Tapered waveguide and waveguide assembly

Abstract

This invention provides a tapered waveguide and a waveguide composite structure. The tapered waveguide includes a waveguide core layer with a gradually varying thickness, an upper cladding layer covering the top and sides of the waveguide core layer, a buried oxide layer covering the bottom of the waveguide core layer, a substrate, and lateral constraint structures disposed within the waveguide cladding layer composed of the upper cladding layer and the buried oxide layer, located on both sides of the waveguide core layer. The refractive indices of the waveguide core layer, the waveguide cladding layer, and the lateral constraint structures decrease sequentially, and the lateral constraint structures cover the area of ​​the waveguide core layer in the vertical direction to enhance the lateral constraint of the optical field. The lateral constraint structures can be in the form of trench-filled low-refractive-index dielectric or subwavelength grating structures, and can be designed as discontinuous, tilted, or variable-angle arrangements. This structure can maintain the adiabatic evolution of the optical field while shortening the length of the tapered waveguide, improve integration density, and reduce mode leakage and inter-waveguide crosstalk, making it suitable for applications such as high-density integrated optical interconnects and mode switching.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonics technology, specifically relating to a tapered waveguide and a waveguide combination structure. Background Technology

[0002] In integrated photonic devices and optical communication systems, waveguides are fundamental for optical field coupling, transmission, and control. As chip integration density continues to increase, the contradiction between device miniaturization and high performance robustness becomes increasingly prominent. This contradiction is particularly acute in mode-spot converters: the fiber mode field is large, while the waveguide mode field is small, making coupling between the fiber and the optical chip difficult. To achieve low-loss coupling between the fiber and the waveguide, traditional solutions typically employ inverted conical waveguides with constant thickness, relying on a sufficiently gentle gradient length to achieve adiabatic evolution. This directly leads to excessively large device sizes, severely limiting chip integration density. Therefore, how to significantly reduce device size while maintaining or even enhancing the waveguide's ability to confine the optical field and achieving rapid adiabatic evolution has become a core challenge in this field for achieving higher integration levels.

[0003] To address these challenges, the industry generally focuses on the optical field confinement capabilities of the waveguide structure itself. However, existing solutions have significant limitations: First, during the adiabatic evolution of the conical waveguide, the TE mode (transverse electric mode) and TM mode (transverse magnetic mode) have different mode field distribution characteristics, resulting in inherently different requirements for waveguide confinement. The TE mode requires enhanced transverse confinement, while the TM mode requires enhanced longitudinal confinement. Generally, a conical waveguide with a constant thickness cannot simultaneously guarantee the coupling effect of the optical fields of the two polarizations. Second, common methods for enhancing confinement, such as using materials with extremely high refractive index contrast or complex cladding structures, often involve a sharp increase in process complexity and additional insertion loss.

[0004] Therefore, how to provide a waveguide solution that can simultaneously meet the constraints of TE mode and TM mode and also meet the requirements of rapid thermal evolution through structural innovation has become a key technical challenge that urgently needs to be overcome in this field.

[0005] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of this application and facilitating understanding by those skilled in the art. It should not be assumed that these technical solutions are known to those skilled in the art simply because they have been described in the background section of this application. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a tapered waveguide and waveguide combination structure to solve the problem that the tapered waveguide has low optical field confinement capability, resulting in excessively large mode conversion device size.

[0007] To achieve the above objectives, the present invention provides a tapered waveguide, comprising:

[0008] Waveguide core layer, the waveguide core layer having a structure with a gradually varying thickness;

[0009] An upper cladding layer covers the top and side regions of the waveguide core layer;

[0010] A buried oxide layer covers the bottom of the waveguide core layer and is in direct contact with the upper cladding; the buried oxide layer and the upper cladding together constitute the waveguide cladding.

[0011] A substrate, wherein the buried oxide layer is formed on the upper surface of the substrate;

[0012] A lateral constraint structure is provided, at least within the upper cladding, and located on both sides of the waveguide core layer along the gradient direction; wherein the refractive indices of the waveguide core layer, the waveguide cladding, and the lateral constraint structure decrease sequentially; and the lower surface of the lateral constraint structure is not higher than the lower surface of the waveguide core layer and not lower than the upper surface of the substrate; the upper surface of the lateral constraint structure is not lower than the upper surface of the waveguide core layer.

[0013] Optionally, the ratio of the length (L) of the waveguide core layer along the gradient direction to the total thickness change (ΔH) of the waveguide core layer is greater than 100:1.

[0014] Optionally, the width of the waveguide core layer varies gradually along the light propagation direction and thickness, or remains constant.

[0015] Optionally, the lateral constraint structure is composed of trenches formed in the waveguide cladding, the trenches being filled with a medium having a refractive index lower than that of the waveguide cladding.

[0016] Optionally, the sidewall of the trench has an inclination angle, and the second angle between the tangent at any point on the contour line of the sidewall and the vertical direction ranges from -45° to 45°.

[0017] Optionally, the medium is selected from at least one of a gaseous medium, a solid medium, or a liquid medium.

[0018] Optionally, the lateral constraint structure is implemented by a subwavelength grating structure, the period (Λ) of which satisfies: Λ<λ / n_clad, where λ is the operating wavelength and n_clad is the refractive index of the waveguide cladding material.

[0019] Optionally, the lateral constraint structure is continuously distributed along the gradient direction of the waveguide core layer.

[0020] Optionally, the first angle between the extension direction of the lateral constraint structure on the horizontal plane and the axial direction of the waveguide core layer is in the range of 0° to 90°, and the value of the first angle varies constantly, continuously or discontinuously along the gradient direction of the waveguide core layer.

[0021] Optionally, the lateral constraint structure is disposed along the light propagation direction in the end region of the gradient section of the waveguide core layer, including the minimum cross section, rather than covering the entire gradient section.

[0022] Optionally, the cross-section of the waveguide core layer is rectangular or trapezoidal.

[0023] Optionally, it further includes a side wing core layer, which is located on both sides of the waveguide core layer and has a gap between them; the side wing core layer is located between the waveguide core layer and the lateral constraint structure; the refractive index of the waveguide core layer and the refractive index of the side wing core layer are both greater than the refractive index of the waveguide cladding.

