Compact, high-efficiency and large-bandwidth thin-film lithium niobate end face coupler for large spot size

Abstract

The invention belongs to the technical field of integrated optical devices, and discloses a compact, high-efficiency and large-bandwidth thin-film lithium niobate end face coupler for large die spots, which comprises a substrate and an insulating layer, the single-layer back taper film lithium niobate waveguide layer, the first double-layer back taper film lithium niobate waveguide layer and the second double-layer back taper film lithium niobate waveguide layer are arranged on the insulating layer; the low-refractive-index waveguide layer is arranged in front of the thin-film lithium niobate waveguide, and the thin-film lithium niobate waveguide is covered with the three-dimensional tapering low-refractive-index waveguide wrapping layer. According to the invention, the coupling efficiency and bandwidth are improved by optimizing the low-refractive-index waveguide layer and matching with the mode of a laser chip large spot; the optical mode is efficiently and compactly coupled to the second double-layer inverted-cone waveguide layer by means of the single-layer and double-layer inverted-cone thin film lithium niobate waveguide layers in cooperation with the three-dimensional tapering low-refractive-index waveguide cladding layer and the transition cladding layer.

Description

Technical Field

[0001] This invention belongs to the field of integrated optical device technology, specifically relating to a compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large pattern size. Background Technology

[0002] Lithium niobate (LNiO) has proven to be a promising optical platform in integrated optics due to its wide transparency window, large electro-optic coefficient, and high second-order nonlinear coefficient. However, traditional LNiO waveguides, fabricated using proton exchange or titanium diffusion methods, have very small refractive index differences, resulting in weak light confinement and low overlap between the electric and optical fields. Consequently, devices based on bulk LNiO are typically large. The advent of thin-film LNiO and breakthroughs in etching technology have changed this situation. Thin-film LNiO not only provides high confinement of the optical field but also retains the inherent excellent properties of LNiO. However, the mode size varies significantly (up to the micrometer level) between different optical material platforms, making it difficult to achieve efficient, compact, and broadband coupling from large-mode-size (3-10 micrometers) laser chips to thin-film LNiO platforms (sub-micrometer mode sizes). There is existing research on coupling technology for large mode spots, such as trident end face couplers and multi-cone end face couplers. However, these couplers have low coupling efficiency and small bandwidth, making it difficult to achieve efficient coupling from the O to L bands simultaneously (1dB bandwidth is only 200-300nm). Moreover, in order to achieve the conversion from large mode spots to small mode spots, these devices are too long (350-700um). Summary of the Invention

[0003] To overcome the technical problems of low coupling efficiency, small bandwidth, and excessive length for large mode spots, this invention provides a compact, high-efficiency, and high-bandwidth thin-film lithium niobate end-face coupler for large mode spots. It improves mode matching with the large mode spots of the laser chip by designing a low-refractive-index waveguide layer. Then, by designing a single-layer inverted-tapered thin-film lithium niobate waveguide layer, a first double-layer inverted-tapered thin-film lithium niobate waveguide layer, and a three-dimensionally tapered low-refractive-index waveguide cladding and a low-refractive-index transition waveguide cladding above the thin-film lithium niobate waveguide, it achieves efficient and compact coupling of the optical mode from the low-refractive-index waveguide layer to the second double-layer inverted-tapered thin-film lithium niobate waveguide layer, while simultaneously increasing the operating bandwidth.

