Optical waveguide structure, design and preparation method thereof and photon integrated circuit

Abstract

The invention provides an optical waveguide structure and a design and preparation method thereof, and a photon integrated circuit, and belongs to the technical field of integrated photonics, the optical waveguide structure comprises a waveguide segmentation structure, the waveguide segmentation structure has a preset bending radius, and the cross section of the waveguide segmentation structure is an elliptical cross section; the elliptical cross section is defined by ovality and a long axis direction, and a relative included angle is formed between the long axis direction and the bent plane of the waveguide segmented structure; and the ovality and the relative included angle are configured to enable a first mode coupling coefficient introduced by the bending of the waveguide segmented structure and a second mode coupling coefficient introduced by the ovality of the elliptical cross section to be equal in amplitude and opposite in phase. According to the invention, high-purity and crosstalk-free stable transmission of the orbital angular momentum mode in the bent waveguide is realized.

Description

Technical Field

[0001] This invention relates to the field of integrated photonics technology, specifically to an optical waveguide structure, its design, fabrication method, and photonic integrated circuit. Background Technology

[0002] With the rapid development of technologies such as big data, artificial intelligence, and quantum information, unprecedented demands have been placed on the transmission capacity and integration density of on-chip optical interconnects and information processing systems. Traditional multiplexing dimensions such as intensity, wavelength, and polarization are gradually approaching their physical limits, making it difficult to continuously meet the exponentially increasing data throughput demands. Therefore, mode division multiplexing (MDM) technology based on the degrees of freedom of optical field spatial modes, especially utilizing orbital angular momentum modes with orthogonal phase distribution characteristics, is considered one of the key paths to overcome existing capacity bottlenecks. OAM (Orbital Angular Momentum) modes, due to their theoretically infinite topological charge dimension, can transmit multiple independent signals in parallel within a single waveguide, greatly improving the channel capacity and spectral efficiency of on-chip systems.

[0003] To achieve efficient multiplexing and low crosstalk transmission of OAM modes in practical waveguides, the following technical solutions can be adopted, mainly including: compensating for mode degeneracy splitting caused by manufacturing errors by optimizing the geometry of the waveguide cross-section (such as adjusting circular symmetry or designing a gradient profile); artificially creating effective refractive index differences between modes by using gradient refractive index cladding or anisotropic optical materials, thereby suppressing coupling; and introducing mode-selective filters or asymmetric structures into the waveguide to actively filter out or suppress unwanted crosstalk modes. These methods provide, to varying degrees, ways to suppress crosstalk from specific disturbance sources (such as material inhomogeneities or cross-sectional distortion).

[0004] However, the aforementioned existing technologies still have significant limitations. First, most solutions only optimize for a single type of perturbation (such as elliptic distortion in straight waveguides or ideal bending), lacking comprehensive suppression capabilities for the complex crosstalk mechanisms caused by the combined effects of bending and cross-sectional ellipticity. Second, designs such as graded refractive index cladding or complex filter structures are often extremely sensitive to fabrication accuracy, operating wavelength, and polarization state, resulting in small actual fabrication tolerances, unstable device performance, and low yield. Furthermore, some active compensation schemes require external control, increasing system complexity and power consumption. Summary of the Invention

[0005] In view of this, it is necessary to provide an optical waveguide structure, its design, fabrication method, and photonic integrated circuit to solve the shortcomings of existing technologies that cannot effectively suppress mode crosstalk caused by bending and cross-sectional ellipticization, and that generally suffer from complex design and unstable performance.

[0006] To address the aforementioned technical problems, in a first aspect, the present invention provides an optical waveguide structure, comprising: A segmented waveguide structure, wherein the segmented waveguide structure has a preset bending radius and the cross-section of the segmented waveguide structure is an elliptical cross-section; The elliptical cross section is defined by ellipticity and major axis direction, and the major axis direction forms a relative angle with the bending plane of the waveguide segment structure; The ellipticity and the relative angle are configured such that the first mode coupling coefficient introduced by the bending of the waveguide segment structure is equal in magnitude and opposite in phase to the second mode coupling coefficient introduced by the ellipticity of the elliptical cross section.

[0007] In one possible implementation, the value of the ellipticity is negatively correlated with the value of the preset bending radius.

[0008] In one possible implementation, the waveguide segmentation structure is used to transmit orbital angular momentum modes with topology parameters of +1 and -1.

[0009] In one possible implementation, the waveguide segmentation structure includes a core layer and a cladding layer surrounding the core layer.

[0010] In one possible implementation, the core layer is made of glass and is fabricated using femtosecond laser direct writing technology.

