Glass skin waveguide and silicon waveguide coupling structure for three-dimensional integrated chip

Abstract

The application discloses a glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips. A three-dimensional curved skin waveguide is arranged in a glass substrate, and a coupling surface is arranged on a side of the three-dimensional curved skin waveguide close to a silicon substrate; a non-skin section is formed by a section where an incident end of the three-dimensional curved skin waveguide is located and is far away from the coupling surface; a skin section is formed by a section where an emitting end of the three-dimensional curved skin waveguide is located and is close to the coupling surface; and the emitting end is opposite to and horizontally coupled with a silicon-based waveguide. An optical signal is input from a standard optical fiber into the incident end of the horizontally interconnected three-dimensional curved skin waveguide, and then is input into the silicon-based waveguide through the emitting end after passing through the reverse-curved non-skin section and the skin section. The application utilizes the refractive index gradient skin waveguide and the skin section to compress the mode field and form a mode field adaptation area, so that the coupling loss can be reduced, and the coupling efficiency can be optimized by adjusting and controlling the mode field diameters of the skin waveguide and the silicon-based waveguide; and meanwhile, based on the fan-in and fan-out coupling mode of the skin waveguide, the thickness of the glass substrate can be reduced to realize on-chip integration.

Description

Technical Field

[0001] This invention relates to a waveguide coupling structure, and relates to the fields of femtosecond laser processing, semiconductor photonic integrated chips, and optical communication technology. Specifically, it relates to an optical interconnect structure for co-packaged optical modules and for three-dimensional integrated chips, and particularly to a glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips. Background Technology

[0002] With the rapid development of artificial intelligence, cloud computing, and high-performance computing, the data throughput within data centers is growing exponentially, placing unprecedented demands on the bandwidth density and energy efficiency of optical interconnect systems. Compared to electrical interconnects that use copper wires as the transmission medium, optical interconnects, especially waveguide interconnects, have become the best solution for data communication between short-distance data centers, between very short-distance computers, and even between integrated circuits due to their advantages such as high bandwidth, high speed, low cost, low energy consumption, and resistance to electromagnetic interference.

[0003] Currently, silicon-based photonic platforms have become the mainstream choice for on-chip optical communication due to their high compatibility with Complementary Metal-Oxide Semiconductor (CMOS) technology and excellent optoelectronic integration capabilities. As the design and manufacturing processes of silicon-based integrated optics become increasingly sophisticated, developing simple and high-quality fiber optic interconnect structures is crucial for ensuring high-quality signal transmission in the communication chain. Currently, the refractive indices and mode field diameters of silicon waveguides and optical fibers used in silicon-based photonic chips differ significantly. Existing off-chip fiber interconnect technologies for integrated photonic chips mainly employ grating vertical coupling and inverted conical end-face coupling schemes. Grating vertical coupling refers to coupling the optical fiber to the fiber on the chip surface in a manner nearly perpendicular to the chip surface. This scheme has high requirements for alignment accuracy and transmission bandwidth, and also suffers from polarization sensitivity and insufficient integration density. In contrast, end-face coupling allows for horizontal optical transmission and has advantages such as lower manufacturing difficulty, lower coupling loss, polarization insensitivity, wide transmission bandwidth, and high integration density, making it the most mainstream optical coupling method in practical applications. While traditional end-face coupling devices can achieve low insertion loss and high bandwidth coupling transmission with relatively low manufacturing difficulty, their commonly used V-groove structure also places high demands on etching accuracy and the surface quality of the fiber and sidewalls. Therefore, there is an urgent need to develop a low-cost, simple, low-loss, and high-quality end-face coupling method. Summary of the Invention

[0004] To address the problems existing in the background technology, this invention provides a glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips. This invention combines the high precision, customizable path and radius of femtosecond laser processing, high mode field with standard optical fiber, and refractive index matching characteristics. A three-dimensional "S"-shaped curved waveguide with depth-tunable characteristics is fabricated using femtosecond laser direct writing technology as a mode conversion medium. The use of a two-stage coupling mechanism effectively alleviates the coupling loss problem caused by the mode field size mismatch between the silicon-based waveguide and standard optical fiber in traditional solutions.

[0005] Compared to traditional planar processing techniques such as conventional photolithography and electron-beam lithography (EBL), femtosecond lasers can induce multiphoton absorption to alter the internal structure of transparent materials, offering advantages such as high precision, low cost, simple processing, and non-contact in-situ fabrication. Direct-write glass waveguides possess sub-micron spatial resolution, low transmission loss, and packaging potential compatible with silicon-based processes, making them a key technological path for achieving high-density, low-crosstalk three-dimensional photonic interconnects. On one hand, the refractive index of glass-based waveguides is well-matched to that of optical fibers, and they also possess tunable radius and mode field characteristics. On the other hand, glass-based waveguides allow for free control of waveguide depth variations and three-dimensional paths. Furthermore, glass-based waveguides also offer significant advantages in terms of process complexity and resistance to environmental interference.