[0024] Optionally, the spacing between the side wing core layer and the waveguide core layer is constant or continuously varying along the gradient direction of the waveguide core layer.

[0025] Optionally, the width and thickness of the side wing core layer along the light propagation direction can be increased, decreased, or kept constant independently.

[0026] Optionally, the projected length of the side wing core layer in the direction parallel to the length of the waveguide core layer is one of the following: equal to the total length of the waveguide core layer, longer than the total length of the waveguide core layer, or shorter than the total length of the waveguide core layer.

[0027] The present invention also proposes a waveguide assembly structure, wherein the waveguide assembly structure is composed of at least two waveguide segments connected sequentially along the direction of light propagation; wherein at least one of the at least two waveguide segments is a tapered waveguide as described in any of the preceding claims; and the waveguide assembly structure is capable of achieving a monotonically adiabatic change in the effective area of ​​the optical mode field from the input end to the output end of the assembly structure; the monotonically adiabatic change is either monotonically increasing or monotonically decreasing.

[0028] As described above, this invention provides a tapered waveguide and a waveguide composite structure. The tapered waveguide includes a waveguide core layer with a gradually varying thickness, an upper cladding layer covering the top and sides of the core layer, a buried oxide layer covering the bottom of the waveguide core layer, and lateral constraint structures disposed in the waveguide cladding and located on both sides of the waveguide core layer. The refractive indices of the waveguide core layer, waveguide cladding, and lateral constraint structures decrease sequentially, and the lateral constraint structures cover the area of ​​the waveguide core layer in the vertical direction to enhance the lateral constraint of the optical field. The lateral constraint structures can be in the form of trench-filled low-refractive-index media or subwavelength grating structures, and can be designed to be discontinuous, tilted, or arranged at varying angles. Simultaneously, wing core layers can be disposed on both sides of the waveguide core layer, with a gap between them, to further control the lateral distribution of the optical field and optimize the mode conversion efficiency. This structure can maintain the adiabatic evolution of the optical field while shortening the length of the tapered waveguide, improve the integration density and reduce mode leakage. In particular, it can simultaneously improve the constraint of the transverse and longitudinal mode fields, thereby reducing crosstalk between waveguides. It is suitable for applications such as high-density integrated optical interconnects and mode conversion. Attached Figure Description

[0029] Figure 1 The diagram shown is a three-dimensional structural schematic of the tapered waveguide according to an embodiment of the present invention.

[0030] Figure 2 The image shown is a cross-sectional front view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure is a trench.

[0031] Figure 3 The diagram shows a front cross-sectional view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure is a filled trench.

[0032] Figure 4 The diagram shows a cross-sectional front view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure is a trench with a trapezoidal cross section.

[0033] Figure 5 The image shown is a cross-sectional side view of a tapered waveguide according to an embodiment of the present invention, wherein the height of the waveguide core layer gradually changes along the direction of light propagation.

[0034] Figure 6 The diagram shows a top-view cross-sectional view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure has a fixed angle with the direction of light propagation.

[0035] Figure 7 The diagram shows a top-cross view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure has a variable angle with the direction of light propagation.

[0036] Figure 8The diagram shows a top-view cross-sectional view of a tapered waveguide according to an embodiment of the present invention, wherein the lateral constraint structure is disposed in the end region of the waveguide core layer gradient section, including the minimum cross-section, rather than covering the entire gradient section.

[0037] Figure 9 The diagram shown is a three-dimensional structural schematic of the tapered waveguide in an embodiment of the present invention, wherein the lateral constraint structure is a subwavelength grating structure.

[0038] Figure 10 The diagram shows a schematic representation of the structure of the side wing core layer of the tapered waveguide in an embodiment of the present invention, with the core layer parallel to the direction of light propagation.

[0039] Figure 11 The diagram shows a structural schematic of the tapered waveguide described in this embodiment of the invention, where the side core layer is at a certain angle to the direction of light propagation.

[0040] Figure 12 The diagram shows a structure in which the projected length of the side wing core layer in the tapered waveguide of the present invention is equal to the total length of the waveguide core layer.

[0041] Figure 13 The waveguide assembly structure shown in this embodiment of the invention is composed of the tapered waveguide.

[0042] Explanation of reference numerals in the attached figures

[0043] 110 waveguide core layer 210 upper cladding 310 Lateral constraint structure 311 trench 312 Subwavelength grating structure 320 Side core layer 410 Buried oxygen layer 510 substrate 610 First tapered waveguide 620 Second conical waveguide Detailed Implementation

[0044] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0045] In the detailed description of embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged and not to scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0046] For ease of description, spatial relation terms such as “below,” “under,” “lower than,” “below,” “above,” and “upper” may be used herein to describe the relationship between one element or feature shown in the accompanying drawings and other elements or features. It will be understood that these spatial relation terms are intended to include orientations of the device in use or operation other than those depicted in the drawings, and may include embodiments in which the first and second features are formed in direct contact, or embodiments in which additional features are formed between the first and second features, such that the first and second features may not be in direct contact. Furthermore, when a layer is referred to as “between” two layers, it may be the only layer between the two layers, or there may be one or more layers in between.

[0047] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0048] Example 1

[0049] This embodiment provides a tapered waveguide, combined with attached... Figures 1-9 The waveguide is described below. The tapered waveguide includes:

[0050] Waveguide core layer 110, the waveguide core layer 110 having a structure with a gradually varying thickness;

[0051] The upper cladding layer 210 covers at least the top and side regions of the waveguide core layer 110;

[0052] The buried oxide layer 410 covers the bottom of the waveguide core layer 110 and is in direct contact with the upper cladding layer 210; the buried oxide layer 410 and the upper cladding layer 210 together constitute the waveguide cladding layer.

[0053] Substrate 510, wherein the buried oxide layer 410 is formed on the upper surface of the substrate 510;

[0054] A lateral constraint structure 310 is provided at least in the upper cladding 210 and located on both sides of the waveguide core layer 110 along the gradient direction; wherein the refractive index of the waveguide core layer 110, the waveguide cladding, and the lateral constraint structure 310 decreases sequentially; the lower surface of the lateral constraint structure 310 is not higher than the lower surface of the waveguide core layer 110 and not lower than the upper surface of the substrate 510; the upper surface of the lateral constraint structure 310 is not lower than the upper surface of the waveguide core layer 110.