[0004] To achieve the above objectives, the present invention provides the following solution: A compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large pattern size, wherein the coupler comprises, from bottom to top, a substrate, an insulating layer on the substrate, a single-layer thin-film lithium niobate inverted conical waveguide layer on the insulating layer, a first double-layer thin-film lithium niobate inverted conical waveguide layer, a second double-layer thin-film lithium niobate inverted conical waveguide layer, a low-refractive-index waveguide layer in front of the single-layer thin-film lithium niobate inverted conical waveguide layer, a three-dimensional tapered low-refractive-index waveguide cladding covering the single-layer thin-film lithium niobate inverted conical waveguide layer, and a low-refractive-index transition waveguide cladding above the first double-layer thin-film lithium niobate inverted conical waveguide layer; Wherein: the first double-layer thin-film lithium niobate inverted conical waveguide layer includes a first double-layer thin-film lithium niobate planar waveguide layer and an upper first double-layer thin-film lithium niobate ridge waveguide layer; one side of the first double-layer thin-film lithium niobate planar waveguide layer is connected to one side of the single-layer thin-film lithium niobate inverted conical waveguide layer. The second double-layer lithium niobate inverted conical waveguide layer includes a second double-layer lithium niobate planar waveguide layer and an upper second double-layer lithium niobate ridge waveguide layer; one side of the second double-layer lithium niobate planar waveguide layer is connected to one side of the first double-layer lithium niobate planar waveguide layer; one side of the second double-layer lithium niobate ridge waveguide layer is connected to one side of the first double-layer lithium niobate ridge waveguide layer. The three-dimensional tapered low-refractive-index waveguide cladding and one side of the low-refractive-index waveguide layer are connected; The low-refractive-index transition waveguide cladding and the three-dimensional tapered low-refractive-index waveguide cladding are connected on one side.

[0005] Preferably, the heights of the single-layer thin-film lithium niobate inverted conical waveguide layer, the first double-layer thin-film lithium niobate planar waveguide layer, and the second double-layer thin-film lithium niobate planar waveguide layer are all equal and less than 250 nm. The heights of the first double-layer lithium niobate ridge waveguide layer and the second double-layer lithium niobate ridge waveguide layer are equal and less than 250 nm. The widths on one side of the second double-layer lithium niobate ridge waveguide layer and the first double-layer lithium niobate ridge waveguide layer are equal.

[0006] Preferably, the length of the single-layer thin-film lithium niobate inverted conical waveguide layer and the length of the three-dimensionally tapered low-refractive-index waveguide cladding are equal and less than 120 μm, and simultaneously meet the following conditions: ; In the formula, The fundamental mode propagation constant of a waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer and a three-dimensionally tapered low-refractive-index waveguide cladding. The propagation constants of adjacent modes of the fundamental mode of a waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer and a three-dimensionally tapered low-refractive-index waveguide cladding are given. It is the length of the single-layer thin-film lithium niobate inverted conical waveguide layer.

[0007] Preferably, the lengths of the first double-layer thin-film lithium niobate planar waveguide layer, the first double-layer thin-film lithium niobate ridge waveguide layer, and the low-refractive-index transition waveguide cladding are equal. The low-refractive-index transition waveguide cladding and the three-dimensional tapered low-refractive-index waveguide cladding have the same width and height on one side. The low-refractive-index waveguide layer and the three-dimensionally tapered low-refractive-index waveguide cladding have the same width and height on one side.

[0008] Preferably, the width and height of the low-refractive-index waveguide layer should be set to match the mode field of the laser chip, specifically: ; In the formula, This represents the overlap integral between the input mode field of the laser source and the low-refractive-index waveguide layer. It is the input mode field of the laser source. It is the mode field of the low-refractive-index waveguide layer. It is the complex conjugate of the input mode field of the laser source. The complex conjugate of the mode field of the low-refractive-index waveguide layer, where x and y represent the spatial coordinates of the horizontal and vertical directions of the end face cross section, respectively.

[0009] Preferably, the refractive index of the first double-layer thin-film lithium niobate inverted conical waveguide layer is greater than that of the single-layer thin-film lithium niobate inverted conical waveguide layer. The width of the single-layer thin-film lithium niobate inverted conical waveguide layer decreases linearly from one end near the first double-layer thin-film lithium niobate inverted conical waveguide layer to the end far from the first double-layer thin-film lithium niobate inverted conical waveguide layer, forming an inverted conical structure.

[0010] Preferably, the widths of the first double-layer lithium niobate planar waveguide layer and the first double-layer lithium niobate ridge waveguide layer decrease linearly from one end near the second double-layer lithium niobate inverted conical waveguide layer to the end away from the second double-layer lithium niobate inverted conical waveguide layer, forming an inverted conical structure.

[0011] Preferably, the width and height of the three-dimensionally tapered low-refractive-index waveguide cladding decrease linearly from one end near the low-refractive-index waveguide layer to the other end away from the low-refractive-index waveguide layer, forming a wedge shape.

