness has become a key technological requirement in current photonic integration and optical interconnect systems. Summary of the Invention
[0004] In view of the above problems, the present invention provides a lithium niobate end-face coupler.
[0005] An embodiment of the present invention provides a lithium niobate end-face coupler, comprising: a lithium niobate waveguide structure for guiding and processing optical signals; and a vertical transfer module disposed above the lithium niobate waveguide structure. The vertical transfer module includes multiple silicon nitride dielectric layers arranged sequentially in the vertical direction. The vertical transfer module is used to couple optical signals step by step to the top silicon nitride dielectric layer in the multiple silicon nitride dielectric layers, and uses the output optical signal of the top silicon nitride dielectric layer as the output optical field of the lithium niobate end-face coupler.
[0006] According to an embodiment of the present invention, the lithium niobate waveguide structure includes a first lithium niobate waveguide and a second lithium niobate waveguide, wherein the first lithium niobate waveguide is connected to the second lithium niobate waveguide; the first lithium niobate waveguide is used to output an optical signal to the second lithium niobate waveguide; and the second lithium niobate waveguide is used to amplify the mode field of the optical signal.
[0007] According to an embodiment of the present invention, the second lithium niobate waveguide is tapered, and its width gradually decreases in the direction away from the first lithium niobate waveguide.
[0008] According to an embodiment of the present invention, the lithium niobate waveguide structure further includes a third lithium niobate waveguide and a conversion device connecting the first lithium niobate waveguide and the third lithium niobate waveguide; the third lithium niobate waveguide is used to provide an optical signal having a first mode field distribution; the conversion device is used to convert the optical signal having the first mode field distribution into an optical signal having a second mode field distribution, wherein the constraint of the second mode field distribution is lower than that of the first mode field distribution; the first lithium niobate waveguide is used to output an optical signal having the second mode field distribution to the second lithium niobate waveguide.
[0009] According to an embodiment of the present invention, a first lithium niobate waveguide is symmetrically disposed on both sides of a third lithium niobate waveguide.
[0010] According to an embodiment of the present invention, the projection of the second lithium niobate waveguide and the bottommost silicon nitride dielectric layer in the multilayer silicon nitride dielectric layer overlaps in the vertical direction; the multilayer silicon nitride dielectric layers are arranged collinearly along the central axis in the width direction; adjacent silicon nitride dielectric layers are spaced apart in the horizontal direction and their projections overlap in the vertical direction.
[0011] According to an embodiment of the present invention, the lithium niobate waveguide structure and the multilayer silicon nitride dielectric layer are both symmetrically arranged with respect to the central axis along the width direction, and the central axes are collinear.
[0012] According to an embodiment of the present invention, the vertical transfer module further includes an interlayer coupling structure; the interlayer coupling structure is respectively disposed between two adjacent silicon nitride dielectric layers and between the lithium niobate waveguide structure and the bottommost silicon nitride dielectric layer in the multilayer silicon nitride dielectric layers; the interlayer coupling structure is used to realize the directional transfer of optical signals between two adjacent silicon nitride dielectric layers and between the lithium niobate waveguide structure and the bottommost silicon nitride dielectric layer.
[0013] According to an embodiment of the present invention, the thickness of each silicon nitride dielectric layer is 10 nm to 1 μm.
[0014] According to an embodiment of the present invention, both the lithium niobate waveguide structure and the vertical transfer module are disposed inside the cladding, the cladding material being silicon dioxide; a buried oxide layer is disposed below the cladding, the bottom of the lithium niobate waveguide structure being in contact with the buried oxide layer; a substrate is disposed below the buried oxide layer, the substrate material being silicon or quartz.
[0015] The lithium niobate end-face coupler provided by this invention has at least the following technical advantages:
[0016] 1. When the lithium niobate end-face coupler is used as an output coupler, the optical mode field can be coupled layer by layer from the lithium niobate waveguide structure to the upper silicon nitride waveguide, realizing the gradual upward shift of the optical field. This significantly increases the vertical distance between the optical signal mode field and the substrate, effectively suppresses optical loss caused by substrate absorption, and improves the end-face fiber coupling efficiency. It can also be adapted to large mode field fibers to achieve high-efficiency end-face coupling.