[0055] For details, please refer to Figure 1 The tapered waveguide, from bottom to top, includes the substrate 510, the buried oxide layer 410, and the upper cladding 210 formed on the buried oxide layer 410. The waveguide core layer 110 and the lateral constraint structure 310 are integrated within the waveguide cladding formed by the upper cladding 210 and the buried oxide layer 410. The buried oxide layer 410, serving as the lower cladding, constitutes an optical isolation layer, confining the light field within the upper waveguide functional region and preventing vertical leakage of light energy to the substrate 510, thus ensuring low transmission and coupling loss. Simultaneously, the buried oxide layer 410 is materially compatible with the waveguide core layer 110, and its flat and stable interface provides a technological basis for fabricating the waveguide core layer 110 with precisely controllable cross-sectional dimensions. In one specific embodiment, the waveguide core layer 110 is made of silicon, and the buried oxide layer 410 is made of silicon dioxide, whose refractive index is lower than that of the waveguide core layer 110. This creates an effective refractive index difference in the vertical direction, confining the light field within the waveguide core layer 110. In actual manufacturing processes, the buried oxide layer 410 and the upper cladding layer 210 are typically made of the same material (e.g., silicon dioxide). However, due to different fabrication sequences, their refractive indices may have slight differences. Nevertheless, this difference does not affect their function as a low-refractive-index cladding (i.e., waveguide cladding) to confine the light field within the core layer.

[0056] This technical solution achieves low-loss mode field evolution for both TE and TM modes through the synergistic effect of the three-dimensionally gradient-designed waveguide core layer 110, the uniformly wrapped waveguide cladding (composed of an upper cladding layer 210 and a buried oxide layer 410), and the lateral confinement structure 310 (e.g., air, matching liquid / glue, water, oil, or other low-refractive-index media). The waveguide core layer 110 employs a thickness gradient to provide low-loss transmission, ensuring a smooth evolution of the optical field during the gradient process and providing a low-loss transmission path for both transverse electric and transverse magnetic modes. The lateral confinement structure 310 enhances the optical field confinement capability of both modes by introducing an additional transverse refractive index barrier, modulates the mode field distribution, and significantly improves the matching degree between the output beam and the fiber mode field. This design physically improves the confinement capability of transverse electric and transverse magnetic modes and provides greater design freedom in the system, allowing devices to achieve equivalent performance in shorter lengths, or to obtain superior optical field confinement capability and process adaptability in the same length. The overall structure ensures low loss and wide bandwidth operation while also achieving high integration and manufacturing feasibility. The substrate 510 is made of materials such as silicon, silicon carbide, and silicon nitride, to provide support during the process, and is not limited to these materials.

[0057] As an example, the width of the waveguide core layer 110 varies gradually along the light propagation direction in the same trend as its thickness, or remains constant.

[0058] For details, please refer to Figure 1The waveguide core layer 110 adopts a three-dimensional gradient design. To adapt to different mode conversion requirements, its thickness and width can change synchronously or asynchronously along the light propagation direction. Synchronous changes include the simultaneous increase or decrease of the thickness and width of the waveguide core layer 110; asynchronous changes include the non-simultaneous increase or decrease of the thickness and width of the waveguide core layer 110 during the gradient process, for example, a change from initially decreasing thickness with constant width, followed by constant thickness and decreasing width. As the main channel for light field transmission, the gradient geometry of the waveguide core layer 110 is the physical basis for realizing the adiabatic evolution of the mode field. Preferably, the thickness and width of the waveguide core layer 110 can change gradually and synergistically. The waveguide core layer 110 can be made of materials selected from, for example, silicon, silicon nitride, lithium niobate, indium phosphide, or silicon dioxide.

[0059] In this invention, the core of the thickness and width gradient lies in achieving mode field conversion by designing the variation of the waveguide core layer 110's dimensions along the propagation direction. In a preferred embodiment of this invention, the variation is monotonic to achieve the most efficient and low-loss optical performance. Those skilled in the art will understand that any non-constant dimensional variation design serving the above objectives falls within the scope of the "gradient" described in this invention.

[0060] As an example, the ratio of the length L of the waveguide core layer 110 along the gradient direction to its total thickness change ΔH is greater than 100:1.

[0061] Specifically, to ensure that the light wave achieves adiabatic mode conversion during propagation, the geometric gradient of the waveguide core layer 110 must meet adiabatic conditions. For details, see [link to relevant documentation]. Figure 5 The ratio of the gradually changing length L along the light propagation direction to the total thickness change ΔH is greater than 100:1, i.e., L / ΔH > 100:1. This constraint ensures that the change in waveguide cross-section is sufficiently gradual, thereby reducing mode conversion loss to an acceptable level (typically <0.5dB) and suppressing higher-order mode excitation. Of course, the specific value of this ratio depends on the material system, operating wavelength, performance targets (such as loss, bandwidth, and size), and whether an auxiliary constraint structure is used, and is not limited here.

[0062] It should be noted that the L / ΔH > 100:1 ratio itself does not directly reflect the integration or compactness of the device. For example, if mode switching is achieved solely by changing the waveguide width, a long gradient length L is often required to meet this ratio, resulting in a large device size. However, this invention increases the thickness of the gradient waveguide core layer 110 and introduces a side constraint structure, allowing for a significantly shorter actual gradient length L while still meeting the L / ΔH > 100:1 requirement, thus achieving more compact integration. Therefore, the advantage of this invention lies in significantly improving the device's integration while meeting thermal insulation requirements through multi-dimensional geometric gradients and auxiliary constraints.

[0063] As an example, the cross-section of the waveguide core layer 110 is rectangular or trapezoidal.

[0064] For details, please refer to Figure 3 A rectangular cross-section has the advantages of clear geometric definition and high sidewall perpendicularity, which facilitates precise fabrication and predictable mode distribution. When a trapezoidal cross-section is used, its sloping sidewalls help improve the interface quality between the waveguide core layer 110 and the upper cladding layer 210, reducing sidewall scattering loss. In addition, the trapezoidal structure can further enhance the waveguide's birefringence modulation freedom and has a positive effect on the constraint capability of balancing TE and TM modes.