[0006] The technical solution adopted in this invention is: The glass skin waveguide-silicon waveguide coupling structure for 3D integrated chips of the present invention includes a glass substrate on which at least one 3D curved skin waveguide is directly written by femtosecond laser, at least one silicon waveguide with a specific cross-sectional size, and a silicon substrate. The final packaged glass skin waveguide-silicon waveguide coupling structure serves as a 3D integrated chip. The glass substrate is mounted on one side of the top surface of the silicon substrate. Two vertical end faces of the glass substrate are the incident surface and the exit surface, respectively. The incident end and the exit end of each 3D curved skin waveguide coincide with the incident surface and the exit surface, respectively. The surface of the glass substrate closest to the silicon substrate is defined as the coupling surface. The section containing the incident end of each 3D curved skin waveguide is far from the coupling surface and is designated as the non-skin segment. The section containing the exit end of each 3D curved skin waveguide is close to the coupling surface and is designated as the skin segment. Each silicon waveguide is directly opposite and horizontally coupled to the exit end of its respective 3D curved skin waveguide. At least one silicon waveguide is mounted on the other side of the top surface of the silicon substrate. Each silicon waveguide is directly opposite to and horizontally coupled to the output end of a three-dimensional curved skin waveguide. The input ends of each three-dimensional curved skin waveguide are horizontally optically interconnected with their respective incident standard optical fibers. The optical signal is input from the input ends of one or more three-dimensional curved skin waveguides through the incident standard optical fibers, passes sequentially through the non-skin segment and skin segment of the reverse curve, and then enters the silicon waveguide through the output end. This constructs an on-chip integrated silicon optical coupler to realize single-line or parallel transmission.

[0007] The length direction of the three-dimensional curved skin waveguide is perpendicular to the incident and exit surfaces. The middle part of the three-dimensional curved skin waveguide is curved. The skin segment near the exit end and the non-skin segment near the incident end are parallel and both are parallel to the bottom surface of the glass substrate. The skin segment near the exit end is coaxially arranged with the silicon waveguide directly opposite it. The distance between the center of the skin segment near the exit end and the coupling surface is the skin distance, which ranges from 1 to 15 µm, preferably 2 µm, 6 µm or 10 µm. The distance between the center of the non-skin segment near the incident end and the coupling surface is greater than the skin distance.

[0008] The diameter of the skin segment of the three-dimensional curved skin waveguide is the same as or gradually distributed as that of the non-skin segment, gradually decreasing from the end near the non-skin segment to the output end. The diameter of the skin segment varies from 1 to 10 µm.

[0009] The silicon-based waveguide has an inverted conical tapered waveguide structure at one end near the three-dimensional curved skin waveguide, with its cross-sectional width gradually increasing along the direction of light propagation.

[0010] The aforementioned three-dimensional curved skin waveguide is composed of several waveguide trajectories of the same diameter that are closely parallel to each other along a straight line / curve, obtained by multiple direct writing operations in a glass substrate using a femtosecond laser.

[0011] When multiple three-dimensional curved skin waveguides are used, they form a three-dimensional multi-core array structure. The glass skin waveguide coupled with the silicon-based waveguide serves as a fan-in / fan-out device for femtosecond laser direct-write waveguides. The three-dimensional curved skin waveguides are evenly spaced, and the number of silicon-based waveguides is the same as the number of three-dimensional curved skin waveguides. The silicon-based waveguides are also evenly spaced. The depth of the skin segment near the output end of each three-dimensional curved skin waveguide is the same, while the depth of the non-skin segment near the input end of each three-dimensional curved skin waveguide may be the same or different. The number N of three-dimensional curved skin waveguides can be 1-100. When N≥2, the center distance between adjacent three-dimensional curved skin waveguides is 20-250 µm, preferably 127 µm. The output end of the skin segment of each three-dimensional curved skin waveguide is arranged in a one-to-one correspondence with its corresponding silicon-based waveguide.

[0012] A metal interconnect layer is provided between the three-dimensional curved skin waveguide and the silicon substrate. The metal interconnect layer includes one or more pairs of mutually adsorbable metal sheets, which are respectively arranged on the opposite side of the three-dimensional curved skin waveguide and the silicon substrate by sputtering or other methods. During installation, the three-dimensional curved skin waveguide is adsorbed and bonded to the silicon substrate by the pairs of opposite metal sheets, so that one or more three-dimensional curved skin waveguides are aligned with their respective silicon-based waveguides. In specific implementation, the three-dimensional curved skin waveguide is marked with a cross by femtosecond laser, and the cross marks should correspond one-to-one with the alignment marks / interconnection structures on the silicon substrate, thereby ensuring the horizontal alignment between the skin end of the silicon-based waveguide and the three-dimensional curved skin waveguide.

[0013] The beneficial effects of this invention are: 1) Compared to traditional fiber end-face coupling methods such as microlens focusing coupling and cylindrical lens beam shaping, coupling technology based on three-dimensional waveguide structures can achieve one-time customized fabrication of couplers by programming and controlling the spatial trajectory of the laser focus. This technology not only has lower process complexity and a simplified system architecture, but also achieves compact and efficient coupling, demonstrating significant advantages in the field of integrated optical devices.

[0014] 2) Compared with the mainstream tapered end-face coupling method, this scheme eliminates the need for standard CMOS processes such as mask fabrication, development, and etching, effectively reducing fabrication complexity and production costs. The refractive index gradient waveguide structure and the skin-end compressed mode field can naturally form a mode field adaptation region, reducing coupling loss.

