A multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices

Abstract

This invention relates to the field of micro-nano optoelectronic device technology, specifically to a multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices, comprising a 3×2 multimode interference coupler, a connecting single-mode waveguide, and a 2×1 multimode interference coupler; the 3×2 multimode interference coupler includes three input tapered waveguides, a first multimode waveguide, and a dual-output tapered waveguide connected in sequence; the 2×1 multimode interference coupler includes a dual-input tapered waveguide, a second multimode waveguide, a single-output tapered waveguide, and a single-mode waveguide connected in sequence; the middle input tapered waveguide and the side input tapered waveguides are symmetrically distributed along the optical axis, and the first and second multimode waveguides include an interference region and a transition region, the width of which narrows along the direction of light propagation; this invention can reduce the transmission loss of the end-face coupler, increase the alignment tolerance, and reduce the fabrication difficulty.

Description

Technical Field

[0001] This invention relates to the field of micro-nano optoelectronic device technology, and more specifically to a multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices. Background Technology

[0002] Since on-chip laser technology is still immature, optical coupling between optical fibers and integrated optical waveguide structures is a necessary input / output interface for photonic integrated circuits. Efficient coupling of optical signals plays a crucial role in many applications such as optical interconnects, optical switching, and integrated quantum optics.

[0003] However, direct coupling between traditional single-mode fiber and on-chip waveguide structures results in various losses, including Fresnel reflection loss caused by differences in end-face refractive indices or the presence of air gaps, alignment misalignment loss due to horizontal or vertical offsets during connection, mode field mismatch loss between the fiber and waveguide end-faces, and transmission loss introduced by the end-face coupler. For Fresnel reflection loss, a common technique is to use a refractive index matching fluid to replace the air gap at the fiber-waveguide connection. Alignment misalignment loss is often evaluated using a 3dB or 1dB tolerance of the coupling structure. For SMF-28 fiber, the core material is often doped SiO2, while the cladding is pure SiO2, resulting in a relatively small core-cladding refractive index difference. In contrast, to achieve small-size, low-loss, and highly constrained waveguide devices, on-chip passive waveguide devices often use materials with higher refractive indices, such as Si or Si3N4, as the core layer, while the cladding remains pure SiO2. This results in a larger core-cladding refractive index difference and a more concentrated optical field distribution. The difference in effective refractive index between the optical fiber and the end face of the on-chip waveguide device leads to a significant difference in the optical field distribution.

[0004] Various coupling structures have been studied, mainly including single-layer end-face coupling, vertical coupling based on gratings, and 3D coupling primarily using multi-layer or varying thickness structures. Compared to end-face coupling, vertical coupling has lower coupling efficiency and often requires an additional reflective layer to address substrate leakage; while 3D structures require more complex fabrication processes, resulting in high manufacturing difficulty and complexity. Therefore, achieving efficient light coupling using single-layer end-face coupling structures is of great significance. Summary of the Invention

[0005] In view of the above problems, the present invention provides a multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices, which solves the technical problems of long coupling length, small alignment tolerance, low coupling efficiency and high processing difficulty of the existing end-face coupler.

[0006] This invention provides a multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices, including a 3×2 multimode interference coupler, a single-mode waveguide 6, and a 2×1 multimode interference coupler;

[0007] The 3×2 multimode interference coupler includes two side-input tapered waveguides 1, a middle-input tapered waveguide 2, a first multimode waveguide, and a dual-output tapered waveguide 5. The output ends of the side-input tapered waveguides 1 and the middle-input tapered waveguide 2 are both connected to the input end of the first multimode waveguide, and the output end of the first multimode waveguide is connected to the input end of the dual-output tapered waveguide 5.

[0008] The 2×1 multimode interference coupler includes a dual-input tapered waveguide 7, a second multimode waveguide, a single-output tapered waveguide 10, and a single-mode waveguide 11. The dual-output tapered waveguide 7 is connected to the input end of the dual-input tapered waveguide 7 via a single-mode waveguide 6. The output end of the dual-input tapered waveguide 7 is connected to the input end of the second multimode waveguide. The output end of the second multimode waveguide is connected to the input end of the single-mode waveguide 11 via the single-output tapered waveguide 10.