[0012] Preferably, the low-refractive-index waveguide layer, the three-dimensionally tapered low-refractive-index waveguide cladding, and the low-refractive-index transition waveguide cladding comprise a low-refractive-index polymer.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention improves mode matching with the large mode of the laser chip by setting a low-refractive-index waveguide layer, reduces reflection caused by refractive index mismatch, and broadens the bandwidth to improve coupling efficiency; it achieves efficient conversion from low-refractive-index waveguide mode to lithium niobate ridge waveguide mode by setting an inverted conical structure composed of a single-layer lithium niobate inverted conical waveguide layer and a first double-layer lithium niobate inverted conical waveguide layer; and it achieves efficient coupling of the device in a compact structure by setting a three-dimensional tapered low-refractive-index waveguide cladding and a low-refractive-index transition waveguide cladding. Attached Figure Description

[0014] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. 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 effort.

[0015] Figure 1 This is a schematic diagram of the structure of a compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large pattern size according to an embodiment of the present invention. Figure 2 This is a top view of a compact, high-efficiency, and high-bandwidth thin-film lithium niobate end-face coupler for large pattern size according to an embodiment of the present invention; Figure 3 This is a front view of a compact, high-efficiency, and high-bandwidth thin-film lithium niobate end-face coupler for large pattern size according to an embodiment of the present invention; Figure 4 The following are simulation experiment diagrams of embodiments of the present invention; wherein, (I) is a schematic diagram of the effect of the length of the measurement device on the change of coupling efficiency by designing simulation; (II) is a schematic diagram of the effect of coupling loss caused by alignment deviation in the actual measurement process; and (III) is a schematic diagram of the change of coupling efficiency at different wavelengths. Wherein, 1-substrate, 2-insulating layer, 3-single-layer thin-film lithium niobate inverted conical waveguide layer, 4-first double-layer thin-film lithium niobate inverted conical waveguide layer, 5-second double-layer thin-film lithium niobate inverted conical waveguide layer, 6-low refractive index waveguide layer, 7-three-dimensional tapered low refractive index waveguide cladding, 8-low refractive index transition waveguide cladding, 9-first double-layer thin-film lithium niobate planar waveguide layer, 10-first double-layer thin-film lithium niobate ridge waveguide layer, 11-second double-layer thin-film lithium niobate planar waveguide layer, 12-second double-layer thin-film lithium niobate ridge waveguide layer. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] Example 1 This invention provides a compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large-pattern coverage. From bottom to top, the structure comprises: a substrate 1; an insulating layer 2 on the substrate 1; a single-layer thin-film lithium niobate inverted tapered waveguide layer 3 on the insulating layer 2; a first double-layer thin-film lithium niobate inverted tapered waveguide layer 4; a second double-layer thin-film lithium niobate inverted tapered waveguide layer 5; a low-refractive-index waveguide layer 6 in front of the single-layer thin-film lithium niobate inverted tapered waveguide layer 3; a three-dimensionally tapered low-refractive-index waveguide cladding 7 covering the single-layer thin-film lithium niobate inverted tapered waveguide layer 3; and a low-refractive-index transition waveguide cladding 8 above the first double-layer thin-film lithium niobate inverted tapered waveguide layer 4; wherein: The first double-layer thin-film lithium niobate inverted conical waveguide layer 4 includes a first double-layer thin-film lithium niobate planar waveguide layer 9 and an upper first double-layer thin-film lithium niobate ridge waveguide layer 10; one side of the second double-layer thin-film lithium niobate planar waveguide layer 11 is connected to one side of the single-layer thin-film lithium niobate inverted conical waveguide layer 3. The second double-layer lithium niobate inverted conical waveguide layer 5 includes a second double-layer lithium niobate planar waveguide layer 11 and an upper second double-layer lithium niobate ridge waveguide layer 12; one side of the second double-layer lithium niobate planar waveguide layer 11 is connected to one side of the first double-layer lithium niobate planar waveguide layer 9; one side of the second double-layer lithium niobate ridge waveguide layer 12 is connected to one side of the first double-layer lithium niobate ridge waveguide layer 10. The three-dimensional tapered low-refractive-index waveguide cladding 7 and the low-refractive-index waveguide layer 6 are connected on one side. The low-refractive-index transition waveguide cladding 8 and the three-dimensional tapered low-refractive-index waveguide cladding 7 are connected on one side.