[0015] 3) Compared with the existing glass waveguide fan-in and fan-out structures for coupling optical fibers and silicon-based waveguides, the fan-in and fan-out coupling method based on skin waveguides can ensure that the thickness of the glass block is less than 50 µm, thus truly achieving on-chip integration.

[0016] 4) This invention adopts a bonding method, which uses femtosecond laser to mark the position of the optical waveguide and aligns it with the alignment marks / interconnection structure of the silicon photonic chip to achieve high-precision alignment between the silicon-based waveguide and the skin waveguide. Compared with the traditional V / U groove alignment method, it effectively reduces the requirements for etching accuracy and surface quality of optical fiber and sidewall, and achieves high-precision alignment with lower process complexity.

[0017] 5) This invention is applicable to optical interconnection between optical fibers with different mode fields and silicon-based waveguides of different materials, exhibiting excellent adaptability. By adjusting the mode field diameters of the glass waveguide and the silicon-based waveguide, coupling efficiency can be effectively optimized.

[0018] 6) The skin waveguide of the present invention is 1-15µm away from the glass surface, which is suitable for the bonding requirements in silicon photonic chips and fully demonstrates the integration advantages of glass skin waveguide.

[0019] 7) This invention employs a passive alignment strategy based on visual imaging and photolithography, achieving high-precision integrated alignment of femtosecond laser-written glass skin waveguide fan-in / fan-out devices and integrated photonics chips in five degrees of freedom (three translations: X, Y, Z; two rotations: pitch and yaw). Compared to active alignment techniques in off-chip free space, this passive strategy significantly reduces the process complexity of multi-device coupling alignment.

[0020] In summary, this invention utilizes a refractive index-gradient skin waveguide and a skin segment to compress the mode field and form a mode field adaptation region, which can reduce coupling loss. Furthermore, it optimizes coupling efficiency by adjusting the mode field diameter of the skin waveguide and the silicon-based waveguide. Simultaneously, based on the fan-in and fan-out coupling method of the skin waveguide, it can ensure that the thickness of the glass substrate is reduced to achieve on-chip integration. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the fabrication method of a three-dimensional curved skin waveguide in a glass substrate by femtosecond laser direct writing according to the present invention; Figure 2 Schematic diagram of a multi-core three-dimensional curved skin waveguide for direct writing with femtosecond laser; Figure 3 This is a schematic diagram showing the interconnection between the integrated photonics glass chip containing the three-dimensional curved skin waveguide written by femtosecond laser in the glass substrate and the two-dimensional silicon photonics chip containing the silicon-based waveguide, as described in this invention. Figure 4 The images show a three-dimensional curved skin waveguide end face micrograph and a mode field diagram of the silicon-based waveguide output end face, as presented in this invention. Figure 4 (a) is a micrograph of the output end face of the three-dimensional curved skin waveguide of the present invention. Figure 4 (b) is the mode field diagram of the output end face of the silicon-based waveguide of the present invention; Figure 5 A physical diagram of a three-dimensional curved skin waveguide coupled with a silicon-based waveguide; Figure 6 This is a schematic diagram of the fiber-skin waveguide-silicon waveguide interconnect. In the figure: 1. Femtosecond laser focusing objective, 2. Femtosecond laser beam, 3. Three-dimensional curved skin waveguide, 4. Metal interconnect layer, 5. Silicon waveguide, 6. Silicon substrate, 7. Collecting objective, 8. Power meter, 9. Incident standard fiber. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] like Figure 1 , Figure 2 and Figure 3As shown, the glass skin waveguide-silicon waveguide coupling structure for three-dimensional integrated chips of the present invention includes a glass substrate on which at least one three-dimensional curved skin waveguide 3 is directly written by femtosecond laser, at least one silicon waveguide 5 with a specific cross-sectional size, and a silicon substrate 6. The final packaged glass skin waveguide-silicon waveguide coupling structure serves as a three-dimensional integrated chip. The glass substrate is mounted on one side of the top surface of the silicon substrate 6. Two vertical end faces of the glass substrate facing each other are the incident surface and the exit surface, respectively. The incident end and the exit end of each three-dimensional curved skin waveguide 3 coincide with the incident surface and the exit surface, respectively. The surface of the glass substrate closest to the silicon substrate 6 is defined as the coupling surface. The section where the incident end of each three-dimensional curved skin waveguide 3 is located is far from the coupling surface and is considered a non-skin segment. The section where the exit end of each three-dimensional curved skin waveguide 3 is located is close to the coupling surface and is considered a skin segment. At least one silicon waveguide 5 with a specific cross-sectional size is mounted on the other side of the top surface of the silicon substrate 6. Each silicon waveguide 5 is directly opposite to and horizontally coupled to the output end of a three-dimensional curved skin waveguide 3. The input ends of each three-dimensional curved skin waveguide 3 are horizontally optically interconnected with their respective incident standard optical fibers 9. The optical signal is input from the input ends of one or more three-dimensional curved skin waveguides 3 through the incident standard optical fibers 9, and after passing through the non-skin segment and skin segment of the reverse curvature in sequence, it is input into the silicon waveguide 5 through the output end. This constructs an on-chip integrated silicon optical coupler to realize single-line or parallel transmission.