[0011] In one possible implementation, the relative angle is configured such that the phase difference between the second mode coupling coefficient and the first mode coupling coefficient is π.

[0012] In a second aspect, the present invention also provides a photonic integrated circuit, wherein the photonic integrated circuit integrates the optical waveguide structure described in the first aspect, for realizing signal transmission or processing in orbital angular momentum mode.

[0013] Thirdly, the present invention also provides a design method for the optical waveguide structure described in the first aspect, the design method comprising: A preset bending radius is determined for a waveguide segment structure; the cross-section of the waveguide segment structure is an elliptical cross-section. Based on the preset bending radius, the first mode coupling coefficient generated by the waveguide bending effect is calculated; Key parameters are adjusted to calculate the second mode coupling coefficient introduced by the elliptical cross section, the key parameters including the ellipticity and relative angle of an elliptical cross section model, the relative angle being the angle between the major axis direction of the elliptical cross section and the bending plane of the waveguide segment structure; The final ellipticity and the final relative angle that satisfy the preset conditions are determined as the final design parameters of the waveguide segment structure. The preset conditions are that the first mode coupling coefficient and the second mode coupling coefficient are equal in amplitude and opposite in phase.

[0014] In one possible implementation, the value of the ellipticity is negatively correlated with the value of the preset bending radius.

[0015] Fourthly, the present invention also provides a method for fabricating an optical waveguide structure, which is based on the design method for the optical waveguide structure described in the third aspect. The fabrication method includes: obtaining and fabricating the optical waveguide structure based on the final design parameters.

[0016] The beneficial effects of the present invention are: the optical waveguide structure provided by the present invention, firstly... Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of a curved waveguide structure with a conventional circular cross-section; Figure 2 A schematic diagram of an embodiment of the optical waveguide structure provided by the present invention; Figure 3 A schematic flowchart of an embodiment of the design method for the optical waveguide structure provided by the present invention; Figure 4 A schematic flowchart of an embodiment of the fabrication method of the optical waveguide structure provided by the present invention; Figure 5 These are schematic diagrams illustrating two different types of crosstalk in this invention; Figure 6 This is a schematic diagram showing the crosstalk-free mode evolution result of the optical waveguide structure with an elliptical cross section according to the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] In the description of the embodiments of the present invention, unless otherwise stated, "multiple" means two or more. "And / or" describes the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0021] The terms "first," "second," etc., used in the embodiments of this invention are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a technical feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] like Figure 1 As shown, a conventional circular curved waveguide when carrying l When the OAM mode of the +1 topological charge is incident, energy will couple (crosstalk) to the ground during propagation due to the bending effect. l =-1 mode, and vice versa. In a conventional circular curved waveguide, the OAM beam cannot be transmitted stably. The angular momentum information it carries (i.e., the topological charge l value) will become disordered and distorted as the path bends, making it unusable for reliable on-chip communication or computation. Figure 1 Because OAM mode experiences periodic crosstalk and energy exchange, it cannot maintain stable and clean transmission.

[0024] To address the aforementioned problems, this invention provides an optical waveguide structure, its design, fabrication method, and a photonic integrated circuit, which will be described below.

[0025] Figure 2 This is a schematic diagram of an embodiment of the optical waveguide structure 1 provided by the present invention, as shown below. Figure 2 As shown, the optical waveguide structure 1 includes: Waveguide segment structure D2, wherein the waveguide segment structure D2 has a preset bending radius and the cross section of the waveguide segment structure D2 is an elliptical cross section; The elliptical cross section is defined by ellipticity and major axis direction, and the major axis direction forms a relative angle θ with the bending plane of the waveguide segment structure D2; The ellipticity and the relative angle θ are configured such that the first mode coupling coefficient introduced by the bending of the waveguide segment structure D2 is equal in magnitude and opposite in phase to the second mode coupling coefficient introduced by the ellipticity of the elliptical cross section.

[0026] It should be noted that: such as Figure 2 As shown, the overall optical waveguide structure 1 is an on-chip integrated optical path, consisting of three waveguide segments connected sequentially: an input straight waveguide segment D1, an output straight waveguide segment D3, and a curved waveguide segment (i.e., waveguide segmentation structure D2) located between the input straight waveguide segment D1 and the output straight waveguide segment D3. The input straight waveguide segment D1 is a straight waveguide used to transmit the pure OAM mode (e.g., generated by an external light source) to the input straight waveguide segment D3. l (+1 mode) The optical signal is smoothly coupled and guided to the intermediate curved waveguide section. The cross-section of the input straight waveguide section D1 is typically circular or elliptical to ensure mode matching and reduce access loss. The curved waveguide section is the waveguide section that enables the optical path to turn or loop; its cross-section is elliptical. The output straight waveguide section D3 is a straight waveguide used to receive and output the optical signal transmitted from the curved waveguide section, guiding it to the next optical element (such as a detector, demultiplexer, etc.) or chip output.