[0017] 2. The number, thickness, and refractive index distribution of the multilayer silicon nitride dielectric layers can be flexibly adjusted according to the mode field size of the input optical fiber and the system requirements, thereby achieving wide adaptability to different mode field sizes and significantly enhancing the device's tolerance to process deviations, making the end-face coupling structure have low loss, high robustness, and strong process compatibility. Attached Figure Description
[0018] The above-described features, other objects, and advantages of the present invention will become clearer from the following description of embodiments of the invention with reference to the accompanying drawings, in which:
[0019] Figure 1 A schematic diagram of a lithium niobate end-face coupler according to an embodiment of the present invention is shown.
[0020] Figure 2 A schematic top view of a lithium niobate end-face coupler according to an embodiment of the present invention is shown;
[0021] Figure 3 A schematic cross-sectional view of a lithium niobate end-face coupler according to an embodiment of the present invention is shown.
[0022] Figure reference numerals: 1-Lithium niobate waveguide structure; 11-First lithium niobate waveguide; 12-Second lithium niobate waveguide; 13-Third lithium niobate waveguide; 2-Vertical transfer module; 21-Silicon nitride dielectric layer; 3-Clad layer; 4-Buried oxide layer; 5-Substrate. Detailed Implementation
[0023] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the invention for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.
[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.
[0025] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.
[0026] Figure 1 A schematic diagram of a lithium niobate end-face coupler according to an embodiment of the present invention is shown.
[0027] like Figure 1 As shown, an embodiment of the present invention provides a lithium niobate end-face coupler, comprising: a lithium niobate waveguide structure 1 for guiding and processing optical signals; and a vertical transfer module 2 disposed above the lithium niobate waveguide structure 1. The vertical transfer module 2 includes multiple silicon nitride dielectric layers 21 arranged sequentially in the vertical direction. The vertical transfer module 2 is used to couple the optical signal step by step to the top silicon nitride dielectric layer 21 in the multiple silicon nitride dielectric layers 21, and uses the output optical signal of the top silicon nitride dielectric layer 21 as the output optical field of the lithium niobate end-face coupler.
[0028] Among them, the lithium niobate waveguide structure 1 utilizes the strong electro-optic effect of lithium niobate, integrating electrodes on both sides or above it. Applying an electric field can rapidly change the refractive index of the waveguide, thereby modulating the phase, intensity, or polarization of the transmitted light wave.
[0029] When the lithium niobate end-face coupler provided in the embodiments of the present invention is used as an output coupler, the vertical transfer module 2 couples the optical signal in the lithium niobate waveguide structure 1 from the lithium niobate waveguide structure 1 close to the substrate 5 to the topmost silicon nitride dielectric layer 21 far away from the substrate 5. The topmost silicon nitride dielectric layer 21 serves as the final output waveguide of the lithium niobate end-face coupler. Since the topmost silicon nitride dielectric layer 21 is located at the top of the vertical stacked structure, the physical distance between the optical field and the lossy substrate 5 below is the largest, effectively eliminating the absorption loss of the substrate 5.
[0030] For example, the cross-section of the top silicon nitride dielectric layer 21 can be optimized to support a large-sized, nearly circularly symmetric fundamental mode, maximizing the overlap integral between this mode and the fiber mode field, thereby achieving high-efficiency coupling. Furthermore, by directly increasing the cross-sectional size of the output waveguide of the top silicon nitride dielectric layer 21, the diameter of the supported optical mode field can be linearly increased, making the output mode field size adjustable. By designing the top silicon nitride dielectric layer 21 to have a square or near-square cross-section and using sufficient thickness, the light can be constrained similarly in the horizontal and vertical directions, resulting in a highly circularly symmetric output mode field that perfectly matches the circular mode field of the outer fiber, thus adapting to large-mode-field fibers.