[0024] Glass substrates include, but are not limited to, all transparent glass materials, such as: quartz glass, borosilicate glass, porous glass, phosphate glass, tellurate glass, germanate glass, bismuthate glass, fluorosilicate glass, fluorogermanate glass, fluoride glass, etc.

[0025] The length direction of the three-dimensional curved skin waveguide 3 is perpendicular to the incident and exit surfaces. The middle part of the three-dimensional curved skin waveguide 3 is curved. The skin segment near the exit end and the non-skin segment near the incident end are parallel and both are parallel to the bottom surface of the glass substrate. The skin segment near the exit end is coaxially arranged with the silicon-based waveguide 5 facing it. The distance between the center of the skin segment near the exit end and the bottom surface of the glass substrate is 1-15µm and is smaller than the distance between the non-skin segment near the incident end and the top surface of the glass substrate.

[0026] The three-dimensional curved skin waveguide has an adjustable depth from the surface. The principle behind this adjustable depth and surface thermal / optical interaction is based on femtosecond laser direct writing (FLDW) technology. By controlling the three-dimensional movement of the laser focus, waveguides can be directly written at any depth within a glass substrate. The fundamental physical mechanism behind the adjustable depth of the skin waveguide lies in evanescent wave coupling and the control of optical field leakage at the micro-nano scale. As the waveguide depth (e.g., 1-15µm) gradually approaches or even falls below the wavelength of light, the refractive index profile changes its ability to confine the optical field, allowing some of the optical field (evanescent field) to penetrate the glass-air (or glass-cladding) interface and leak into the external environment. This precise adjustability of depth allows the light transmitted within the waveguide to directly interact strongly with the active material covering the surface (evanescent field) through the evanescent field. The principle of multi-pass direct writing to resist surface ablation: Traditional single-pass femtosecond laser direct writing is prone to severe spherical aberration and surface ablation when close to the surface due to the huge refractive index contrast at the air-glass interface. By employing multi-scan overlap (such as the OCMS method) or photonic lattice (PLLSW) writing strategies, high-contrast and well-symmetrical waveguide structures can be formed at very shallow depths (1-10 µm) with extremely low pulse energy (15-17 nJ) through fine stacking of structures, ensuring that the surface is not damaged.

[0027] The vertical height difference between the skin-approaching and non-skin-approaching sections is adjustable, balancing low transmission loss and high surface interaction: the non-skin-approaching section (incident end) is buried deeper, effectively reducing scattering loss caused by surface defects and ensuring low-loss and stable beam transmission after entering the chip; while the skin-approaching section (exit end) is close to the surface (1-15µm). Three-dimensional topology wiring and spatial transformation: the vertical height difference is achieved through a mid-section bend, showcasing the advantages of the glass substrate's 3D topology capabilities, transforming traditional 2D coupling into three-dimensional coupling, greatly improving the chip's space utilization and integration density.

[0028] The coupling end of the three-dimensional curved skin waveguide 3 to the incident standard fiber 9 is far from the surface, while the coupling end to the silicon waveguide 5 is close to the surface. The purpose of this design is to ensure that the glass substrate can fit into the narrow space of the on-chip integrated bonding, and at the same time meet the dimensional requirements of the incident standard fiber 9 and the silicon waveguide 5 in the thickness direction. The glass substrate can be made very thin, less than 50µm.

[0029] The waveguide mode field of the 3D curved skin waveguide 3 can be set to single-mode, multi-mode, and vortex modes. The principle of 3D mode field customization is as follows: through femtosecond laser direct writing technology such as Overlapped Controlled Multi-scan (OCMS), the scanning overlap ratio (OR) at different positions of the waveguide cross-section can be precisely adjusted, thereby achieving sub-micron spatial resolution customization of the refractive index (RI) distribution (such as step type, gradient GRIN, "W" type, etc.). This ability to arbitrarily shape the refractive index cross-section allows the waveguide to not only support single-mode transmission, but also to support and select multi-mode and vortex modes (such as LP) by matching specific Laguerre-Gaussian modes through the central symmetry of the refractive index distribution. 11 LP 21 LP 31 Stable transmission in higher-order modes. Mode multiplexing scalability: Waveguides can be customized for vortex modes or multimodes, providing core hardware support for future large-capacity integrated photonic circuits based on space division multiplexing (SDM) or mode division multiplexing (MDM).

[0030] The diameter of the skin segment in the 3D curved skin waveguide 3 is the same as or gradually varies with the diameter of the non-skin segment, decreasing from the end closest to the non-skin segment towards the output end. The diameter of the skin segment varies from 1 to 10 µm. This design addresses thickness mismatch and enables ultra-thin, high-density packaging: Standard single-mode fiber (such as SMF-28) typically has a diameter of 125 µm and a core diameter of approximately 8 µm. Direct bonding to a silicon substrate presents a significant physical space conflict. In this design, the incident end is buried at a greater depth, providing sufficient space for fiber docking; the output end is extremely shallow, facilitating bonding or adhesion (such as evanescent wave coupling or direct end-face coupling) to the silicon waveguide. This solves the 3D dimensional mismatch problem between the "thick fiber" and the "thin silicon wafer," supporting high-density 3D packaging on ultra-thin (<50 µm) glass substrates.