[0027] Based on the layout requirements of photonic integrated circuits, the preset bending radius that the waveguide segment structure D2 needs to achieve is determined. The bending of the waveguide segment structure D2 will introduce a first-mode coupling coefficient. Kbend First mode coupling coefficient Kbend This represents the inherent coupling strength and phase between certain OAM modes (such as OAM+1 and OAM-1) caused by waveguide bending; the first-mode coupling coefficient. Kbend This determines the speed and pattern of the periodic oscillation of energy between +1 and -1 modes. The first-mode coupling coefficient... Kbend The magnitude of the amplitude depends primarily on the preset bending radius R. The smaller R is (the sharper the bend), the greater the refractive index gradient and the stronger the coupling. Kbend ∝1 / R. Simultaneously, the elliptical cross-section introduces a second-mode coupling coefficient. Kellipse Second mode coupling coefficient Kellipse This represents the coupling strength and phase introduced between the same OAM modes because the waveguide cross-section is elliptical (rather than circular). Second mode coupling coefficient Kellipse The magnitude of the amplitude mainly depends on the ellipticity ρ (which depends on the semi-major axis length a and semi-minor axis length b of the ellipse's cross-section). The greater the deviation of ρ from 1 (the more flattened the ellipse), the stronger the asymmetry and the stronger the coupling. Kellipse ∝ρ, Second-mode coupling coefficient KellipseThe phase can be adjusted by the relative angle θ between the major axis of the ellipse and the bending plane. With a fixed preset bending radius R, the coupling coefficient of the first mode can be calculated and determined. Kbend Therefore, the ellipticity and relative angle (ρ, θ) can be set such that the second mode coupling coefficient calculated based on the ellipticity and relative angle (ρ, θ) is such that... Kellipse Coupling coefficient with first mode Kbend Satisfy: | Kellipse ∣=∣ Kbend |and Kellipse Kbend = π This means achieving equal amplitude and opposite phase.

[0028] In summary, the optical waveguide structure 1 provided in this embodiment of the invention first sets the ellipticity and relative angle (ρ, θ) to make the amplitudes equal and the phases opposite, so that the negative coupling introduced by the elliptical cross section (i.e., the second mode coupling coefficient) is reduced. Kellipse The amplitude of the waveguide is positively coupled with the bending of the waveguide (i.e., the first-mode coupling coefficient). Kbend The amplitudes of the two modes are matched to ensure destructive interference during waveguide propagation, thus suppressing crosstalk between OAM modes. Furthermore, the entire scheme is achieved entirely through the waveguide's own geometry, eliminating the need for additional active control components (such as thermo-optic or electro-optic modulators), complex multilayer materials, or external filters. This significantly reduces system complexity, power consumption, and potential failure points, while improving the device's long-term stability and environmental robustness. Without relying on external control or complex materials, the crosstalk between a pair of orbital angular momentum (OAM) modes with opposite rotational directions, caused by the combined effects of waveguide bending and cross-sectional ellipticity, is fundamentally suppressed through the waveguide's own geometric design, thereby achieving high-purity and stable transmission of these modes along the curved path.

[0029] In some embodiments of the present invention, such as Figure 2 As shown, the value of the ellipticity is negatively correlated with the value of the preset bending radius.

[0030] It should be noted that a larger preset bending radius results in a larger ellipticity, and a smaller preset bending radius results in a smaller ellipticity. This embodiment designs the ellipticity value to be negatively correlated with the preset bending radius, thus eliminating dependence on a single specific bending radius. Regardless of whether the chip layout requires the waveguide to have a sharp bend (small radius) or a gentle bend (large radius), the optimal solution can be quickly found by adjusting the single variable of ellipticity. This allows for flexible adaptation to the ever-changing needs of actual photonic integrated circuit layouts, enabling the customization of the optimal ellipticity for each waveguide with different curvatures, thereby ensuring that all OAM channels on the entire chip achieve optimal performance.

[0031] In some embodiments of the present invention, the waveguide segment structure D2 is used to transmit orbital angular momentum modes with topological charge numbers of +1 and -1.