[0019] In this embodiment, the heights of the single-layer thin-film lithium niobate inverted conical waveguide layer 3, the first double-layer thin-film lithium niobate planar waveguide layer 9, and the second double-layer thin-film lithium niobate planar waveguide layer 11 are all equal and less than 250 nm.

[0020] In this embodiment, the heights of the first double-layer lithium niobate ridge waveguide layer 10 and the second double-layer lithium niobate ridge waveguide layer 12 are equal and less than 250 nm.

[0021] In this embodiment, the widths on one side of the second double-layer lithium niobate ridge waveguide layer 12 and the first double-layer lithium niobate ridge waveguide layer 10 are equal.

[0022] In this embodiment, the length of the single-layer thin-film lithium niobate inverted conical waveguide layer 3 and the length of the three-dimensional tapered low-refractive-index waveguide cladding layer 7 are equal and less than 120 μm, and simultaneously satisfy the condition shown in equation (I): (I) In formula (Ⅰ), The fundamental mode propagation constant of the waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer 3 and a three-dimensionally tapered low-refractive-index waveguide cladding layer 7 is given. The propagation constants of adjacent modes of the fundamental mode of the waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer 3 and a three-dimensionally tapered low-refractive-index waveguide cladding layer 7 are given. It is the length of the single-layer thin-film lithium niobate inverted conical waveguide layer 3.

[0023] In this embodiment, the lengths of the first double-layer thin-film lithium niobate planar waveguide layer 9, the first double-layer thin-film lithium niobate ridge waveguide layer 10, and the low-refractive-index transition waveguide cladding layer 8 are equal.

[0024] In this embodiment, the low-refractive-index transition waveguide cladding 8 and the three-dimensional tapered low-refractive-index waveguide cladding 7 have the same width and height on one side.

[0025] In this embodiment, the low-refractive-index waveguide layer 6 and the three-dimensionally tapered low-refractive-index waveguide cladding layer 7 have the same width and height on one side.

[0026] In this embodiment, the refractive index of the first double-layer thin-film lithium niobate inverted conical waveguide layer 4 is greater than that of the single-layer thin-film lithium niobate inverted conical waveguide layer 3.

[0027] In this embodiment, the width and height of the low-refractive-index waveguide layer 6 are set to match the mode field of the laser chip.

[0028] In this embodiment, the widths of the first double-layer lithium niobate planar waveguide layer 9 and the first double-layer lithium niobate ridge waveguide layer 10 decrease linearly from one end near the second double-layer lithium niobate inverted conical waveguide layer 5 to the end away from the second double-layer lithium niobate inverted conical waveguide layer 5, forming an inverted conical structure.

[0029] In this embodiment, the width of the single-layer thin-film lithium niobate inverted conical waveguide layer 3 decreases linearly from one end near the first double-layer thin-film lithium niobate inverted conical waveguide layer 4 to the end away from the first double-layer thin-film lithium niobate inverted conical waveguide layer 4, forming an inverted conical structure.

[0030] In this embodiment, the width and height of the three-dimensionally tapered low-refractive-index waveguide cladding 7 decrease linearly from one end near the low-refractive-index waveguide layer 6 to the end away from the low-refractive-index waveguide layer 6, forming a wedge shape.

[0031] In this embodiment, the low-refractive-index waveguide layer 6, the three-dimensionally tapered low-refractive-index waveguide cladding layer 7, and the low-refractive-index transition waveguide cladding layer 8 comprise low-refractive-index polymers.

[0032] Example 2 This embodiment proposes a compact, high-efficiency, and high-bandwidth thin-film lithium niobate end-face coupler for large-pattern areas, such as... Figures 1-3 The diagram shown is a structural schematic of a compact, high-efficiency, and high-bandwidth thin-film lithium niobate end-face coupler for large pattern size in this embodiment.