[0031] The silicon-based waveguide 5, near the end of the three-dimensional curved skin waveguide 3, is an inverted tapered waveguide structure with its cross-sectional width gradually increasing along the light propagation direction. Silicon-based waveguide 5 employs an inverted taper coupling physical mechanism, utilizing a classic inverted taper waveguide structure. At the narrowest tip of the inverted taper, the silicon waveguide can barely confine the light field, which diffuses completely outward and is similar in size to the mode field of the external environment (glass waveguide). As the width of the silicon waveguide gradually increases along the light propagation direction, the light field is adiabatically pulled back and tightly confined again within the high-refractive-index core layer of the silicon waveguide. This enables polarization insensitivity and ultra-wideband coupling: the end-face coupling method of the inverted taper structure reduces polarization-dependent loss, and structural optimization can improve the coupling efficiency of TE and TM polarization states, simultaneously supporting high-efficiency TE and TM polarization state coupling, which significantly reduces polarization-dependent loss (PDL). Reduced sensitivity to process tolerance: Allowing different diameters and cross-sectional shapes at the coupling ends indicates that the coupling mechanism mainly relies on the macroscopic overlap of the mode field (through adiabatic gradual adaptive matching) rather than rigid geometric face-to-face splicing. This not only reduces the stringent requirements for end-face alignment (increasing lateral and axial alignment tolerances), but also greatly reduces the difficulty of micro-nano fabrication and bonding processes.

[0032] Furthermore, the cross-sectional dimensions of the emitting end of the silicon-based waveguide 5 and the coupling end of the three-dimensional curved skin waveguide 3 can be the same or different. The cross-sectional shapes of the three-dimensional curved skin waveguide 3 and the silicon-based waveguide 5 are not fixed and can be circular or rectangular.

[0033] The waveguide material of the silicon-based waveguide 5 can be at least one of the following: elemental silicon, group IV compound semiconductors, group III-V compound semiconductors, inorganic oxides, inorganic nitrides, inorganic nitrides, and ferroelectric crystal materials; preferably including silicon (Si), amorphous silicon, polycrystalline silicon, silicon nitride, silicon carbide, germanium, silicon germanium, silicon dioxide, silicon oxynitride, aluminum oxide, tantalum oxide, titanium oxide, hafnium oxide, lithium niobate, lithium tantalate, barium titanate, barium carbonate, indium phosphide, gallium arsenide, or gallium nitride. Material universality and hybrid integration principle: Different high-refractive-index semiconductors or dielectric materials possess different band structures and optical properties (e.g., lithium niobate has a very strong linear electro-optic effect, indium phosphide has a direct band gap and can be used as a gain-emitting medium, and silicon nitride has ultra-low loss and no two-photon absorption). The physical essence of the end-face coupling structure is mode field matching brought about by geometric adiabatic scaling. This physical process mainly depends on mode field matching and equivalent refractive index gradient. Therefore, it has a certain adaptability to different silicon-based photonic material systems and can realize a highly compatible hybrid photonic integration platform. This structure gives the glass substrate strong compatibility as a motherboard, and can seamlessly couple light-emitting active devices (III-V group), high-speed modulators (lithium niobate), and low-loss passive devices (silicon nitride / silicon dioxide) into the same 3D glass network, breaking through the technical bottleneck that single silicon-based materials cannot emit light efficiently and have weak electro-optic modulation capabilities.

[0034] The three-dimensional curved skin waveguide 3 is composed of several waveguide trajectories of the same diameter that are closely parallel to a straight line / curve obtained by multiple direct writing operations in a glass substrate using a femtosecond laser. The number of waveguide trajectories can be adjusted from 1 to 100. In specific implementation, the femtosecond laser focusing objective 1 is placed directly above the glass substrate, which is fixed by a three-dimensional displacement platform. The femtosecond laser beam 2 is output from the laser above the femtosecond laser focusing objective 1 and enters the glass substrate for direct writing. By controlling the three-dimensional displacement platform, the femtosecond laser focus moves inside the glass substrate, controlling the arrangement and skin depth of the single waveguide trajectory, thereby directly writing out the three-dimensional curved skin waveguide 3.

[0035] Multiple three-dimensional curved skin waveguides 3 form a three-dimensional multi-core array structure. The glass skin waveguide and silicon-based waveguide coupling structure serves as a fan-in and fan-out device for femtosecond laser direct writing waveguides. The three-dimensional curved skin waveguides 3 are evenly spaced. The number of silicon-based waveguides 5 is the same as the number of three-dimensional curved skin waveguides 3, and the silicon-based waveguides 5 are evenly spaced. The depth of the skin segment near the output end of each three-dimensional curved skin waveguide 3 is the same, and the depth of the non-skin segment near the incident end of each three-dimensional curved skin waveguide 3 is the same or different.