[0032] It should be noted that in modular division multiplexing systems, these two channels are often used as the most basic channels because they are simple to generate and have relatively good anti-interference capabilities. This invention designs a waveguide segment structure D2 to transmit orbital angular momentum modes with topological charge numbers of +1 and -1. Since the l=±1 mode is most sensitive to structural disturbances, the elliptical deformation required to cancel its crosstalk is extremely small and precise.

[0033] In some embodiments of the present invention, the waveguide segment structure D2 includes a core layer and a cladding surrounding the core layer.

[0034] It should be noted that the core layer is located at the center of the waveguide structure and is wrapped by the cladding material in all lateral directions (i.e., within the cross-section). In integrated waveguides, the cladding is usually the same material used for both the substrate and the top cover. The core layer and the cladding together form a single, integrated waveguide. The cross-section of the waveguide segment structure D2 refers to the cross-sectional shape of the core layer.

[0035] In some embodiments of the present invention, the core layer is made of glass and is prepared by femtosecond laser direct writing technology.

[0036] It should be noted that the core layer can be made of optical glass sensitive to femtosecond lasers, such as germanium-doped quartz glass or fluorophosphate glass. Under femtosecond laser pulse irradiation, the core layer material undergoes structural changes through nonlinear absorption effects, resulting in a slight increase in refractive index. The cladding material uses pure optical glass with the same or similar composition as the core layer substrate as the substrate, thus constituting a natural cladding itself.

[0037] In some embodiments of the present invention, the relative angle θ is configured such that the phase difference between the second mode coupling coefficient and the first mode coupling coefficient is π.

[0038] It should be noted that regardless of whether the waveguide bends to the left or right, the phase difference condition of π can be satisfied by adjusting the relative angle θ (for example, from 45° to 135°). This allows for flexible adaptation to various curved waveguides with different orientations on the chip, greatly enhancing the practical value of this technology in complex photonic integrated circuits.

[0039] The present invention also provides a photonic integrated circuit, wherein the photonic integrated circuit integrates the optical waveguide structure 1 described in the first aspect for realizing signal transmission or processing in orbital angular momentum mode.

[0040] The specific implementation principle of the photonic integrated circuit can be found in the corresponding content of the above-mentioned optical waveguide structure 1 embodiment, and will not be repeated here.

[0041] To provide a systematic, streamlined, and universal design process for optical waveguide structures. Figure 3 A schematic flowchart of an embodiment of the design method for the optical waveguide structure 1 provided by the present invention is shown below. Figure 3 As shown, the design method includes: S301. Determine the preset bending radius of a waveguide segment structure D2; the cross-section of the waveguide segment structure D2 is an elliptical cross-section.

[0042] It should be noted that, based on the overall layout and wiring density requirements of the photonic integrated circuit, the preset bending radius that this waveguide segment must achieve must be clearly defined, for example... R =1cm. To ensure the optimization starting point has a reasonable physical meaning, the initial shape of the elliptical cross-section is set to be equal to the core area of ​​the reference straight waveguide (a theoretical model used for design calculations, an abstract idealized structure used to determine basic optical parameters), i.e., the initial ellipticity. ρ 0≈1, initial relative angle θ 0 = 0.

[0043] S302. Based on the preset bending radius, calculate the first mode coupling coefficient generated by the waveguide bending effect.

[0044] It should be noted that: once the preset bending radius of the waveguide segment is determined (e.g., ...), R =1cm) and set the initial parameters for the elliptical cross-section ( ρ 0≈1, initial relative angle θ After setting the value to 0, create a model with a preset bending radius in electromagnetic simulation software (such as Lumerical MODE or COMSOL Wave Optics). R A waveguide model with an ideal circular cross-section is used. Based on the requirements of waveguide structure 1, the input material parameters (e.g., glass refractive index) and operating wavelength (e.g., 808 nm) of the waveguide model are set to be consistent with the parameters of the aforementioned reference straight waveguide. Based on the preset bending radius, operating wavelength, and waveguide material and geometric parameters, the mode coupling strength caused by bending is calculated using coupled-mode theory, i.e., obtaining the eigenmode fields and propagation constants of a pair of orbital angular momentum modes with opposite topological charges in the unbending state. Waveguide bending is equivalent to a perturbation on the straight waveguide, the intensity of which is a function of the preset bending radius. The first mode coupling coefficient is obtained by calculating the overlap integral of the two eigenmode fields under the perturbation. In summary, the eigenmode solver is run to calculate the coupling coefficient of the bent waveguide model. l =±1 mode ( l =+1 pattern and l The propagation constants β corresponding to the -1 mode are respectively +1 and β 1. Then, the first mode coupling coefficient Kbend = (β) is calculated. +1 β 1) / 2.