[0009] Preferably, the centerline of the intermediate input tapered waveguide 2 is arranged along the optical axis of the multi-stage end-face coupler, and the centerlines of the two side input tapered waveguides 1 are symmetrically distributed with the optical axis as the axis of symmetry; the widths of the side input tapered waveguides 1 and the intermediate input tapered waveguide 2 both increase linearly from the input end to the output end; the centerline of the side input tapered waveguide 1 is offset from the input end to the output end in a direction away from the optical axis.

[0010] Preferably, the first multimode waveguide includes a first multimode waveguide interference region 3 and a first multimode waveguide transition region 4. Light input from the side input tapered waveguide 1 and the middle input tapered waveguide 2 interferes in the first multimode waveguide interference region 3 and then connects to the dual-output tapered waveguide 5 through the first multimode waveguide transition region 4. The width of the first multimode waveguide interference region 3 remains constant along the light propagation direction, while the width of the first multimode waveguide transition region 4 narrows along the light propagation direction, and the width of the output end is consistent with the input width of the dual-output tapered waveguide 5.

[0011] The second multimode waveguide includes a second multimode waveguide interference region 8 and a second multimode waveguide transition region 9. Light input from the dual-input tapered waveguide 7 is interfered with in the second multimode waveguide interference region 8, passes through the second multimode waveguide transition region 9, and is output to the single-output tapered waveguide 10. The width of the second multimode waveguide interference region 8 remains unchanged along the light propagation direction, while the width of the second multimode waveguide transition region 9 narrows along the light propagation direction, and the width of the output end is consistent with the width of the single-output tapered waveguide 10.

[0012] Preferably, the widths of the side input tapered waveguide 1 and the middle input tapered waveguide 2 range from 90nm to 120nm, the spacing between the side input tapered waveguide 1 and the middle input tapered waveguide 2 ranges from 0.8μm to 1.2μm, and the output widths of the side input tapered waveguide 1 and the middle input tapered waveguide 2 are the same; the lengths of the side input tapered waveguide 1 and the middle input tapered waveguide 2 along the light propagation direction range from 30μm to 50μm.

[0013] Preferably, the width range of the first multimode waveguide interference region 3 is 6μm-20μm, and the width variation profile curves of the first multimode waveguide transition region 4 and the second multimode waveguide transition region 9 are: power function, exponential function or trigonometric function curves.

[0014] Preferably, the dual-output tapered waveguide 5 includes two tapered waveguides symmetrically distributed about the optical axis of the multi-stage end-face coupler. The width of both tapered waveguides of the dual-output tapered waveguide 5 decreases linearly along the light propagation direction. The output end of the dual-output tapered waveguide 5 is connected to the connecting single-mode waveguide 6. The width of the output end of the dual-output tapered waveguide 5 is the same as the width of the connecting single-mode waveguide 6. The width of the input end of the dual-output tapered waveguide 5 is in the range of 1.4μm-2μm. The length of the dual-output tapered waveguide 5 along the light propagation direction is 10μm, and the length of the connecting single-mode waveguide 6 along the light propagation direction is 10μm.

[0015] Preferably, the dual-input tapered waveguide 7 includes two tapered waveguides symmetrically distributed about the optical axis of the multi-stage end-face coupler. The width of the input end of the two tapered waveguides of the dual-input tapered waveguide 7 is consistent with the width of the single-mode waveguide 6, and the width of both waveguides increases linearly along the direction of light propagation. The width of the output end of the dual-input tapered waveguide 7 is in the range of 0.8μm-1.2μm, and the length of the dual-input tapered waveguide 7 along the direction of light propagation is 10μm.

[0016] Preferably, the width of the single-output tapered waveguide 10 narrows linearly along the direction of light propagation, the width of the input end of the single-output tapered waveguide 10 is the same as the width of the output end of the dual-input tapered waveguide 7, and the output end of the single-output tapered waveguide 10 is connected to the single-mode waveguide 11, with the same width as the single-mode waveguide 11.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects:

[0018] (1) Compared with the existing end-face coupler structure, the present invention creatively uses two multimode waveguides cascaded together and a three-cone input end, which can not only meet the mode spot size matching of the fiber-waveguide end face, but also reduce the transmission loss of light in the coupler.