[0036] The skin segments of each three-dimensional curved skin waveguide 3 have the same diameter variation. The adiabatic evolution of the mode field and the principle of crosstalk suppression are as follows: In a multi-core array architecture, optical crosstalk between channels is a core challenge. Maintaining a uniform spacing between skin waveguides and using three-dimensional space to increase the spacing at the incident end (i.e., the depth can be different) can drastically attenuate the evanescent field overlap integral between channels by increasing the physical spacing, thus cutting off the coupling interference between modes at the source. At the exit end, the skin segments have consistent depth and the same diameter variation, ensuring that all channels have highly consistent group velocity evolution conditions and mode compression rates, maintaining phase consistency.

[0037] High-density fan-in / fan-out and channel expansion: Current silicon photonics-based chips are developing towards multi-core and multi-channel architectures. Multi-core array structures, as 3D fan-in / fan-out devices, cleverly utilize the longitudinal depth (Z-axis dimension) of the glass. The incident end (non-skin segment) can be arranged in a 3D array to match high-density multi-core fiber (MCF) or fiber array (FA); while the exit end skin segment is arranged flush with the surface in a one-dimensional linear array to interface with planar silicon gratings or inverted conical arrays. Ensuring batch consistency and reducing array packaging difficulty: The uniform depth and identical diameter gradient of the skin segments ensure that the mode field size and center position of each waveguide channel in the array are highly consistent at the coupling interface. During actual optical packaging alignment, only one overall six-degree-of-freedom alignment adjustment is needed to achieve synchronous and efficient alignment coupling of all channels, greatly reducing the difficulty and manufacturing cost of optical packaging for array-level devices.

[0038] An Au / Ni metal interconnect layer 4 is disposed between the three-dimensional curved skin waveguide 3 and the silicon substrate 6. The metal interconnect layer 4 includes one or more pairs of mutually adsorbable metal sheets, which are respectively arranged on the opposite side of the three-dimensional curved skin waveguide 3 and the silicon substrate 6 by sputtering or other methods. During installation, the three-dimensional curved skin waveguide 3 is adsorbed and bonded to the silicon substrate 6 by the metal sheets in pairs, so that one or more three-dimensional curved skin waveguides 3 are aligned with their respective silicon waveguides 5. In specific implementation, the three-dimensional curved skin waveguide 3 is marked with a cross by femtosecond laser, and the cross marks should correspond one-to-one with the alignment marks / interconnection structures of the silicon substrate 6, thereby ensuring the horizontal alignment between the skin end of the silicon waveguide 5 and the three-dimensional curved skin waveguide 3.

[0039] The three-dimensional curved skin waveguide of the present invention adopts an optimized S-shaped curved structure, in which one end is bent toward the glass surface to form a skin segment. The skin segment is coupled with the silicon-based waveguide. The planar alignment between the two is achieved by using the pre-set alignment mark structure on the surface of the integrated photonics chip and the femtosecond laser direct writing device. The axial alignment criterion is precisely controlled by the femtosecond laser direct writing waveguide technology.

[0040] The preparation process of the present invention is as follows: by adjusting the power, pulse width, wavelength, repetition frequency and polarization of the femtosecond laser, the scanning speed and focusing depth of the femtosecond laser beam 2 in the sample, as well as the magnification and numerical aperture of the femtosecond laser focusing objective 1, the writing paths of various submicrometer or micrometer-scale tracks are planned to obtain a bent three-dimensional curved skin waveguide 3 with low insertion loss; after marking the waveguide position with femtosecond laser, the glass substrate provided with at least one three-dimensional curved skin waveguide 3 is bonded onto the silicon-based photonic chip, that is, onto the silicon-based waveguide 5 and the silicon substrate 6.

[0041] For femtosecond laser processing parameters, femtosecond lasers with different output wavelengths (e.g., 355, 515, 800, 1030 nm, etc.), different pulse widths (40 fs-10 ps), different repetition frequencies (1 kHz-10 MHz), different powers and different polarization outputs are used; femtosecond laser focusing objectives 1 with different numerical apertures (0.4<NA<1.5) are adopted; different scanning speeds (0.1-50 mm / s) and different intervals for multiple direct writing (0.5-1.5 µm) are applied. After optimization, a three-dimensional curved skin waveguide 3 with low loss and good mode matching with the silicon-based waveguide 5 can be obtained.

[0042] The principle of the present invention is that during processing, femtosecond laser pulses undergo nonlinear absorption processes including multiphoton ionization, tunneling effect and avalanche ionization at the focused focal spot to form plasma, and the plasma further absorbs energy, thereby realizing non-thermal-melting "cold processing" of the material and forming an optical waveguide with positive refractive index change. The program-controlled three-dimensional displacement platform performs multiple repeated scans to orderly arrange and stack scanning tracks, so as to realize an "S-shaped" three-dimensional curved skin waveguide 3 with a diameter of about 6-50 µm that has relatively good mode field matching with the incident standard optical fiber 9. Further, by adjusting the femtosecond laser parameters, the stress field distribution in the glass substrate is controlled, and a three-dimensional curved skin waveguide 3 with a depth of about 1-15 µm whose skin section mode field is relatively well matched with that of the silicon-based waveguide 5 is prepared.