[0031] For example, the shape of the silicon nitride dielectric layer 21 can be any shape of waveguide in a broad range, such as rectangular, tapered, or parabolic waveguides.
[0032] For example, the coupling end between the top silicon nitride dielectric layer 21 and the external optical fiber can be optimized through a mode adaptation structure, which may include a tapered waveguide, a focused waveguide, or a spot conversion structure to further reduce coupling loss and increase coupling bandwidth.
[0033] For example, each silicon nitride dielectric layer 21 can be composed of any chemical composition and can be prepared using any process, including but not limited to low-pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, sputtering or atomic layer deposition. The thickness, refractive index and spacing of each silicon nitride dielectric layer 21 can be adjusted according to the mode field size of the input optical fiber and system requirements, thereby achieving a wide range of adaptability to optical fibers with different mode fields, while improving the device's tolerance to process errors and overall robustness.
[0034] Figure 2 A schematic top view of a lithium niobate end-face coupler according to an embodiment of the present invention is shown.
[0035] like Figure 2 As shown, the lithium niobate waveguide structure 1 includes a first lithium niobate waveguide 11 and a second lithium niobate waveguide 12, with the first lithium niobate waveguide 11 connected to the second lithium niobate waveguide 12; the first lithium niobate waveguide 11 is used to output optical signals to the second lithium niobate waveguide 12; and the second lithium niobate waveguide 12 is used to amplify the mode field of the optical signals.
[0036] For example, the first lithium niobate waveguide 11 can be a lithium niobate strip waveguide. The lithium niobate strip waveguide is a rectangular waveguide formed by etching. It has a larger mode field size and weaker confinement than the lithium niobate ridge waveguide. Therefore, the mode field of the optical signal in the lithium niobate strip waveguide is more easily coupled with other waveguide modes.
[0037] It is important to understand that adiabatic conditions refer to the conditions under which a system can maintain its eigenstate (or mode) unchanged during parameter changes.
[0038] In some embodiments, the second lithium niobate waveguide is tapered and its width gradually decreases in the direction away from the first lithium niobate waveguide 11.
[0039] In the second lithium niobate waveguide 12, because the waveguide width changes gradually and slowly, the optical field can maintain its initial mode characteristics during propagation, thus achieving adiabatic transmission. This adiabatic transmission characteristic ensures that the optical field does not suffer energy loss due to mode mismatch or scattering when propagating in the waveguide, thereby guaranteeing the amplification effect of the optical mode field.
[0040] When the width of the second lithium niobate waveguide 12 is reduced to a value much smaller than the wavelength of the optical signal, the optical field of the optical signal cannot be effectively confined and will diffuse violently into the surrounding cladding material 3, thereby achieving adiabatic amplification of the optical signal.
[0041] like Figure 2 As shown, the lithium niobate waveguide structure 1 also includes a third lithium niobate waveguide 13 and a conversion device connecting the first lithium niobate waveguide 11 and the third lithium niobate waveguide 13; the third lithium niobate waveguide 13 is used to provide an optical signal with a first mode field distribution; the conversion device is used to convert the optical signal with the first mode field distribution into an optical signal with a second mode field distribution, the second mode field distribution having a lower degree of constraint than the first mode field distribution; the first lithium niobate waveguide 11 is used to output an optical signal with a second mode field distribution to the second lithium niobate waveguide 12.
[0042] For example, the third lithium niobate waveguide 13 can be a lithium niobate ridge waveguide. In a lithium niobate ridge waveguide, the ridge structure provides a high refractive index difference contrast in both the horizontal and vertical directions, thereby tightly confining the optical signal in the ridge region for functions such as modulation and wavelength conversion.
[0043] For example, the conversion device is a longitudinal conical structure. Along the direction of light propagation, the etching depth or ridge width of the lithium niobate ridge waveguide changes slowly, gradually evolving into the geometry of the lithium niobate strip waveguide. This slow change forces the optical mode field to evolve adiabatically, without radiation loss.