[0033] The compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large pattern size proposed in this embodiment includes: Substrate 1; Insulating layer 2 on substrate 1; A single-layer thin-film lithium niobate inverted conical waveguide layer 3 on insulating layer 2, a first double-layer thin-film lithium niobate inverted conical waveguide layer 4, and a second double-layer thin-film lithium niobate inverted conical waveguide layer 5; Low-refractive-index waveguide layer 6 in front of single-layer thin-film lithium niobate inverted conical waveguide layer 3; Three-dimensional tapered low-refractive-index waveguide cladding 7 on a single-layer thin-film lithium niobate inverted tapered waveguide layer 3; Low-refractive-index transition waveguide cladding 8 on the second double-layer thin-film lithium niobate inverted conical waveguide layer 5; The first double-layer thin-film lithium niobate inverted conical waveguide layer 4 includes a first double-layer thin-film lithium niobate planar waveguide layer 9 and an upper first double-layer thin-film lithium niobate ridge waveguide layer 10; one side of the first double-layer thin-film lithium niobate planar waveguide layer 9 is connected to one side of the single-layer thin-film lithium niobate inverted conical waveguide layer 3. The second double-layer lithium niobate inverted conical waveguide layer 5 includes a second double-layer lithium niobate planar waveguide layer 11 and an upper second double-layer lithium niobate ridge waveguide layer 12; one side of the second double-layer lithium niobate planar waveguide layer 11 is connected to one side of the first double-layer lithium niobate planar waveguide layer 9; one side of the second double-layer lithium niobate ridge waveguide layer 12 is connected to one side of the first double-layer lithium niobate ridge waveguide layer 10. The three-dimensional tapered low-refractive-index waveguide cladding 7 and the low-refractive-index waveguide layer 6 are connected on one side.

[0034] One side of the low-refractive-index transition waveguide cladding 8 and the three-dimensional tapered low-refractive-index waveguide cladding 7 are connected.

[0035] According to the formula , This represents the overlap integral of the input mode field of the laser source and the low-refractive-index waveguide layer 6. It is the input mode field of the laser source. It is the complex conjugate of the input mode field of the laser source. It is the mode field of low-refractive-index waveguide layer 6. The complex conjugate of the mode field of the low-refractive-index waveguide layer 6, where x and y represent the spatial coordinates of the horizontal and vertical directions of the end face cross-section, respectively. In this embodiment, by setting the width and height of the low-refractive-index waveguide layer 6, the mode field of the low-refractive-index waveguide layer 6 is changed, thereby improving the high-mode overlap with the laser source mode field, increasing coupling efficiency, and simultaneously reducing effective refractive index mismatch and reducing reflectivity.

[0036] To ensure efficient coupling of modes from the low-refractive-index waveguide layer 6 to the monolayer lithium niobate inverted conical waveguide layer 3, the tip of the monolayer lithium niobate inverted conical waveguide layer 3 is pointed towards the low-refractive-index waveguide layer 6. To mitigate losses caused by abrupt structural changes, the tip width should be set to less than 200 nm. Due to the small tip width, modes cannot be confined to the tip of the monolayer lithium niobate inverted conical waveguide layer 3 and will diffuse into the three-dimensionally tapered low-refractive-index matching waveguide cladding 7. As the width of the monolayer lithium niobate inverted conical waveguide layer 3 gradually increases, the modes are gradually confined within the monolayer lithium niobate inverted conical waveguide layer 3.

[0037] To further transition the mode to a more confined ridge waveguide mode, a first double-layer lithium niobate planar waveguide layer 9 and a first double-layer lithium niobate ridge waveguide layer 10 are introduced. The tip of the first double-layer lithium niobate ridge waveguide layer 10 should point towards the single-layer lithium niobate inverted conical waveguide layer 3. Similarly, to reduce losses caused by abrupt structural changes in the vertical direction, the tip width of the first double-layer lithium niobate ridge waveguide layer 10 is set to be less than 200 nm. As the widths of the first double-layer lithium niobate planar waveguide layer 9 and the first double-layer lithium niobate ridge waveguide layer 10 increase, the waveguide mode of the first double-layer lithium niobate planar waveguide layer 9 gradually converts to a ridge waveguide mode. Then, other thin-film lithium niobate devices are connected via a second double-layer lithium niobate inverted conical waveguide layer 5. During the coupling of the low-refractive-index waveguide layer 6 mode to the single-layer lithium niobate inverted conical waveguide layer 3 mode, to avoid the fundamental mode shifting to unwanted adjacent modes while reducing the device length, based on length scale and weak power transfer criteria... , It is the fundamental propagation mode. It is the propagation constant of adjacent modes. It is a tapered length. In this embodiment, by designing a three-dimensional tapered low-refractive-index waveguide cladding 7, the height and width of the three-dimensional tapered low-refractive-index waveguide cladding 7 are gradually varied to form a wedge structure, which changes the effective refractive index of the mode, thereby increasing the refractive index difference between the fundamental mode and adjacent modes, making the length required for the device to achieve thermal coupling shorter.