[0043] As Figure 1 shown, in specific implementation, the present invention uses Eagle glass as the material of the glass substrate. The processing of the three-dimensional curved skin waveguide 3 of "photonic lattice-like" processed by femtosecond laser direct writing mainly includes the following steps: Step 1) Place the glass substrate on a three-dimensional displacement stage, focusing the femtosecond laser at the target position. The femtosecond laser directly writes a submicron or micron-scale trajectory in the glass substrate in a single pass. At this point, the single-pass direct-write trajectory of the positive refractive index change does not possess light-guiding properties. The femtosecond laser parameters used are: center wavelength 1030nm, repetition rate 1MHz, pulse width 213fs. An oil-immersed lens with a magnification of 100x and a numerical aperture of 1.2 is used, with a laser power of 10-40mW, a scanning speed of 20mm / s, and a focusing depth of 10μm.

[0044] Step 2) According to the design program, the single-strand trajectory is repeatedly scanned multiple times according to the set parameters to form a cylindrical skin waveguide matching the single-mode fiber mode. The laser direct writing spacing is 0.5-1.5μm each time, and a total of 40-1500 scans are used to form a cylindrical three-dimensional curved skin waveguide 3 with a diameter of about 6μm that matches the single-mode fiber mode field. The end face is as follows. Figure 4 As shown in (a), one end of the three-dimensional curved skin waveguide 3 is connected to the surface by two opposing curved structures with a bending radius of 10 mm. The distance between the skin segments and the glass surface ranges from 2 to 10 μm. Specifically, the depth of the skin segment is 10 µm and the skin length ranges from 0.15 mm. The insertion loss of the three-dimensional curved skin waveguide 3 formed is as low as less than 1 dB.

[0045] The integrated photonic glass chip housing the three-dimensional curved skin waveguide 3 has high integration potential and can be considered an effective platform for constructing compact, multifunctional photonic systems. For example... Figure 2 and Figure 3 As shown, by directly writing multi-channel skin waveguide trajectories into a glass substrate, efficient end-face coupling with multi-core optical fibers or multi-channel incident standard optical fiber arrays (9) is achieved, while simultaneously achieving efficient end-face coupling with multi-channel silicon-based waveguides (5). This approach allows for the simultaneous carrying and manipulation of multiple independent optical signal paths, enabling parallel data transmission and providing a key technical path for improving the channel density and functional complexity of optical communication and sensing systems.

[0046] Then proceed with the following steps: Step 1) First, positioning is achieved using crosshairs formed by femtosecond laser dot-matrix marking, such as... Figure 5 As shown, the upper right corner is a femtosecond laser processing mark diagram, on which a metal interconnect layer 4 is constructed on the upper surface of a glass substrate with a three-dimensional curved skin waveguide 3 directly written on it.

[0047] Step 2) Fabricate a silicon-based waveguide chip sample comprising a silicon-based waveguide 5 and a silicon substrate 6, and process the silicon-based waveguide chip metal interconnect layer 4. Invert the glass substrate and directly align the skin segment with the corresponding end face of the silicon-based waveguide 5. Based on a passive alignment strategy using visual imaging and photolithography, achieve the bonding of the glass substrate and the silicon-based waveguide chip, and ensure accurate relative positioning through imaging. The final bonded and integrated coupling result is shown in the image. Figure 5 As shown.

[0048] Step 3) Perform coupling tests on the high-efficiency coupling structure between the glass-based skin waveguide and the silicon-based waveguide for 3D integrated chips, such as... Figure 6 As shown, A laser with a center wavelength of 1550nm is used as the light source and is incident on the standard optical fiber 9. A high-precision three-dimensional six-axis displacement platform supports the optical fiber clamp. By adjusting the spatial position of the standard optical fiber 9, its end is aligned with the cross-section of the three-dimensional curved skin waveguide 3. A CCD camera and a matching imaging system are suspended above the three-dimensional curved skin waveguide 3. This imaging system is used to observe the alignment between the standard optical fiber 9 and the three-dimensional curved skin waveguide 3. When the light couples to the non-skin segment of the three-dimensional curved skin waveguide 3, which matches its mode field, it is transmitted through the three-dimensional curved skin waveguide 3 to the skin segment. At this point, the mode field is compressed into an elliptical mode field shape, thereby achieving efficient mode matching and coupling with the target silicon-based waveguide 5.

[0049] Specifically, another displacement platform is placed on one side of the silicon-based waveguide 5. A lens placed on this platform can function as a magnifying glass. A collecting objective lens 7 with a magnification of 20× and a numerical aperture of 0.42 is used to focus the light onto the target surface of the power meter 8, with the collecting objective lens 7 aligned with the silicon-based waveguide 5. A laser with a center wavelength of 1550 nm is used for testing to measure the output power. The details are as follows: in, P For power, show the power conversion. , All are units; Insertion loss of the entire structure , The input power is measured at end 9 of the incident standard optical fiber. The output power collected by power meter 8.