[0019] (2) The overall thickness of the present invention remains unchanged, and the device design is carried out only on a single-layer structure. It is a symmetrical structure, which can avoid the complexity and uncertainty of the process of multi-layer structures.

[0020] (3) The end face coupler proposed in this invention is based on the principle of multimode interference coupler. It has a small size and a length of only about 150μm, which can reduce the size of the device and provide more allowable area for subsequent effective devices.

[0021] (4) The present invention adopts a three-cone input end structure, which can reduce the effective refractive index of the end face mode, expand the waveguide mode area, achieve mode field matching, and increase the alignment tolerance. In addition, the design of the side conical center offset structure can prevent the problem of high processing difficulty caused by the narrow spacing of the wide end structure. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0023] Figure 1 This is a top view schematic diagram of the end-face coupler structure provided by the present invention.

[0024] Figure 2 This is a 3D schematic diagram of the end-face coupler structure provided by the present invention.

[0025] Figure 3 This is an internal optical field transmission diagram of one embodiment of the end-face coupler provided by the present invention.

[0026] Figure 4 The end-face mode field distribution diagram of the three input terminals of the end-face coupler provided by the present invention.

[0027] Figure reference numerals: 1-Side input tapered waveguide, 2-Middle input tapered waveguide, 3-First multimode waveguide interference region, 4-First multimode waveguide transition region, 5-Dual-output tapered waveguide, 6-Connecting single-mode waveguide, 7-Dual-input tapered waveguide, 8-Second multimode waveguide interference region, 9-Second multimode waveguide transition region, 10-Single-output tapered waveguide, 11-Single-mode waveguide, 12-Single-mode optical fiber. Detailed Implementation

[0028] To better understand the above-described objectives, features, and advantages of the present invention, the 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. Furthermore, the present invention can be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0029] To illustrate the effectiveness of the method proposed in this invention, the following detailed description of the above technical solution is provided through a specific embodiment, such as... Figure 1 , Figure 2 As shown, a multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices is disclosed, including a 3×2 multimode interference (MMI) coupler and a 2×1 multimode interference coupler. The 3×2 multimode interference coupler includes two side-input tapered waveguides 1, a middle-input tapered waveguide 2, a first multimode waveguide, and a dual-output tapered waveguide 5. The outputs of the side-input tapered waveguides 1 and the middle-input tapered waveguide 2 are both connected to the input of the first multimode waveguide. The output of the first multimode waveguide is connected to the input of the dual-output tapered waveguide 5. The 2×1 multimode interference coupler includes a single-mode waveguide 6, a dual-input tapered waveguide 7, a second multimode waveguide, a single-output tapered waveguide 10, and a single-mode waveguide 11. The dual-output tapered waveguide 5 is connected to the input of the dual-input tapered waveguide 7 via the single-mode waveguide 6. The output of the dual-input tapered waveguide 7 is connected to the input of the second multimode waveguide. The output of the second multimode waveguide is connected to the input of the single-mode waveguide 11 via the single-output tapered waveguide 10.

[0030] The thickness of the overall structure of the multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices provided by this invention is consistent with the thickness of the single-mode waveguide. Taking a Si3N4 waveguide with a wavelength of 850nm as an example, when the thickness of the single-mode waveguide is 300nm, the width of the single-mode condition is calculated to be 500nm by the finite difference characteristic mode method (FDE). That is, when the width of the single-mode waveguide is 500nm, it can ensure that it only supports the fundamental mode transmission at a wavelength of 850nm.

[0031] In some embodiments, the three input terminals formed by the two side input tapered waveguides 1 and the middle input tapered waveguide 2 of the present invention are symmetrically distributed. Specifically, the centerline of the middle input tapered waveguide 2 is arranged along the optical axis of the multi-stage end-face coupler, and the centerlines of the two side input tapered waveguides 1 are symmetrically distributed about the optical axis. The widths of both the side input tapered waveguides 1 and the middle input tapered waveguide 2 increase linearly from the input end to the output end. The centerline of the side input tapered waveguide 1 is offset away from the optical axis from the input end to the output end. With this structure, the three input terminals of the present invention can avoid the additional losses caused by the excessively narrow distance between the wide ends of the three tapered waveguides.