[0044] Based on the lithium niobate end-face coupler provided in this embodiment of the invention, the optical signal is gradually transmitted from the third lithium niobate waveguide 13 to the first lithium niobate waveguide 11, which reduces the constraint of the difficult-to-extract strong-constraint optical signal with low loss in the first stage, providing a physical basis for subsequent mode field amplification and vertical coupling.
[0045] It's important to understand that thin-film lithium niobate is classified into two types based on the crystal cutting direction: X-cut and Z-cut. In Z-cut thin-film lithium niobate, the Z-axis (optical axis) of the crystal is perpendicular to the chip surface. This means that the polarization direction of light propagating within the chip plane can be flexibly designed relative to the Z-axis. In X-cut thin-film lithium niobate, the X-axis of the crystal is perpendicular to the chip surface, while the Z-axis (optical axis) lies within the chip plane. X-cut lithium niobate is mainly used in phase modulators, intensity modulators, and nonlinear optical devices.
[0046] In some embodiments, the first lithium niobate waveguide is symmetrically disposed on both sides of the third lithium niobate waveguide 13.
[0047] For example, the thickness of the lithium niobate waveguide structure 1 is 100 nm to 1 μm.
[0048] In some embodiments, the projection of the second lithium niobate waveguide 12 and the bottommost silicon nitride dielectric layer 21 in the multilayer silicon nitride dielectric layer 21 in the vertical direction overlaps; the multilayer silicon nitride dielectric layer 21 is arranged collinearly along the central axis in the width direction; adjacent silicon nitride dielectric layers 21 are spaced apart in the horizontal direction and their projections in the vertical direction overlap.
[0049] The projection of the second lithium niobate waveguide 12 and the bottom silicon nitride dielectric layer 21 in the multilayer silicon nitride dielectric layer 21 overlaps in the vertical direction, which means that the optical signal after mode field amplification can penetrate into the bottom silicon nitride dielectric layer 21. At this time, the length of the overlapping area is the coupling length, which provides sufficient interaction space for thermal insulation or phase matching coupling.
[0050] As light moves vertically upwards layer by layer, each layer of coupling requires the mode field centers of the upper and lower waveguides to be aligned horizontally. The collinear central axes of the multilayer silicon nitride dielectric layers 21 ensure that each stage of coupling is performed under optimal alignment, avoiding mode mismatch and radiation loss caused by lateral offset.
[0051] The two adjacent silicon nitride dielectric layers 21 are not in direct contact in the vertical direction, but are separated by a silicon dioxide spacer layer. The horizontal projections of the two adjacent silicon nitride waveguides partially overlap, forming a parallel coupling region. The coupling region provides an interaction length along the direction of light propagation for energy transfer.
[0052] like Figure 2 As shown, the lithium niobate waveguide structure and the multilayer silicon nitride dielectric layer 21 are both symmetrically arranged with their central axes along the width direction, and the central axes are collinear.
[0053] It is important to understand that light propagates along its central axis (i.e., the trajectory of the energy center of the optical mode field) within a waveguide. When the central axes of all functional waveguides are collinear, it means that the propagation axis of light is a straight line without lateral offset throughout the entire process from the lithium niobate waveguide structure 1 to the vertical coupling to the silicon nitride dielectric layer 21. This provides the most direct, stable, and controllable evolution path for mode field transformation of the optical signal (whether it is tapered amplification in the horizontal direction or interlayer coupling in the vertical direction). Simultaneously, the efficiency of optical signal coupling from the lower lithium niobate waveguide structure 1 to the upper silicon nitride dielectric layer 21 depends on the degree of overlap of the two waveguide mode fields in the vertical direction. When their central axes are strictly coincident in the horizontal projection, their symmetrical mode fields can achieve maximum overlap, thereby maximizing the coupling coefficient of the interlayer coupler.