[0065] These two cross-sectional shapes offer complementary design freedom for mode field manipulation. Rectangular waveguides are highly compatible with photolithography processes and are the cornerstone for realizing ultra-compact, highly integrated devices; trapezoidal waveguides, due to their scattering suppression effect caused by the tilted sidewalls, are often used in scenarios where loss requirements are stringent. Of course, the cross-section of the waveguide core layer 110 is not limited to the description above.

[0066] The upper cladding 210 completely encloses the top and sides of the waveguide core layer 110, forming a uniform dielectric cladding layer. It typically uses a low-refractive-index material such as silicon dioxide to create a suitable refractive index difference with the waveguide core layer 110. The upper cladding 210 provides basic optical field confinement for the tapered waveguide while isolating the waveguide core layer 110 from the external environment, improving the structural stability and reliability of the device. Additionally, it provides a carrier or embedded substrate for the lateral constraint structure 310.

[0067] Further, participation Figures 1-4The lateral constraint structure 310 is precisely embedded in the waveguide cladding (composed of the upper cladding 210 and the buried oxide layer 410) and distributed on both sides along the gradient direction of the waveguide core layer 110. The lower surface of the lateral constraint structure 310 is flush with or lower than the lower surface of the waveguide core layer 110, and the upper surface of the lateral constraint structure 310 is flush with or higher than the upper surface of the waveguide core layer 110, ensuring complete coverage of the mode field distribution area of ​​the waveguide core layer 110 in the vertical direction. The refractive index of the lateral constraint structure 310 is lower than that of the waveguide cladding, thus achieving a three-order refractive index decrease from waveguide core layer 110 to waveguide cladding to lateral constraint structure 310. The lateral constraint structure 310 provides an additional lateral "focusing" effect when the width of the waveguide core layer 110 decreases, preventing excessive mode field diffusion, effectively suppressing lateral energy leakage that may be caused by the narrowing of the mode field, and enabling the output beam spot to achieve a higher mode field matching degree with the fiber mode field.

[0068] The lateral constraint structure 310 described in this invention can be implemented in various ways. Two representative specific embodiments are described below, both of which, together with the waveguide core layer 110, the upper cladding layer 210, the buried oxide layer 410, and the substrate 510, constitute the complete tapered waveguide. The first sub-implementation is the lateral constraint structure 310 based on the trench 311.

[0069] In this embodiment, the lateral constraint structure 310 is composed of the trench 311 formed in the waveguide cladding, and the trench 311 is filled with a medium with a refractive index lower than that of the waveguide cladding.

[0070] For details, please refer to Figures 2-4 The trench 311 is fabricated on both sides of the waveguide core layer 110 using a selective etching process. Its interior is filled with a medium whose refractive index is lower than that of the waveguide cladding, such as air, matching liquid / glue, water, oil, or other low-refractive-index media, thereby forming a physical barrier with significant refractive index contrast. In one specific embodiment, the waveguide core layer 110 in the tapered waveguide is made of silicon, the upper cladding 210 is made of silicon dioxide, the buried oxide layer 410 is made of silicon dioxide, and the material within the trench 311 is an air gap.

[0071] Furthermore, the vertical depth of the trench 311 is designed to be no less than the thickness of the waveguide core layer 110, ensuring that the low-refractive-index region can completely cover the vertical distribution range of the core layer mode field, thereby achieving sufficient constraint on the optical field. For example, the bottom of the trench 311 can be flush with the upper surface of the buried oxide layer 410, or it can be flush with the lower surface of the buried oxide layer 410, or it can even extend into the buried oxide layer 410. (See reference...) Figure 2, the depth of said structure 311 is D, and the bottom of said trench 311 may be flush with the lower surface of said buried oxide layer 410. In addition, said waveguide cladding with a controllable thickness is retained between the inner side wall of said trench 311 and the side wall of said waveguide core layer 110 as a first buffer layer. This design not only ensures the mechanical stability of the waveguide body, but also can finely regulate the strength of lateral confinement by adjusting the width W1 of said first buffer layer. As a specific embodiment, the width W1 of said first buffer layer ranges from 0 < a ≤ 50 μm, for example, any value within this range such as 1 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, etc. The range of the width W2 of said trench 311 can be specifically selected as required, and is not limited herein. In addition, the process for forming said trench 311 is compatible with integrated processes. It can not only effectively suppress mode field diffusion and reduce leakage loss through a high refractive index difference, but also provides a flexible and reliable physical carrier for subsequently introducing a tunable medium to realize dynamically controllable optical field confinement.

[0072] By way of example, the medium is selected from at least one of a gaseous medium, a solid medium and a liquid medium.

[0073] Specifically, the medium can be flexibly selected from at least one of gas, solid or liquid media according to specific optical performance requirements, process compatibility and functional expansion requirements. Gaseous media (such as air or inert gas) can provide the largest refractive index difference and the strongest static lateral confinement with their lowest refractive index (about 1.0), and the process is simple, and they often exist in the form of air gaps; solid media (such as silicon dioxide, optical polymers, etc.) have excellent mechanical and thermal stability, their refractive index can be selected within a certain range, which facilitates fine regulation of confinement force, and is fully compatible with standard semiconductor deposition processes; liquid media (such as refractive index matching liquid, liquid crystal, etc.) endow the structure with dynamic tuning capability, their refractive index can be continuously adjusted by external fields (electricity, heat, etc.), which provides a physical basis for constructing adaptive and reconfigurable photonic devices.

[0074] Furthermore, no matter what medium form is selected, its final refractive index must strictly satisfy the three-level refractive index decreasing relationship of "waveguide core layer 110 - waveguide cladding - lateral confinement structure 310", which is the optical prerequisite for ensuring effective lateral confinement and suppressing mode field leakage. This diversified medium strategy enables said structure to achieve optimal design in different dimensions such as high performance, high reliability and functional flexibility.

[0075] By way of example, the side wall of said trench 311 has an inclination angle, and the second angle range between the tangent to any point on the contour line of said side wall and the vertical direction is -45° to 45°.