[0045] S303. Adjust key parameters to calculate the second mode coupling coefficient introduced by the elliptical cross section. The key parameters include the ellipticity of an elliptical cross section model and the relative angle θ, where the relative angle θ is the angle between the major axis direction of the elliptical cross section and the bending plane of the waveguide segment structure D2.

[0046] It should be noted that the waveguide model is modified to an elliptical cross section. In the software, the ellipticity ρ and the relative angle θ are set as adjustable variables. Within a certain parameter space (e.g., ρ: [1.000, 1.050], θ: [0°, 180°]), optimization algorithms (such as the simplex method, genetic algorithm, or parameter sweep) are used to systematically change ρ and θ. For each set of (ρ, θ) candidate values, the simulation calculates the corresponding elliptical curved waveguide... l The second-mode coupling coefficient Kellipse between the ±1 modes is calculated. In each iteration, the amplitude difference and phase difference between the current Kellipse and the reference Kbend are calculated.

[0047] S304. Determine the final ellipticity and the final relative angle θ that meet the preset conditions. opt The final design parameters for the waveguide segment structure D2 are: the preset condition is that the coupling coefficients of the first mode and the coupling coefficients of the second mode are equal in amplitude and opposite in phase.

[0048] It should be noted that: when the optimization algorithm finds a set of candidate values ​​for parameters (ρ, θ), a complete three-dimensional beam propagation simulation is performed. This is done with a pure... l Using the +1 mode as input, observe the output at the preset bending length. l =+1 mode purity and crosstalk level. If simulation verification meets the performance indicators, then these candidate values ​​of parameters (ρ, θ) are determined as the final ellipticity ρ of the waveguide segment structure D2. opt The final relative angle θ opt These parameters, together with the preset bending radius R, constitute the complete manufacturing specifications.

[0049] In this embodiment, by quantifying the physical objective of coupling cancellation, the design of the high-performance crosstalk-free optical waveguide structure 1 can be reliably reproduced, lowering the design threshold. Compared to traditional trial-and-error methods, systematic iterative optimization algorithms (such as genetic algorithms) can perform intelligent searches within a broad parameter space, avoiding getting trapped in local optima, and thus are more likely to find the global optimum or near-optimal (ρ). opt θ optThis combination shortens the design cycle and ensures that the designed waveguide performance reaches the optimal level under theoretical conditions. The final output is a set of precise geometric parameters (R, ρ). opt θ opt It can be directly used as input for computer-aided manufacturing to drive a femtosecond laser direct writing system for processing, enabling the efficient and high-quality manufacture of excellent optical waveguide structures.

[0050] In some embodiments of the present invention, the value of the ellipticity is negatively correlated with the value of the preset bending radius. It should be noted that a larger preset bending radius results in a larger ellipticity, and a smaller preset bending radius results in a smaller ellipticity. This embodiment, by designing the ellipticity value to be negatively correlated with the preset bending radius, eliminates the dependence on a single specific bending radius. Regardless of whether the chip layout requires the waveguide to have a sharp bend (small radius) or a gentle bend (large radius), the optimal solution can be quickly found by adjusting the single variable of ellipticity. This flexibly adapts to the ever-changing needs of actual photonic integrated circuit layouts, allowing for the individual customization of the optimal ellipticity for each waveguide with different curvatures, thereby ensuring that all OAM channels on the entire chip achieve optimal performance.

[0051] To provide a systematic, streamlined, and universal process for fabricating optical waveguide structures. Figure 4 This is a schematic flowchart of an embodiment of the fabrication method of the optical waveguide structure 1 provided by the present invention, as shown below. Figure 4 As shown, the design method includes: S401. Determine the preset bending radius of a waveguide segment structure D2; the cross-section of the waveguide segment structure D2 is an elliptical cross-section; S402. Based on the preset bending radius, calculate the first mode coupling coefficient generated by the waveguide bending effect; S403. Adjust key parameters to calculate the second mode coupling coefficient introduced by the elliptical cross section. The key parameters include the ellipticity of an elliptical cross section model and the relative angle θ, where the relative angle θ is the angle between the major axis direction of the elliptical cross section and the bending plane of the waveguide segment structure D2. S404. Determine the final ellipticity and the final relative angle θ that satisfy the preset conditions. opt The final design parameters for the waveguide segmented structure D2 are: the preset condition is that the first mode coupling coefficient and the second mode coupling coefficient are equal in amplitude and opposite in phase. S405. Obtain and fabricate the optical waveguide structure 1 based on the final design parameters.