[0032] The width and spacing of the side-input tapered waveguide 1 and the middle-input tapered waveguide 2 in this invention determine the mode overlap integral (Overlap) with the fiber coupling end face. Taking coupling with an 850nm single-mode fiber with a mode field diameter (MFD) of 2.5μm as an example, the width range of the side-input tapered waveguide 1 and the middle-input tapered waveguide 2 is 90nm-120nm, and the spacing range is 0.8μm-1.2μm, wherein the width of the middle waveguide can be slightly wider than that of the side waveguide, for example, about 10nm wider. The resulting end-face mode field diagram is shown in Figure 4, at which point the Overlap reaches approximately 93%. Its length ranges from 30μm to 50μm.

[0033] In some embodiments, the output widths of the two side-input tapered waveguides 1 and the middle-input tapered waveguide 2 are the same, and the output widths of the side-input tapered waveguides 1 and the middle-input tapered waveguide 2 are determined by the size of the interference enhancement region in the first multimode waveguide. In a multimode waveguide of fixed width, the interference behavior of the same input mode at the same wavelength is fixed. Therefore, the position of the output end of the side-input tapered waveguide 1 is consistent with the side position of the output end of the 1×3MMI beamsplitter of the same width. The calculated output end spacing of the 1×3MMI beamsplitter under the same width and wavelength conditions is the output end spacing between the side-input tapered waveguide 1 and the middle-input tapered waveguide 2.

[0034] In some embodiments, the first multimode waveguide includes a first multimode waveguide interference region 3 and a first multimode waveguide transition region 4. The width of the first multimode waveguide interference region 3 remains constant along the light propagation direction. Light input from the three input terminals interferes in the first multimode waveguide interference region 3 and then propagates through the first multimode waveguide transition region 4 to the dual-output tapered waveguide 5. The width of the first multimode waveguide transition region 4 gradually narrows along the light propagation direction, with the narrow end width being the same as the outer width of the dual-output tapered waveguide 5. In this way, optical loss caused by light exiting at the output end face of the multimode waveguide can be prevented.

[0035] The width of the first multimode waveguide interference region 3 in this invention ranges from 6μm to 20μm. The width of the multimode waveguide interference region affects the offset position of the input tapered waveguide center and the length of the multimode waveguide. The wider the width, the longer the required length of the multimode waveguide, resulting in a larger device size. Conversely, a narrow width leads to too close spacing between the output ends, increasing the manufacturing difficulty. Therefore, a trade-off needs to be struck between device size and output end spacing. The width variation profile of the first multimode waveguide transition region 4 in this invention can be set as various function curves, such as power functions, exponential functions, trigonometric functions, etc. Taking a power function with an exponent of m as an example, where m varies from 0 to 1, when m is 0, the width variation profile of the transition region is a horizontal line continuing the multimode waveguide. When m is 1, the shape of the transition region is a connection between the first multimode waveguide and the dual-output tapered waveguide 6. Figure 3 The optical field transmission diagrams are shown for the transition regions of the first and second multimode waveguides, with m values ​​of 0.7 and 1, respectively.

[0036] In some embodiments, the dual-output tapered waveguide 5 consists of two identical tapered waveguides symmetrically distributed along the optical axis, with the width of both waveguides narrowing linearly along the light propagation direction. The output end of the dual-output tapered waveguide 5 is connected to the single-mode waveguide 6. The width of the input end of the dual-output tapered waveguide 5 is determined by the size of the interference enhancement region in the first multimode waveguide, and the width of the output end is consistent with the width of the single-mode waveguide 6. This structure enables efficient light output guidance and reduces transmission loss through single-mode waveguide transmission.

[0037] The input width of the dual-output tapered waveguide 5 of the present invention is determined by the size of the interference enhancement region in the first multimode waveguide. Taking the 8μm multimode waveguide width as an example, the input width of the dual-output tapered waveguide 5 ranges from 1.4μm to 2μm. The narrow end is connected to the single-mode waveguide and has the same width as the single-mode waveguide. Its length ranges from about 10μm.