[0076] Specifically, refer to Figure 4The second angle α characterizes the geometry of the sidewall of the groove 311. When the sidewall slopes inward from top to bottom towards the interior of the groove 311 (i.e., the groove 311 has a trapezoidal cross-section that is wider at the top and narrower at the bottom), α takes a positive value; when the sidewall slopes outward from top to bottom towards the exterior of the groove 311 (i.e., the groove 311 has an inverted trapezoidal cross-section that is narrower at the top and wider at the bottom), α takes a negative value. See also... Figures 2-3 When α = 0°, the sidewall of the groove 311 is an ideal vertical plane; as Figure 4 The groove 311 has a sidewall with an inclination angle α of 10°, and the sidewall is a slope with a constant inclination angle. When -45°≤α≤45°, the sidewall of the groove 311 can be a slope with a constant inclination angle or a curved surface with continuously varying curvature.

[0077] This geometric feature can be controlled by adjusting parameters of the dry or wet etching process (such as anisotropic etching ratio and mask lateral drilling degree). When α > 0°, it is beneficial to the uniformity and integrity of subsequent dielectric filling processes, avoiding filling voids caused by excessive aspect ratio; secondly, regardless of the sign of α, a specific sidewall tilt angle can optimize the spatial distribution of the constraint barrier, achieving more precise control over the lateral expansion behavior of the optical field; finally, when the sidewall has continuously varying curvature (α gradually changes along the contour line), it can serve as an additional degree of design freedom to compensate for process errors or further optimize mode field matching. The introduction of the second angle α enhances the overall advantages of the lateral constraint structure 310 in terms of the adjustability of manufacturing optical performance.

[0078] In a preferred embodiment of the present invention, the lateral constraint structure 310 is continuously distributed along the gradient direction of the waveguide core layer 110.

[0079] For details, please refer to Figures 6-7 The lateral constraint structure 310 is a strip or band-shaped structure that extends uninterruptedly along the light propagation path, completely covering the gradient region of the waveguide core layer 110 where enhanced lateral constraint is required. This continuous structure, within the waveguide cladding (composed of the upper cladding 210 and the buried oxide layer 410), forms a pair of parallel or continuously low-refractive-index boundaries on both sides of the waveguide core layer 110, from the initial change position of the waveguide core layer 110 to the position where the mode field evolution reaches or exceeds the expected target. The vertical range of this continuous structure completely covers the waveguide core layer 110; that is, its upper surface is not lower than the upper surface of the waveguide core layer 110, and its lower surface is not higher than the lower surface of the waveguide core layer 110, thereby ensuring effective constraint on the core layer mode field in the vertical direction. The sidewalls of the lateral constraint structure 310 can be parallel to the sidewalls of the waveguide core layer 110, or can change synchronously with the width of the waveguide core layer 110 according to a specific functional relationship.

[0080] The lateral constraint structure 310 provides a uniform and consistent additional lateral refractive index barrier along the entire gradient section. This continuous structure compensates for the naturally weakened lateral optical constraint force caused by the increase in the width of the waveguide core layer 110. It ensures that the optical mode field is consistently subjected to stable "guide walls" from both sides during adiabatic expansion, thereby continuously suppressing lateral diffusion and energy leakage of the mode field and maintaining the smoothness and symmetry of mode field evolution. Furthermore, this continuous distribution structure is easy to implement, typically achieved through a single patterning and etching process, offering good repeatability and providing reliable and predictable lateral constraint for the tapered waveguide, which helps ensure consistent device performance.

[0081] In another specific embodiment of the present invention, the first angle between the extension direction of the lateral constraint structure 310 on the horizontal plane and the axial direction of the waveguide core layer 110 is in the range of 0° to 90°, and the value of the first angle θ varies constantly, continuously or discontinuously along the gradient direction of the waveguide core layer 110 (e.g., segmented change with jumps between segments).

[0082] Specifically, the design of the first angle significantly improves the degree of freedom in adjusting the performance of the waveguide structure. By forming a variable first angle θ between the lateral constraint structure 310 and the axis of the tapered waveguide, wherein the first angle θ is in the range of 0°≤θ<90°, such as, but not limited to, values ​​within this range such as 0°, 15°, 20°, 40°, 60°, 80°, 89°, etc., and allowing the included angle to change continuously or discontinuously along the gradient direction, this change in lateral constraint strength compensates for the trend of optical field change caused by the cross-sectional geometric deformation or material anisotropy of the waveguide core layer 110.

[0083] Furthermore, the first angle θ may remain unchanged, may change smoothly and continuously along the tapered waveguide, or may change in segments with jumps between segments. In a specific embodiment, see [reference needed]. Figure 6 The first angle θ has a value of θ1 and remains unchanged. In another specific embodiment of the invention, the lateral constraint structure 310 is a groove 311, and the extending direction of the groove 311 is not fixed. (See also...) Figure 7 In the transition section of the waveguide core layer 110, the width of the waveguide core layer 110 remains constant, while the thickness gradually increases along the light propagation direction. The angle θ between the groove 311 and the axis starts from an initial value θ2 and increases, for example, to θ3 or θ4. The trend of this angle θ change matches the increasing trend of the thickness of the waveguide core layer 110.

[0084] In a specific embodiment of the present invention, see [reference needed]. Figure 8The lateral constraint structure 310 is disposed along the light propagation direction in the end region of the gradient section of the waveguide core layer 110, including the minimum cross section, rather than covering the entire gradient section.

[0085] Specifically, the lateral constraint structure 310 is discontinuously disposed on both sides of the waveguide core layer 110, aiming to precisely reinforce optically weak points during the thermal gradient process. (Reference) Figure 8 When the waveguide core layer 110 begins to tighten but the lateral constraint force has not yet increased synchronously, a transition region with relatively insufficient constraint will be formed, becoming a high-risk section for mode field diffusion and energy leakage. The discontinuous structure allows high-strength constraint units to be concentrated in these critical sections, thereby maintaining stable optical field transmission with higher material and area efficiency and suppressing lateral leakage.

[0086] The width of the waveguide core layer 110 linearly transitions from a first width W3 to a second width W4 along the light propagation direction, where the first width W3 is greater than the second width W4, and the smallest cross-section is located at the output end with the smaller width. In a local region near the output end (corresponding to a specific proportion of the total length of the transition section), the lateral expansion of the mode field is significantly enhanced, resulting in the highest risk of lateral leakage. Therefore, the lateral constraint structure 310 is only disposed on both sides of this end region, specifically as a pair of trenches 311, the depth of which is equal to that of the waveguide core layer 110, and the spacing between the trenches 311 remains constant along the light propagation direction. This local arrangement can control the transmission loss within an acceptable range, achieving performance comparable to that of a comparative device with lateral constraint throughout the entire length, but significantly reducing the total length of the lateral constraint structure 310, which is beneficial for reducing device size and process complexity.