[0052] It should be noted that S401 to S404 refer to S301 to S304 above. The final ellipticity and the final relative angle θ are determined through S401 to S404. opt Then, obtain the preset bending radius. R Final ellipticity ρopt Final relative angle θopt The system obtains the three-dimensional coordinates of the waveguide's start and end points, as well as the material's refractive index distribution. Dedicated waveguide direct-write path planning software converts this parameterized description into machine instructions recognizable by the manufacturing equipment. A smooth three-dimensional curved path along the waveguide's central axis is generated based on the preset bending radius R and the three-dimensional coordinates of the waveguide's start and end points. At each point on this curved path, the instantaneous direction of the elliptical cross-section is calculated based on the tangent direction of point Pi (defining the local bending plane) and θopt. Centered on point Pi, a closed laser scanning loop is generated along the calculated elliptical direction according to the major and minor axis ratio defined by ρopt. This elliptical loop represents the cross-section of the waveguide at that location. A high-purity fused silica glass sheet, optically polished, is selected as the waveguide's substrate (cladding). A pre-established process database is invoked, and the optimal combination of matching laser parameters, such as pulse energy, repetition frequency, and scanning speed, is retrieved from the database based on the core layer refractive index increment and dimensions of the waveguide segment structure D2. The generated processing instructions and the retrieved optimal laser process parameters are then loaded into the femtosecond laser direct-write system. The system starts up, and the laser beam is focused inside the glass through the objective lens, driving the laser focus to scan strictly according to the planned three-dimensional curved path and dynamically rotating elliptical ring path. A CCD camera or diffraction efficiency monitoring module integrated into the optical path is used to observe the processing area or monitor the writing effect in real time, and the laser power is fine-tuned to compensate for minor fluctuations in the material or environment, ensuring the uniformity of the refractive index distribution and geometric fidelity of the written waveguide. The prepared sample is placed in a programmable annealing furnace for low-temperature annealing. A vertical end face with excellent optical quality is prepared using precision cutting or polishing techniques. The laser is coupled to the waveguide output end through a single-mode fiber and a microscope objective. Observation at the output end using an infrared camera shows that if a clear annular spot (OAM mode characteristic) can be immediately observed, it can be preliminarily determined that the optical waveguide structure 1 has been successfully prepared and supports mode transmission.

[0053] In this embodiment, since the entire process, from design parameters to processing instructions, is digital, once the design needs to be modified (e.g., changing the bending radius R), only the design file needs to be updated and the processing instructions regenerated, and a new optical waveguide structure 1 that meets the requirements can be quickly fabricated. This allows for rapid optimization, iteration, and customized production of the optical waveguide structure 1, and improves the quality of the produced optical waveguide structure 1.

[0054] In conventional bent waveguides, the bending effect introduces a radial refractive index gradient across the waveguide cross-section, leading to coupling between orbital angular momentum modes. When carrying... l When the OAM mode of the +1 topological charge is incident, it will excite during propagation. l = Mode 1, and vice versa. The coupling strength is inversely proportional to the bending radius; the smaller the bending radius, the greater the refractive index gradient, and the more pronounced the crosstalk between modes. Therefore, this invention introduces an elliptical cross-section waveguide segment structure D2 into the optical waveguide structure 1, and uses the ellipticity of the waveguide segment structure D2 and bending to collaboratively control mode coupling. The relative angle between the major axis of the ellipse and the bending direction is denoted as θ. The ellipticity introduces an additional mode coupling term, whose coupling coefficient's phase and angle... θ Related. When the phase is π, the coupling introduced by the ellipse is negative. If the amplitude of the elliptical coupling is equal to the amplitude of the bending coupling, they can undergo destructive interference, achieving coupling cancellation. Under this condition, when l When the +1 order OAM mode is incident, it can maintain stable propagation throughout the entire curved waveguide with almost no occurrence of [missing information]. l = The mode switching effectively suppresses crosstalk between OAM modes, significantly improving the purity and stability of OAM modes in curved waveguides.