[0054] Furthermore, the vertical transfer module 2 also includes an interlayer coupling structure; the interlayer coupling structure is respectively disposed between two adjacent silicon nitride dielectric layers 21 and between the lithium niobate waveguide structure 1 and the bottommost silicon nitride dielectric layer 21 in the multilayer silicon nitride dielectric layers 21; the interlayer coupling structure is used to realize the directional transfer of optical signals between two adjacent silicon nitride dielectric layers 21 and between the lithium niobate waveguide structure 1 and the bottommost silicon nitride dielectric layer 21.
[0055] For example, the interlayer coupling structure can be an adiabatic directional coupler or a non-adiabatic directional coupler.
[0056] Setting up interlayer coupling structures between silicon nitride dielectric layers 21 can achieve higher single-stage transfer efficiency; interlayer coupling structures (especially thermally adiabatic ones) are relatively less sensitive to process errors (such as small changes in waveguide width and etching depth) and have better robustness.
[0057] In the lithium niobate waveguide structure 1, the second lithium niobate waveguide 12 performs adiabatic amplification of the optical signal mode field in the horizontal plane, and the interlayer coupling structure couples this optical signal to the silicon nitride dielectric layer 21. Lithium niobate and silicon nitride are two materials with completely different refractive indices, dispersion, and nonlinear characteristics. Directly causing light to undergo drastic mode conversion at the interface of the two materials (such as end-face coupling) is extremely inefficient. By introducing an interlayer coupling structure, the lithium niobate part can be independently optimized to obtain the best amplification effect, and the silicon nitride part can also be independently optimized to obtain the lowest transmission loss or the best upward coupling conditions.
[0058] In some embodiments, the thickness of the silicon nitride dielectric layer 21 is 10 nm to 1 μm.
[0059] Figure 3 A schematic cross-sectional view of a lithium niobate end-face coupler according to an embodiment of the present invention is shown.
[0060] like Figure 3As shown, the lithium niobate waveguide structure and the vertical transfer module 2 are both disposed inside the cladding 3, and the material of the cladding 3 includes silicon dioxide; a buried oxide layer 4 is disposed below the cladding 3, and the bottom of the lithium niobate waveguide structure 1 is in contact with the buried oxide layer 4; a substrate 5 is disposed below the buried oxide layer 4, and the material of the substrate 5 includes silicon or quartz.
[0061] In this structure, substrate 5 serves as a mechanical support. Optically, the buried oxide layer 4 confines the optical mode field within the upper lithium niobate film, preventing light leakage into the lossy substrate 5. Electrically, it acts as a good insulator, isolating the upper waveguide and electrodes from the conductive silicon substrate 5, preventing electric field leakage and crosstalk. Cladding 3 surrounds and covers the lithium niobate waveguide structure 1, the mode field adaptation module, and the vertical transfer module 2. Together with the lower buried oxide layer 4, it completely confines the optical field within the predetermined low-loss dielectric (lithium niobate, silicon nitride) from four directions (up, down, left, and right), forming a highly efficient waveguide. This further reduces mode leakage and scattering loss, achieving both physical protection and electrical isolation.
[0062] The lithium niobate end-face coupler provided by this invention can be integrated into an on-chip photonic integration platform and tightly coupled with other photonic devices (such as modulators, beam splitters, delay lines or micro-ring resonators) to form a highly integrated, low-loss optical interconnect or optical signal processing system.
[0063] Those skilled in the art will understand that the features described in the various embodiments of the present invention can be combined or combined in various ways, even if such combinations or combinations are not explicitly described in the present invention. In particular, the features described in the various embodiments of the present invention can be combined and / or combined in various ways without departing from the spirit and teachings of the present invention. All such combinations and / or combinations fall within the scope of the present invention.
[0064] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. The scope of the invention is defined by the appended claims and their equivalents. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.