[0087] Unlike the trench 311 scheme mentioned above, see [link to relevant documentation]. Figure 9 The present invention also provides another lateral constraint scheme based on the principle of equivalent medium. In the second sub-implementation, the lateral constraint structure 310 is implemented by a subwavelength grating structure 312, the period Λ of which satisfies: Λ<λ / n_clad, where λ is the operating wavelength and n_clad is the refractive index of the waveguide cladding material.

[0088] The subwavelength grating structure 312 is composed of nanostructure units arranged in a periodic lattice (such as a triangular lattice or a square lattice) on a two-dimensional plane. The nanostructure units can be cylinders, square pillars, holes, or other geometric shapes.

[0089] Furthermore, the equivalent refractive index n_eff of the subwavelength grating structure 312 is determined by the grating's duty cycle, the geometry of the nanostructure unit, and the material, and can be precisely calculated and designed based on the effective medium theory. To achieve effective lateral light confinement, this invention can adjust the above parameters so that the final obtained equivalent refractive index n_eff is lower than the intrinsic refractive index n_clad of the waveguide cladding (the waveguide cladding includes the buried oxide layer 410 and the upper cladding 210, both of which are made of the same material and have similar refractive indices), i.e., n_eff < n_clad.

[0090] In this invention, the operating wavelength λ refers to the target wavelength or band for which the device is designed. For example, in optical communication applications, λ can be 1550 nm or cover the C-band (1530 nm to 1565 nm); in broader integrated photonics applications, λ can also cover the visible light band or other communication bands. Those skilled in the art will understand that the subwavelength condition must be satisfied within the target operating wavelength or band.

[0091] As an example, such as Figures 10-12 As shown, the structure also includes a side wing core layer 320, which is located on both sides of the waveguide core layer 110 and has a gap between them; the side wing core layer 320 is located between the waveguide core layer 110 and the lateral constraint structure 310; the refractive index of the waveguide core layer 110 and the refractive index of the side wing core layer 320 are both greater than the refractive index of the waveguide cladding.

[0092] In this embodiment, the refractive index decreases sequentially from the side core layer 320 to the waveguide cladding and then to the lateral constraint structure 310. It should be noted that the refractive indices of the waveguide core layer 110 and the side core layer 320 are not strictly ordered; they can be formed from the same material, in which case their refractive indices are equal. Based on the refractive index gradient distribution, this embodiment can achieve a synergistic effect: First, in the narrow tip region of the waveguide core layer 110, the side core layer 320 can guide the optical field to expand further outward and maintain a stable mode field morphology, thereby significantly improving the mode overlap integral with external large-mode optical signals (such as single-mode optical fibers or lasers) and effectively reducing coupling loss. Second, the side core layer 320 is disposed between the waveguide core layer 110 and the lateral constraint structure 310, which helps to form a lateral refractive index gradient. As the light field extends from the waveguide core layer 110 to both sides, it can sense the gradually decreasing refractive index environment, thereby effectively suppressing the excitation of higher-order modes, achieving low-loss mode-field conversion, and further reducing the overall size of the device.

[0093] In one specific embodiment, the waveguide core layer 110 and the side wing core layer 320 are formed simultaneously from the same material layer through a single etching process, and both have equal thicknesses in the direction perpendicular to the substrate 510. The side wing core layer 320 and the waveguide core layer 110 are separated by a preset lateral spacing, which is filled by the material of the waveguide cladding. By optimizing the width of the spacing, the coupling efficiency and intensity of the evanescent wave between the waveguide core layer 110 and the side wing core layer 320 can be precisely controlled, thereby achieving precise control over the lateral distribution of the optical field. This design can not only be used to achieve adiabatic transfer of optical energy in a conical transition region, but also to achieve a specific ratio of optical power distribution according to the selection of the coupling length. It should be noted that the thickness of the side wing core layer 320 can be varied according to actual needs and is not limited to being equal to that of the waveguide core layer 110.

[0094] As an example, the spacing between the side wing core layer 320 and the waveguide core layer 110 is constant and continuously varied along the gradient direction of the waveguide core layer 110.

[0095] In some embodiments of the present invention, see [reference] Figure 10 The side wing core layer 320 is shown to be parallel to the light propagation direction of the waveguide core layer 110, and the spacing between the side wing core layer 320 and the waveguide core layer 110 remains constant along the gradient direction of the waveguide core layer 110. This constant setting helps to maintain uniform coupling strength in the coupling region, which is suitable for scenarios requiring stable optical field interaction.

[0096] In other embodiments of the present invention, the spacing varies continuously along the gradient direction. By pre-setting a variation law (e.g., linear increase, exponential decrease, or variation according to a specific function curve), the coupling coefficient between the side wing core layer 320 and the waveguide core layer 110 can be flexibly adjusted, thereby achieving dynamic adjustment of mode conversion efficiency and transmission bandwidth. See also Figure 11 The side wing core layer 320 is positioned at a certain angle to the light propagation direction in the waveguide core layer 110. In one specific embodiment, this spacing can be linearly increased along the light transmission direction to gradually reduce the disturbance of the side wing core layer 320 to the waveguide core layer 110, thereby achieving thermal mode evolution. By precisely setting the rate of change of the spacing (such as the slope or curvature), the bandwidth and loss characteristics of the device can be optimized. (See also...) Figure 11 The side wing core layer 320 can also have a multi-segment structure.

[0097] As an example, the width and thickness of the side core layer 320 along the light propagation direction can be independently increased, decreased, or kept constant. This ability to independently adjust provides greater design freedom for device performance optimization.

[0098] Specifically, by independently adjusting the width of the side core layer 320, its lateral confinement capability of the optical field can be controlled. Increasing the width enhances lateral mode confinement; decreasing the width achieves weak coupling or serves as a guiding structure for thermal adiabatic transition. By independently adjusting the thickness of the side core layer 320, the mode distribution and polarization characteristics in the vertical direction can be affected: increasing the thickness enhances vertical mode confinement and may introduce birefringence; decreasing the thickness causes the mode field to extend towards the waveguide cladding, reducing vertical confinement.