[0050] The final fabricated three-dimensional integrated chip exhibits an insertion loss as low as 2.98 dB for its highly efficient coupling structure between a glass-based skin waveguide and a silicon-based waveguide. The local coupling efficiency reaches as high as 63.2%. Furthermore, the power meter 8 can be replaced with a beam quality analyzer for measurement and collection, yielding the mode field pattern at the output end of the silicon-based waveguide 5, as shown below. Figure 4 As shown in (b), the converted width of the mode field at the exit end is 23.79 µm and the height is 12.72 µm. This measurement result intuitively demonstrates that the flip-chip coupling structure of the present invention successfully achieves a smooth transition of the optical field and mode field expansion, effectively completing the mode conversion.

[0051] In summary, this invention, based on the unique mode field distribution characteristics of skin waveguides and combined with the advantages of low transmission loss, path curvature radius, and independently controllable mode characteristics of the three-dimensional curved skin waveguide 3 written by femtosecond lasers, achieves efficient coupling between optical fiber and silicon photonic chip while significantly reducing process complexity and optical alignment accuracy requirements. This effectively improves the channel capacity of optical communication systems and the computational dimension of optical signal processors, highlighting the application value of femtosecond laser direct writing technology in achieving high-density three-dimensional optical interconnects and multifunctional integration in the field of integrated photonics.

[0052] The above embodiments are merely specific examples of the present invention, and their descriptions are intended to clearly illustrate the technical solutions. They should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, any equivalent modifications, structural variations, process substitutions, or combinations of technical features made based on the core design principles and innovative spirit of the present invention, as long as they do not depart from the technical concept defined in the claims of the present invention, shall be deemed to fall within the patent protection scope of the present invention.

Claims

1. A glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips, characterized in that: The system comprises a glass substrate on which at least one three-dimensional curved skin waveguide (3) is directly written by a femtosecond laser, at least one silicon-based waveguide (5), and a silicon substrate (6). The glass substrate is mounted on one side of the top surface of the silicon substrate. Two opposite vertical end faces of the glass substrate are the incident surface and the exit surface, respectively. The surface of the glass substrate closer to the silicon substrate (6) is the coupling surface. The segment where the incident end of each three-dimensional curved skin waveguide (3) is located is away from the coupling surface and serves as a non-skin segment. The segment where the exit end of each three-dimensional curved skin waveguide (3) is located is close to the coupling surface and serves as a non-skin segment. Skin segment; at least one silicon waveguide (5) is installed on the other side of the top surface of the silicon substrate (6). Each silicon waveguide (5) is directly opposite to and horizontally coupled to the output end of a three-dimensional curved skin waveguide (3). The input ends of each three-dimensional curved skin waveguide (3) are horizontally optically interconnected with their respective input standard optical fibers (9). The optical signal is input from the input end of one or more three-dimensional curved skin waveguides (3) through the input standard optical fiber (9), and after passing through the reverse curved non-skin segment and skin segment in sequence, it is input into the silicon waveguide (5) through the output end to realize single-line or parallel transmission.

2. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: The length direction of the three-dimensional curved skin waveguide (3) is perpendicular to the incident surface and the exit surface. The middle part of the three-dimensional curved skin waveguide (3) is curved. The skin segment near the exit end and the non-skin segment near the incident end are parallel and both are parallel to the bottom surface of the glass substrate. The skin segment near the exit end is coaxially arranged with the silicon waveguide (5) facing it. The distance between the center of the skin segment near the exit end and the coupling surface is the skin distance, which ranges from 1 to 15 µm. The distance between the center of the non-skin segment near the incident end and the coupling surface is greater than the skin distance.

3. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: The diameter of the skin segment of the three-dimensional curved skin waveguide (3) is the same as or gradually distributed as the non-skin segment, and gradually decreases from the end near the non-skin segment to the output end. The diameter of the skin segment varies from 1 to 10 µm.

4. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: The silicon-based waveguide (5) has an inverted conical tapered waveguide structure at one end near the three-dimensional curved skin waveguide (3), and its cross-sectional width gradually increases along the direction of light propagation.

5. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: The three-dimensional curved skin waveguide (3) is composed of several parallel waveguide trajectories with the same diameter obtained by multiple direct writings in a glass substrate using a femtosecond laser.

6. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: When there are multiple three-dimensional curved skin waveguides (3), a three-dimensional multi-core array structure is formed. Each three-dimensional curved skin waveguide (3) is evenly spaced. The number of silicon-based waveguides (5) is the same as the number of three-dimensional curved skin waveguides (3). Each silicon-based waveguide (5) is evenly spaced. The depth of the skin segment of each three-dimensional curved skin waveguide (3) near the emission end is the same. The depth of the non-skin segment of each three-dimensional curved skin waveguide (3) near the incident end is the same or different.

7. The glass skin waveguide and silicon-based waveguide coupling structure for three-dimensional integrated chips according to claim 1, characterized in that: A metal interconnect layer (4) is provided between the three-dimensional curved skin waveguide (3) and the silicon substrate (6). The metal interconnect layer (4) includes one or more pairs of metal sheets that can be attracted to each other, which are respectively arranged on the opposite side of the three-dimensional curved skin waveguide (3) and the silicon substrate (6). During installation, the three-dimensional curved skin waveguide (3) is attached to the silicon substrate (6) by the attraction of the two pairs of opposite metal sheets, so that one or more three-dimensional curved skin waveguides (3) are opposite to their respective silicon waveguides (5).

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