[0038] In some embodiments, the dimensions of the connecting single-mode waveguide 6 are specially designed to only meet the requirements for fundamental mode transmission. This effectively reduces transmission loss by avoiding crosstalk in higher-order modes. The length of the connecting single-mode waveguide 6 is approximately 10 μm.

[0039] In some embodiments, the dual-input tapered waveguide 7 is composed of two identical tapered waveguides. The input width of the two tapered waveguides of the dual-input tapered waveguide 7 is the same as the width of the connecting single-mode waveguide 6, and the width of both increases linearly along the direction of light propagation. The output width of the dual-input tapered waveguide 7 is determined by the size of the interference enhancement region in the second multimode waveguide, ranging from 0.8 μm to 1.2 μm, and the length ranges from approximately 10 μm.

[0040] In some embodiments, the second multimode waveguide includes a second multimode waveguide interference region 8 and a second multimode waveguide transition region 9. The width of the second multimode waveguide interference region 8 remains constant along the light propagation direction. Light input from the dual-input tapered waveguide 7 is interfered with in the second multimode waveguide interference region 8, passes through the second multimode waveguide transition region 9, and is output to the single-output tapered waveguide 10. The width of the second multimode waveguide transition region 9 gradually narrows along the light propagation direction, with the narrow end having the same width as the single-output tapered waveguide 10. In this way, optical loss caused by direct emission from the output end face of the second multimode waveguide can be prevented. The width of the second multimode waveguide interference region 8 is determined by an optimized 1×2 MMI beamsplitter multimode waveguide width that satisfies the same output waveguide spacing as the dual-input tapered waveguide 8 spacing.

[0041] In some embodiments, the width of the single-output tapered waveguide 10 is linearly narrowed along the light propagation direction. The width of the input end of the single-output tapered waveguide 10 is the same as the width of the output end of the dual-input tapered waveguide 7. The output end of the single-output tapered waveguide 10 is connected to the single-mode waveguide 11, and its width is the same as that of the single-mode waveguide 11, for subsequent light transmission.

[0042] It should be understood that the linear tapered waveguides involved in this invention include: the transition forms of the side-input tapered waveguide 1, the middle-input tapered waveguide 2, the dual-output tapered waveguide 5, the dual-input tapered waveguide 7, and the single-output tapered waveguide 10 can all be optimized to quadratic, exponential, trigonometric, or other forms.

[0043] The multi-stage end-face coupler implementation provided by this invention for interconnecting optical fibers and on-chip photonic integrated devices achieves an optimal transmittance of approximately 85%, a horizontal 3dB alignment tolerance of approximately 3.1 μm, and a vertical 3dB alignment tolerance of approximately 2.6 μm. Simultaneously, the overall device size is approximately 150 μm, significantly reducing the device size compared to the approximately 700 μm length of a single-cone end-face coupler under the same conditions. It is evident that the structure of two cascaded multimode interference couplers, combined with a three-pointed inverted conical structure, achieves high coupling efficiency, large alignment tolerance, and small size, providing support for efficient optical coupling between optical fibers and photonic integrated chips.

[0044] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0045] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0046] In this invention, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. The term "multiple" refers to two or more unless otherwise expressly defined.