[0038] In this embodiment, the heights of the first double-layer lithium niobate planar waveguide layer 9 and the second double-layer lithium niobate planar waveguide layer 11 are set to be the same and less than 250 nm. The heights of the first double-layer lithium niobate ridge waveguide layer 10 and the second double-layer lithium niobate ridge waveguide layer 12 are set to be the same and less than 250 nm. The width of the second double-layer lithium niobate ridge waveguide layer 12 is set to be the same as the width of one side of the first double-layer lithium niobate ridge waveguide layer 10.

[0039] In this embodiment, the low-refractive-index waveguide layer 6, the three-dimensional tapered low-refractive-index waveguide cladding layer 7, and the low-refractive-index transition waveguide cladding layer 8 are made of low-refractive-index polymers.

[0040] This embodiment can achieve compact, efficient, and wide-bandwidth coupling for large-spot conditions such as lasers, with a total device length of less than 200 micrometers, achieving coupling loss of less than 0.5dB in the O to L bands.

[0041] like Figure 4 The diagram shown is a simulation experiment diagram of this embodiment. Figure 4 (I) This embodiment demonstrates that the device only needs a length of less than 200 micrometers to achieve high-efficiency coupling by designing a simulation measurement of the effect of device length on the change of coupling efficiency, which is beneficial to the packaging of the device. Figure 4 (II) This shows the effect of coupling loss caused by alignment deviation during the actual measurement process of this embodiment. It can be seen that the device has a high tolerance for errors caused by alignment. Figure 4 (III) shows the change in coupling efficiency of this embodiment at different wavelengths, indicating that the coupling loss of the device in the 0-L band is less than 0.5dB and has a very large operating bandwidth.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A compact, high-efficiency, and wide-bandwidth thin-film lithium niobate end-face coupler for large pattern sizes, characterized in that, The coupler, from bottom to top, consists of a substrate (1), an insulating layer (2) on the substrate (1), a single-layer thin-film lithium niobate inverted conical waveguide layer (3) on the insulating layer (2), a first double-layer thin-film lithium niobate inverted conical waveguide layer (4), a second double-layer thin-film lithium niobate inverted conical waveguide layer (5), a low-refractive-index waveguide layer (6) in front of the single-layer thin-film lithium niobate inverted conical waveguide layer (3), a three-dimensional tapered low-refractive-index waveguide cladding layer (7) covering the single-layer thin-film lithium niobate inverted conical waveguide layer (3), and a low-refractive-index transition waveguide cladding layer (8) above the first double-layer thin-film lithium niobate inverted conical waveguide layer (4). Wherein: the first double-layer thin-film lithium niobate inverted conical waveguide layer (4) includes a first double-layer thin-film lithium niobate planar waveguide layer (9) and an upper first double-layer thin-film lithium niobate ridge waveguide layer (10); one side of the first double-layer thin-film lithium niobate planar waveguide layer (9) is connected to one side of the single-layer thin-film lithium niobate inverted conical waveguide layer (3); The second double-layer lithium niobate inverted cone waveguide layer (5) includes a second double-layer lithium niobate planar waveguide layer (11) and an upper second double-layer lithium niobate ridge waveguide layer (12); one side of the second double-layer lithium niobate planar waveguide layer (11) is connected to one side of the first double-layer lithium niobate planar waveguide layer (9); one side of the second double-layer lithium niobate ridge waveguide layer (12) is connected to one side of the first double-layer lithium niobate ridge waveguide layer (10); The three-dimensional tapered low-refractive-index waveguide cladding (7) and the low-refractive-index waveguide layer (6) are connected on one side. The low-refractive-index transition waveguide cladding (8) and the three-dimensional tapered low-refractive-index waveguide cladding (7) are connected on one side.