[0055] To illustrate the crosstalk caused by bending, a simulation of a circular bent waveguide was performed. For example... Figure 5 As shown, Figure 5 (a) and Figure 5 (b) is Figure 1 The simulation shows the mode field distribution and corresponding mode energy ratio changes of the optical waveguide structure 1 at different propagation distances with a preset bending radius R = 1 cm. In the simulation, the waveguide along... x Direction bend, z Let be the waveguide propagation distance. Simulation results show that: in z =0, with l When the +1 order OAM mode is incident, the mode field exhibits a typical ring-shaped distribution (donut shape), with no light field intensity in the central region; when propagating to... z When the value is 0.5 cm, the modulus field evolves into a shape that resembles a 0.5 cm² pattern. LP mode with 45° deflection, this mode is composed of l The formation of ±1OAM mode superposition indicates that significant mode crosstalk has occurred at this point; propagation continues to... z When the value is 1.0 cm, the mold field recovers to a ring-shaped distribution, but the corresponding value is... l = Mode 1 indicates that a mode transition has been completed; when z When the value is 1.5cm, the LP mode with a 45° deflection reappears; z When the diameter is 2.0 cm, the mold field returns to its original state. l =+1 mode achieves a complete oscillation cycle.

[0056] To further analyze the mode conversion process, the relationship between propagation distance and mode amplitude was calculated, such as... Figure 5 As shown in (b), the solid line represents l =+1 mode, indicated by a circled line. l = Mode 1. It can be seen that both exhibit a cosine-shaped periodic variation with propagation distance, with an oscillation period of approximately... L ≈2cm. This indicates that waveguide bending introduces inter-mode coupling, leading to energy exchange and crosstalk between OAM modes. To compare and analyze the impact of the elliptical cross-section on OAM mode transmission, a numerical simulation study was further conducted on a straight elliptical cross-section waveguide (hereinafter referred to as "elliptical straight waveguide"). To maintain approximately the same area as the circular reference structure, the semi-major axis was taken as... a = r ×ρ, semi-minor axis b = r / ρ, ellipticity ρ=1.017. Under this structure, for the first-order OAM mode ( l =+1) was used as the incident mode for spatial propagation simulation. Simulation results were presented in the form of... z To determine the propagation distance, different... z The mode field distribution and the variation of OAM mode energy with propagation distance at the location. For example... Figure 5 As shown, Figure 5 (c) and Figure 5 (d) is Figure 2 The waveguide structure 1 with an elliptical cross-section, D2, is shown below. The curves depict the mode field distribution and corresponding mode energy ratio changes at different propagation distances when the preset bending radius R = 1 cm is used. Figure 5 (c) and Figure 5 As shown in (d). Calculations revealed: z =0 (incident surface), with l When the +1 order OAM mode is incident, the initial mode field maintains a typical toroidal (donut) distribution; at z=0.5cm, the mode field evolves into an LP mode deflected at +45°, which can be considered as l =+1 and l = The two OAM components are superimposed with equal amplitude, but their phase relationship is opposite to that of the LP mode (opposite direction) exhibited at the same propagation distance in the circular reference structure. This phenomenon indicates that ellipticity has caused significant coupling between the two modes and a phase shift of π / 2 in their phase relationship, thus producing an LP orientation different from that of the circular structure at this location; z =1.0cm: The mold field recovers to a ring-shaped distribution, but the corresponding ring is l= First-order OAM mode; z=1.5cm: The mode field again exhibits the LP type, but its deflection direction is... 45°; z =2.0cm: The mold field returns to a ring shape and then back to its original state. l=+1 mode, to achieve a complete oscillation cycle. Figure 5 (d) Give two modes of random z The normalized energy curves show that the energy of the two modes oscillates in an approximately cosine manner with the propagation distance.

[0057] Compared to the LP orientations observed at z=0.5cm and z=1.5cm in circular structures, the elliptical straight waveguide exhibits an orientation reversal (+45°) at the same propagation step. The phenomenon observed at 45° is due to the fact that the coupling coefficient introduced by ellipticity not only affects the amplitude of the coupling strength between the two modes but also introduces a considerable phase shift (an additional phase term of approximately π / 2), resulting in mode patterns that are opposite to those in the circular case at different phase points of propagation. This phase characteristic provides a feasible basis for subsequent coupling phase control using the elliptical rotation angle.

[0058] Based on the foregoing analysis, to simultaneously examine the synergistic effect of bending and ellipticity, this embodiment designs an optical waveguide structure 1 with an elliptical cross-section. The waveguide material and optical parameters are the same as described above, with a bending radius R = 1 cm and a slightly elliptical waveguide cross-section with an ellipticity of ρ = 1.017. Numerical simulation results are as follows: Figure 6 As shown in (a). Taking the l=+1 order OAM mode as the incident field, the mode field distribution at different propagation distances can be observed to be: z At different locations such as 0, 0.5cm, 1.0cm, 1.5cm, and 2.0cm, the mode field always maintains a typical ring-shaped distribution, with no light intensity at the center, and the phase distribution direction is consistent, corresponding to a consistent [phase distribution direction]. l = +1st order OAM mode. Unlike the OAM mode oscillations observed in the aforementioned circular curved waveguides, no obvious mode switching or polarization direction rotation was observed in this structure, indicating that crosstalk has been effectively suppressed.