[0099] This decoupled design of width and thickness allows the side wing core layer 320 to simultaneously meet different constraints in the lateral and vertical directions. In some embodiments, the width can be kept constant while only the thickness is adjusted, or the thickness can be kept constant while only the width is adjusted, to achieve fine control over specific optical parameters (such as coupling coefficient, mode field diameter, birefringence, etc.).

[0100] As a further preferred embodiment, in the main region where evanescent wave coupling occurs between the side wing core layer 320 and the waveguide core layer 110, the width of the side wing core layer 320 is controlled to be 30% to 70% of the width of the waveguide core layer 110. This ratio range can ensure good coupling efficiency while avoiding scattering loss or mode interference caused by excessive width difference, significantly improving the optical performance and stability of the device. It should be noted that the above values ​​are only examples, and the actual value range can be adjusted accordingly based on the material system, operating wavelength, and process conditions.

[0101] In some other specific embodiments, based on Figure 10 The width of the side wing core layer 320 increases synchronously with the width of the waveguide core layer 110. In the light propagation direction, as the waveguide core layer 110 gradually widens to enhance the confinement of the light field, the side wing core layer 320 widens accordingly. This coordinated design ensures that the propagation constants of both layers maintain good phase matching within the coupling region. By maintaining this synchronized adiabatic evolution, optical energy can be smoothly transferred from the side wing core layer 320 to the waveguide core layer 110, effectively suppressing higher-order mode excitation and radiation loss caused by phase mismatch, thereby achieving high-efficiency mode switching while shortening the device length.

[0102] In another specific embodiment, based on Figure 10The width of the side wing core layer 320 decreases along the direction in which the width of the waveguide core layer 110 increases. In conventional uniform width or synergistic widening designs, periodic power oscillations may occur between the waveguide core layer 110 and the side wing core layer 320 due to stringent phase matching conditions, leading to wavelength-dependent coupling loss fluctuations. This embodiment introduces a gradual phase mismatch in the light propagation direction by employing a reverse width gradient design. This dynamic mismatch breaks the periodic oscillation mode, forcing light energy to transfer unidirectionally and smoothly from the side wing core layer 320 to the waveguide core layer 110, achieving adiabatic mode field conversion, thereby significantly reducing device loss over a wide spectral range.

[0103] As an example, the projected length of the side wing core layer 320 in the direction parallel to the length of the waveguide core layer 110 is one of the following: equal to the total length of the waveguide core layer 110, longer than the total length of the waveguide core layer 110, or shorter than the total length of the waveguide core layer 110.

[0104] See Figure 12 In this embodiment, the projected length of the side wing core layer 320 is equal to the total length of the waveguide core layer 110, and the side wing core layer 320 and the waveguide core layer 110 completely overlap in the direction of light propagation. This configuration enables uniform coupling strength or refractive index modulation along the entire waveguide length, making it suitable for applications requiring stable long-distance interactions.

[0105] See Figure 10 and Figure 11 In this embodiment, the projected length of the side wing core layer 320 is shorter than the total length of the waveguide core layer 110, and the side wing core layer 320 only covers a portion of the waveguide core layer 110. This allows for specific mode conversion, filtering, or frequency selection functions to be implemented in a localized area, optimizing device performance through fine-tuning of specific sections.

[0106] Example 2

[0107] See Figure 13 The tapered waveguide of the present invention can be used alone or combined with other waveguide segments as a functional unit to form a waveguide composite structure. This embodiment will describe the waveguide composite structure. The composite structure achieves a monotonically adiabatic change in the effective area of ​​the optical mode field from the input end to the output end of the composite structure by sequentially connecting at least one tapered waveguide of the present invention or similar to other waveguide segments along the optical propagation direction. This change can be monotonically increasing (for beam expansion and coupling) or monotonically decreasing (for beam compression and coupling). The composite structure mainly has the following two forms: (1) a combination consisting entirely of the tapered waveguide; (2) a combination containing other types of waveguide segments (such as conventional straight waveguides). The following is a description through specific embodiments.

[0108] First, the first combined embodiment includes a waveguide combination structure with a fully tapered waveguide.

[0109] like Figure 13 The diagram shows the first type of combined structure. The conical waveguide combined structure is formed by sequentially connecting the previously described conical waveguides. In this embodiment, both the first conical waveguide 610 and the second conical waveguide 620 are configured to monotonically change the effective area of ​​their optical mode fields along the light propagation direction. Preferably, both are configured to monotonically decrease the mode field area, thereby achieving a larger overall compression ratio than a single-segment conical waveguide. The output mode field of the first conical waveguide 610 is smoothly connected to the input mode field of the second conical waveguide 620 through mode field matching design, ensuring low reflection and low loss transmission of light energy at the interface. In another embodiment, the first conical waveguide 610 also includes the side wing core layer 320, which is located on both sides of the waveguide core layer 110 of the first conical waveguide 610 and has a gap with it, to further optimize the output mode field of the first conical waveguide 610 and achieve a smooth connection with the input mode field of the second conical waveguide 620.

[0110] Furthermore, the waveguide assembly structure, by directly connecting two high-performance tapered waveguides in series, can seamlessly connect two independent adiabatic transformation processes without inserting other functional segments. This allows the entire assembly structure to achieve an ultra-large mode field transformation ratio that a single tapered waveguide cannot achieve due to adiabatic limitations within a more compact overall device length, while leveraging the inherent low loss and high robustness advantages of each tapered waveguide unit.

[0111] Secondly, the second combined embodiment includes a waveguide combination structure comprising a tapered waveguide and any other waveguide structure.

[0112] Specifically, the waveguide combination structure is formed by sequentially connecting at least one tapered waveguide of the present invention with any other structure in the prior art (such as a strip waveguide); in a specific embodiment, in Figure 13 Based on this, one or more conventional straight waveguides are added between the first tapered waveguide 610 and the second tapered waveguide 620; wherein, the conventional straight waveguide serves as an intermediate functional segment for phase delay and coupling with other components, and its thickness and width match the corresponding tapered waveguides; the tapered waveguides at both ends are responsible for achieving mode field transformation efficiently and with low loss, and the entire combined structure ultimately achieves a monotonically decreasing effective area of ​​the optical mode field from the input end to the output end.