[0047] 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 multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices, characterized in that, It includes a 3×2 multimode interference coupler, a connection between a single-mode waveguide (6) and a 2×1 multimode interference coupler; The 3×2 multimode interference coupler includes two side input tapered waveguides (1), a middle input tapered waveguide (2), a first multimode waveguide, and a dual-output tapered waveguide (5). The output ends of the side input tapered waveguides (1) and the middle input tapered waveguide (2) are both connected to the input end of the first multimode waveguide, and the output end of the first multimode waveguide is connected to the input end of the dual-output tapered waveguide (5). The 2×1 multimode interference coupler includes a dual-input tapered waveguide (7), a second multimode waveguide, a single-output tapered waveguide (10), and a single-mode waveguide (11). The dual-output tapered waveguide (5) is connected to the input end of the dual-input tapered waveguide (7) via a single-mode waveguide (6). The output end of the dual-input tapered waveguide (7) is connected to the input end of the second multimode waveguide. The output end of the second multimode waveguide is connected to the input end of the single-mode waveguide (11) via the single-output tapered waveguide (10). The centerline of the intermediate input tapered waveguide (2) is set along the optical axis of the multi-stage end-face coupler, and the centerlines of the two side input tapered waveguides (1) are symmetrically distributed with the optical axis as the axis of symmetry; the widths of the side input tapered waveguides (1) and the intermediate input tapered waveguide (2) both increase linearly from the input end to the output end; the centerline of the side input tapered waveguide (1) is offset from the input end to the output end in a direction away from the optical axis; The first multimode waveguide includes a first multimode waveguide interference region (3) and a first multimode waveguide transition region (4). Light input from the side input tapered waveguide (1) and the middle input tapered waveguide (2) interferes in the first multimode waveguide interference region (3) and is connected to the dual-output tapered waveguide (5) through the first multimode waveguide transition region (4). The width of the first multimode waveguide interference region (3) remains unchanged along the light propagation direction, while the width of the first multimode waveguide transition region (4) narrows along the light propagation direction. The width of the output end is consistent with the width of the input end of the dual-output tapered waveguide (5). The second multimode waveguide includes a second multimode waveguide interference region (8) and a second multimode waveguide transition region (9). Light input from the dual-input tapered waveguide (7) is interfered in the second multimode waveguide interference region (8), passes through the second multimode waveguide transition region (9), and is output to the single-output tapered waveguide (10). The width of the second multimode waveguide interference region (8) remains unchanged along the light propagation direction, while the width of the second multimode waveguide transition region (9) narrows along the light propagation direction. The width of the output end is consistent with the width of the single-output tapered waveguide (10).

2. The multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices according to claim 1, characterized in that: The width range of the side input tapered waveguide (1) and the middle input tapered waveguide (2) is 90nm-120nm, the spacing between the side input tapered waveguide (1) and the middle input tapered waveguide (2) is 0.8μm-1.2μm, and the output width of the side input tapered waveguide (1) and the middle input tapered waveguide (2) is the same; the length range of the side input tapered waveguide (1) and the middle input tapered waveguide (2) along the light propagation direction is 30μm-50μm.

3. The multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices according to claim 2, characterized in that: The width range of the first multimode waveguide interference region (3) is 6μm-20μm. The width variation profile curves of the first multimode waveguide transition region (4) and the second multimode waveguide transition region (9) are: power function, exponential function or trigonometric function curves.

4. The multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices according to claim 3, characterized in that: The dual-output tapered waveguide (5) includes two tapered waveguides symmetrically distributed about the optical axis of the multi-stage end-face coupler. The width of both tapered waveguides of the dual-output tapered waveguide (5) narrows linearly along the direction of light propagation. The output end of the dual-output tapered waveguide (5) is connected to the connecting single-mode waveguide (6). The width of the output end of the dual-output tapered waveguide (5) is the same as the width of the connecting single-mode waveguide (6). The width of the input end of the dual-output tapered waveguide (5) is in the range of 1.4μm-2μm. The length of the dual-output tapered waveguide (5) along the direction of light propagation is 10μm, and the length of the connecting single-mode waveguide (6) along the direction of light propagation is 10μm.

5. The multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices according to claim 4, characterized in that: The dual-input tapered waveguide (7) includes two tapered waveguides symmetrically distributed about the optical axis of the multi-stage end-face coupler. The width of the input end of the two tapered waveguides of the dual-input tapered waveguide (7) is consistent with the width of the single-mode waveguide (6) connected to it, and the width of both waveguides increases linearly along the direction of light propagation. The width of the output end of the dual-input tapered waveguide (7) is in the range of 0.8μm-1.2μm, and the length of the dual-input tapered waveguide (7) along the direction of light propagation is 10μm.

6. The multi-stage end-face coupler for interconnecting optical fibers and on-chip photonic integrated devices according to claim 5, characterized in that: The width of the single-output tapered waveguide (10) narrows linearly along the direction of light propagation. The width of the input end of the single-output tapered waveguide (10) is the same as the width of the output end of the dual-input tapered waveguide (7). The output end of the single-output tapered waveguide (10) is connected to the single-mode waveguide (11), and its width is the same as that of the single-mode waveguide (11).

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