2. The coupler according to claim 1, characterized in that, The heights of the single-layer thin-film lithium niobate inverted conical waveguide layer (3), the first double-layer thin-film lithium niobate planar waveguide layer (9), and the second double-layer thin-film lithium niobate planar waveguide layer (11) are all equal and less than 250 nm. The heights of the first double-layer thin-film lithium niobate ridge waveguide layer (10) and the second double-layer thin-film lithium niobate ridge waveguide layer (12) are equal and less than 250 nm. The widths on one side of the second double-layer lithium niobate ridge waveguide layer (12) and the first double-layer lithium niobate ridge waveguide layer (10) are equal.

3. The coupler according to claim 1, characterized in that, The length of the single-layer thin-film lithium niobate inverted conical waveguide layer (3) is equal to and less than 120 μm in length with the three-dimensional tapered low-refractive-index waveguide cladding layer (7), and simultaneously satisfies the following conditions: ； In the formula, The fundamental mode propagation constant of the waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer (3) and a three-dimensional tapered low-refractive-index waveguide cladding layer (7) is given. The propagation constants of adjacent modes of the fundamental mode of a waveguide composed of a single-layer thin-film lithium niobate inverted conical waveguide layer (3) and a three-dimensionally tapered low-refractive-index waveguide cladding layer (7) are given. It is the length of the single-layer thin-film lithium niobate inverted cone waveguide layer (3).

4. The coupler according to claim 1, characterized in that, The lengths of the first double-layer thin-film lithium niobate planar waveguide layer (9), the first double-layer thin-film lithium niobate ridge waveguide layer (10), and the low-refractive-index transition waveguide cladding layer (8) are equal. The low-refractive-index transition waveguide cladding (8) and the three-dimensional tapered low-refractive-index waveguide cladding (7) have the same width and height on one side; The low-refractive-index waveguide layer (6) and the three-dimensional tapered low-refractive-index waveguide cladding layer (7) have the same width and height on one side.

5. The coupler according to claim 1, characterized in that, The width and height of the low-refractive-index waveguide layer (6) should be set to match the mode field of the laser chip, specifically: ； In the formula, The integral representing the overlap between the input mode field of the laser source and the low-refractive-index waveguide layer (6) is given. It is the input mode field of the laser source. It is the mode field of the low-refractive-index waveguide layer (6). It is the complex conjugate of the input mode field of the laser source. The complex conjugate of the mode field of the low refractive index waveguide layer (6), where x and y represent the spatial coordinates of the horizontal and vertical directions of the end face cross section, respectively.

6. The coupler according to claim 1, characterized in that, The refractive index of the first double-layer thin-film lithium niobate inverted conical waveguide layer (4) is greater than that of the single-layer thin-film lithium niobate inverted conical waveguide layer (3). The width of the single-layer thin-film lithium niobate inverted conical waveguide layer (3) decreases linearly from one end close to the first double-layer thin-film lithium niobate inverted conical waveguide layer (4) to the other end far from the first double-layer thin-film lithium niobate inverted conical waveguide layer (4), forming an inverted conical structure.

7. The coupler according to claim 1, characterized in that, The width of the first double-layer lithium niobate planar waveguide layer (9) and the first double-layer lithium niobate ridge waveguide layer (10) decreases linearly from one end near the second double-layer lithium niobate inverted cone waveguide layer (5) to the end away from the second double-layer lithium niobate inverted cone waveguide layer (5), forming an inverted cone structure.

8. The coupler according to claim 1, characterized in that, The three-dimensional tapered low-refractive-index waveguide cladding (7) has a width and height that decrease linearly from one end near the low-refractive-index waveguide layer (6) to the other end away from the low-refractive-index waveguide layer (6), forming a wedge shape.

9. The coupler according to claim 1, characterized in that, The low-refractive-index waveguide layer (6), the three-dimensional tapered low-refractive-index waveguide cladding (7), and the low-refractive-index transition waveguide cladding (8) comprise a low-refractive-index polymer.

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