[0113] In summary, this invention provides a tapered waveguide and waveguide combination structure for efficient mode conversion, employing a three-order refractive index decreasing light guide structure to enhance mode field confinement and polarization stability. The tapered waveguide includes a waveguide core layer with gradually varying thickness, a waveguide cladding covering the top and sides of the core layer, and lateral constraint structures disposed on the waveguide cladding and at least on both sides of the core layer, wherein the refractive indices of the waveguide core layer, waveguide cladding, and lateral constraint structures decrease sequentially. The lateral constraint structures completely cover the waveguide core layer in the vertical direction, and their arrangement includes introducing deeply etched trenches into the waveguide cladding and filling them with a low-refractive-index medium, or constructing an equivalent refractive index structure of a subwavelength grating structure; in the horizontal direction, they can be parallel to the waveguide axis, or designed as inclined or non-parallel arrangements varying according to a function. Simultaneously, wing core layers can be disposed on both sides of the waveguide core layer, with a gap between them, to further control the lateral distribution of the light field and optimize mode field conversion efficiency.

[0114] This combined design dynamically compensates for changes in lateral constraint force during the tapering or widening of the waveguide core layer by enhancing the lateral refractive index barrier. This significantly shortens the tapered region length while maintaining the adiabatic evolution of the optical field, effectively suppressing mode leakage and higher-order mode excitation. Specifically, this tapered waveguide with varying thickness can simultaneously support the transmission of both transverse electric and transverse magnetic modes. Furthermore, the lateral constraint structure provides balanced and enhanced lateral confinement of the optical field for both polarization modes, significantly reducing polarization-dependent loss and crosstalk between waveguides. This invention provides a compact and high-performance solution for high-density integrated optical interconnects, polarization-independent mode switching, and multi-mode optical transmission systems, while improving integration density, expanding operating bandwidth, and reducing process tolerance. Therefore, this invention effectively overcomes the shortcomings of existing technologies and has high industrial applicability.

[0115] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A tapered waveguide, characterized in that, include: Waveguide core layer, the waveguide core layer including a thickness-gradient section; In the thickness gradient section, the thickness of the waveguide core layer itself changes continuously and monotonically along the light propagation direction; the ratio of the length L of the thickness gradient section along the light propagation direction to the total thickness change ΔH of the waveguide core layer itself is greater than 100:

1. An upper cladding layer covers the top and side regions of the waveguide core layer; A buried oxide layer covers the bottom of the waveguide core layer and is in direct contact with the upper cladding; the buried oxide layer and the upper cladding together constitute the waveguide cladding. A substrate, wherein the buried oxide layer is formed on the upper surface of the substrate; A lateral constraint structure is provided, at least in the upper cladding, and located on both sides of the waveguide core layer along the gradient direction; wherein the refractive indices of the waveguide core layer, the waveguide cladding, and the lateral constraint structure decrease sequentially; the lower surface of the lateral constraint structure is not higher than the lower surface of the waveguide core layer and not lower than the upper surface of the substrate; the upper surface of the lateral constraint structure is not lower than the upper surface of the waveguide core layer; the lateral constraint structure is continuously distributed along the gradient direction of the waveguide core layer; the extension direction of the lateral constraint structure in the horizontal plane has a first angle with the axial direction of the waveguide core layer, and the first angle is constant along the light propagation direction or increases smoothly and continuously from an initial value.

2. The tapered waveguide according to claim 1, characterized in that: The width of the waveguide core layer varies gradually along the direction of light propagation, similar to its thickness, or remains constant.

3. The tapered waveguide according to claim 1, characterized in that: The lateral constraint structure is composed of trenches formed in the waveguide cladding, the trenches being filled with a medium having a refractive index lower than that of the waveguide cladding.

4. The tapered waveguide according to claim 3, characterized in that: The sidewall of the groove has an inclination angle, and the second angle between the tangent at any point on the contour line of the sidewall and the vertical direction ranges from -45° to 45°.

5. The tapered waveguide according to claim 3, characterized in that: The medium is selected from at least one of gaseous medium, solid medium or liquid medium.

6. The tapered waveguide according to claim 1, characterized in that, The lateral constraint structure is implemented by a subwavelength grating structure, and the period Λ of the subwavelength grating structure satisfies: Λ<λ / n_clad, where λ is the operating wavelength and n_clad is the refractive index of the waveguide cladding material.

7. The tapered waveguide according to claim 1, characterized in that: The first angle between the extension direction of the lateral constraint structure on the horizontal plane and the axial direction of the waveguide core layer is in the range of 0° to 90°.

8. The tapered waveguide according to claim 1, characterized in that: The cross-section of the waveguide core layer is rectangular or trapezoidal.

9. The tapered waveguide according to claim 1, characterized in that: It also includes a side wing core layer, which is located on both sides of the waveguide core layer and has a gap between them; the side wing core layer is located between the waveguide core layer and the lateral constraint structure; the refractive index of the waveguide core layer and the refractive index of the side wing core layer are both greater than the refractive index of the waveguide cladding.

10. The tapered waveguide according to claim 9, characterized in that: The distance between the side wing core layer and the waveguide core layer is constant or continuously varying along the gradient direction of the waveguide core layer.

11. The tapered waveguide according to claim 9, characterized in that: The width and thickness of the side wing core layer along the light propagation direction can be increased, decreased, or kept constant independently.

12. The tapered waveguide according to claim 9, characterized in that, The projected length of the side wing core layer in the direction parallel to the length of the waveguide core layer is one of the following: equal to the total length of the waveguide core layer, longer than the total length of the waveguide core layer, or shorter than the total length of the waveguide core layer.

13. A waveguide composite structure, characterized in that: The waveguide assembly structure is composed of at least two waveguide segments connected sequentially along the direction of light propagation; wherein at least one of the at least two waveguide segments is a waveguide as described in any one of claims 1 to 12; and the waveguide assembly structure is capable of achieving a monotonically adiabatic change in the effective area of ​​the optical mode field from the input end to the output end of the assembly structure; the monotonically adiabatic change is either monotonically increasing or monotonically decreasing.

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