[0059] To further verify this conclusion, the mode field energy composition along the propagation direction was extracted, such as... Figure 6 As shown in (b). l The +1 mode component maintains a near 100% proportion throughout the entire propagation process, while l = The near-zero I component indicates that the coupling between the two modes has been completely canceled. This result shows that when the negative coupling amplitude introduced by the elliptical cross section is equal to the positive coupling amplitude caused by bending and the phase difference is π, they produce destructive interference, thus completely suppressing mode crosstalk induced by geometric effects in the bent waveguide. At this time, the OAM mode can propagate stably in the bent waveguide without energy exchange between degenerate states, thereby achieving high-purity, crosstalk-free orbital angular momentum transfer. The study found that there is a clear compensation relationship between the bending radius and ellipticity. When the bending radius decreases, the radial refractive index gradient introduced by bending increases, thus leading to enhanced bending coupling; conversely, when the bending radius increases, bending coupling weakens. In order to maintain the destructive condition between the two couplings, this invention proposes to achieve precise compensation by adjusting the ellipticity of the waveguide cross section: when the bending radius decreases, the ellipticity is appropriately increased to enhance the negative coupling introduced by the ellipticity; when the bending radius increases, the ellipticity is correspondingly decreased to reduce the negative coupling strength. By matching these parameters, bending coupling and elliptical coupling are made to have equal amplitudes and opposite phases, thus maintaining the pure transmission of the OAM mode under different bending conditions and achieving dynamic suppression and steady-state cancellation of mode crosstalk.

[0060] The optical waveguide structure, its design, fabrication method, and photonic integrated circuit provided by this invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. An optical waveguide structure, characterized in that, include: A segmented waveguide structure, wherein the segmented waveguide structure has a preset bending radius and the cross-section of the segmented waveguide structure is an elliptical cross-section; The elliptical cross section is defined by ellipticity and major axis direction, and the major axis direction forms a relative angle with the bending plane of the waveguide segment structure; The ellipticity and the relative angle are configured such that the first mode coupling coefficient introduced by the bending of the waveguide segment structure is equal in magnitude and opposite in phase to the second mode coupling coefficient introduced by the ellipticity of the elliptical cross section.

2. The optical waveguide structure according to claim 1, characterized in that, The value of the ellipticity is negatively correlated with the value of the preset bending radius.

3. The optical waveguide structure according to claim 1, characterized in that, The waveguide segment structure is used to transmit orbital angular momentum modes with topological charge numbers of +1 and -1.

4. The optical waveguide structure according to claim 1, characterized in that, The waveguide segment structure includes a core layer and a cladding layer surrounding the core layer.

5. The optical waveguide structure according to claim 4, characterized in that, The core layer is made of glass and is prepared using femtosecond laser direct writing technology.

6. The optical waveguide structure according to claim 1, characterized in that, The relative angle is configured such that the phase difference between the second mode coupling coefficient and the first mode coupling coefficient is π.

7. A photonic integrated circuit, characterized in that, The photonic integrated circuit integrates an optical waveguide structure as described in any one of claims 1 to 6, for realizing signal transmission or processing in orbital angular momentum mode.

8. A design method for an optical waveguide structure as described in any one of claims 1 to 6, characterized in that, Design methods include: A preset bending radius is determined for a waveguide segment structure; the cross-section of the waveguide segment structure is an elliptical cross-section. Based on the preset bending radius, the first mode coupling coefficient generated by the waveguide bending effect is calculated; Key parameters are adjusted to calculate the second mode coupling coefficient introduced by the elliptical cross section, the key parameters including the ellipticity and relative angle of an elliptical cross section model, the relative angle being the angle between the major axis direction of the elliptical cross section and the bending plane of the waveguide segment structure; The final ellipticity and the final relative angle that satisfy the preset conditions are determined as the final design parameters of the waveguide segment structure. The preset conditions are that the first mode coupling coefficient and the second mode coupling coefficient are equal in amplitude and opposite in phase.

9. The method for fabricating the optical waveguide structure according to claim 8, characterized in that, The value of the ellipticity is negatively correlated with the value of the preset bending radius.

10. A method for fabricating an optical waveguide structure, characterized in that, The optical waveguide structure is fabricated based on the design method of claim 8 or 9, wherein the fabrication method includes: obtaining and fabricating the optical waveguide structure based on the final design parameters.

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