Claims
1. A lithium niobate end-face coupler, characterized in that, include: A lithium niobate waveguide structure (1) is used to guide and process optical signals; A vertical transfer module (2) is disposed above the lithium niobate waveguide structure (1). The vertical transfer module (2) includes multiple silicon nitride dielectric layers (21) arranged sequentially in the vertical direction. The vertical transfer module (2) is used to couple the optical signal to the top silicon nitride dielectric layer (21) of the multiple silicon nitride dielectric layers (21) step by step, and use the output optical signal of the top silicon nitride dielectric layer (21) as the output optical field of the lithium niobate end coupler.
2. The lithium niobate end-face coupler according to claim 1, characterized in that, The lithium niobate waveguide structure (1) includes a first lithium niobate waveguide (11) and a second lithium niobate waveguide (12), wherein the first lithium niobate waveguide (11) and the second lithium niobate waveguide (12) are connected. The first lithium niobate waveguide (11) is used to output optical signals to the second lithium niobate waveguide (12); The second lithium niobate waveguide (12) is used to amplify the mode field of the optical signal.
3. The lithium niobate end-face coupler according to claim 2, characterized in that, The second lithium niobate waveguide (12) is tapered and its width gradually decreases in the direction away from the first lithium niobate waveguide (11).
4. The lithium niobate end-face coupler according to claim 2, characterized in that, The lithium niobate waveguide structure (1) further includes a third lithium niobate waveguide (13) and a conversion device connecting the first lithium niobate waveguide (11) and the third lithium niobate waveguide (13); The third lithium niobate waveguide (13) is used to provide an optical signal with a first mode field distribution; The conversion device is used to convert the optical signal having a first mode field distribution into an optical signal having a second mode field distribution, wherein the constraint degree of the second mode field distribution is lower than that of the first mode field distribution; The first lithium niobate waveguide (11) is used to output the optical signal with the second mode field distribution to the second lithium niobate waveguide (12).
5. The lithium niobate end-face coupler according to claim 4, characterized in that, The first lithium niobate waveguide (11) is symmetrically arranged on both sides of the third lithium niobate waveguide (13).
6. The lithium niobate end-face coupler according to claim 2, characterized in that, The projection of the second lithium niobate waveguide (12) and the bottom silicon nitride dielectric layer (21) in the multilayer silicon nitride dielectric layer (21) in the vertical direction overlaps; The multilayer silicon nitride dielectric layer (21) is arranged collinearly along the central axis in the width direction; The two adjacent silicon nitride dielectric layers (21) are spaced apart in the horizontal direction and have overlapping regions in their projections in the vertical direction.
7. The lithium niobate end-face coupler according to claim 1, characterized in that, The lithium niobate waveguide structure (1) and the multilayer silicon nitride dielectric layer (21) are both arranged symmetrically with respect to the central axis along the width direction, and the central axes are collinear.
8. The lithium niobate end-face coupler according to claim 1, characterized in that, The vertical transfer module (2) also includes an interlayer coupling structure; The interlayer coupling structures are respectively disposed between two adjacent silicon nitride dielectric layers (21) and between the lithium niobate waveguide structure (1) and the bottommost silicon nitride dielectric layer (21) in the multilayer silicon nitride dielectric layers (21); The interlayer coupling structure is used to realize the directional transfer of optical signals between two adjacent silicon nitride dielectric layers (21) and between the lithium niobate waveguide structure (1) and the bottom silicon nitride dielectric layer (21).
9. The lithium niobate end-face coupler according to claim 1, characterized in that, The thickness of each silicon nitride dielectric layer (21) is 10 nm to 1 μm.
10. The lithium niobate end-face coupler according to claim 1, characterized in that, The lithium niobate waveguide structure (1) and the vertical transfer module (2) are both disposed inside the cladding (3), and the material of the cladding (3) includes silicon dioxide; A buried oxide layer (4) is provided below the cladding (3), and the bottom of the lithium niobate waveguide structure (1) is in contact with the buried oxide layer (4); A substrate (5) is disposed below the buried oxide layer (4), and the material of the substrate (5) includes silicon or quartz.