Design method of metasurface for coupling of optical fiber and on-chip waveguide and coupling device

Abstract

The present disclosure relates to a design method and a coupling device for a fiber-to-chip waveguide coupling super surface. In the technical solution of the present application, by designing the parameters and arrangement of the nanostructure of the fiber end surface super surface, the fiber end surface super surface is used to flexibly control the wavefront of the fiber exit light field, and the coupling of the fiber high-order mode-chip single-mode waveguide is realized.

Description

Technical Field

[0001] This disclosure relates to the field of semiconductor optics, and specifically to a design method and coupling device for a metasurface used for coupling optical fiber and on-chip waveguide. Background Technology

[0002] In existing technologies, coupling between optical fiber and on-chip waveguide is achieved through an on-chip mode converter. This on-chip mode converter mainly comprises two identical tapered waveguides and a Y-junction, based on optical fiber LP... 11 The principle of mode splitting and recombination enables the coupling of optical fiber and on-chip waveguide.

[0003] Existing on-chip mode converters need to be placed at the edge of the chip, i.e. the end face of the on-chip waveguide. Low-loss coupling between the two is achieved through slow mode evolution of the tapered waveguide. The device size is >100μm, which is large and not conducive to the miniaturization of the system.

[0004] Furthermore, the aforementioned on-chip mode converter implements LP 11 -TE1 mode coupling, LP not implemented 11 -TE0 mode coupling, but currently on-chip optical interconnects still use single-mode waveguides as information carriers for stable transmission. Since single-mode waveguides only support the fundamental mode (TE0 mode), an additional on-chip mode converter is still needed to achieve TE1-TE0 coupling. Summary of the Invention

[0005] To address the issues of large size and lack of support for fundamental modes in existing coupling devices, the first aspect of this application provides a design method for a metasurface for coupling optical fiber and on-chip waveguide. The metasurface is disposed between the end face of the optical fiber and the on-chip waveguide to perform wavefront modulation of the emitted light field of the optical fiber. The design method includes the following steps:

[0006] Step S1: Determine the mode characteristics of the optical fiber and the on-chip waveguide;

[0007] Step S2: Based on the transmittance and transmission phase response of the metasurface for the working band, screen the nanostructures that the metasurface can include.

[0008] Step S3: Based on the mode characteristics of the optical fiber, obtain the phase distribution of the metasurface, thereby determining the arrangement of the nanostructures of the metasurface.

[0009] Optionally, the design steps of the metasurface further include:

[0010] Step S4: Perform light field propagation simulation on the metasurface formed in step S3.

[0011] Optionally, the mode characteristics of the optical fiber include the amplitude and phase distribution in the first-order linear polarization mode of the optical fiber, and the mode characteristics of the optical fiber have low-loss coupling requirements.

[0012] Optionally, the mode characteristics of the on-chip waveguide include the amplitude and phase distribution in single-mode waveguide mode.

[0013] Optionally, step S2 specifically includes:

[0014] Step S21: Determine the material of the nanostructure based on the operating wavelength;

[0015] Step S22, according to

[0016]

[0017] Determine the height d of the nanopillar, where, Where n is the emission phase, n is the refractive index of the material, and λ is the wavelength of the operating band.

[0018] Determine the arrangement period of the nanostructure, wherein the arrangement period satisfies the subwavelength condition;

[0019] Step S23: Calculate the spectral response of the periodic grating to obtain the range of nanostructure parameters that meet the requirements of transmittance and transmission phase response.

[0020] Optionally, the nanostructure needs to meet the 2π phase coverage requirement.

[0021] Optionally, step S2 further includes:

[0022] Step S24: Discretize the obtained nanostructure parameter range to obtain the geometric dimensions, transmittance and phase parameters corresponding to the multi-order nanostructure.

[0023] Optionally, the geometric dimensions, transmittance, and phase parameters corresponding to the 8th-order nanopillars can be obtained.

[0024] Optionally, in step S23, the spectral response of the periodic grating is calculated by rigorous coupled-wave analysis or the finite-difference time-domain method.

[0025] Optionally, in step S3, a phase distribution of the metasurface capable of increasing the mode field matching degree of the fiber-on-chip waveguide is obtained, wherein the mode field matching degree satisfies:

[0026]

[0027] Where η is the mode field matching degree, E1(x,y) is the complex amplitude of the fiber output field, and E2(x,y) is the complex amplitude of the chip edge waveguide mode.

[0028] Optionally, in step S3, based on: reducing the mode field size of the fiber output field.

[0029] or

[0030] Increase waveguide end mode size

[0031] The phase distribution of the metasurface is obtained by following the principle of [the principle of obtaining the phase distribution of the metasurface].

[0032] Optionally, in step S3, obtaining the phase distribution of the metasurface includes, based on the focused phase, assigning a phase difference of π in the positive and negative x-axis directions of the metasurface, wherein the phase distribution satisfies:

[0033]

[0034] in, Let f be the metasurface phase, λ be the operating wavelength, and f be the focal length.

[0035] Optionally, in step S3, obtaining the phase distribution of the metasurface includes adding a vortex phase to the focused phase, wherein the phase distribution satisfies:

[0036]

[0037]

[0038] in, θ(x,y) is the metasurface focusing phase, λ is the metasurface vortex phase, f is the working wavelength, l is the focal length, l is the angular quantum number, and θ is the spatial azimuth angle.

[0039] Optionally, in step S4, after the phase distribution is discretized in multiple orders, a full-wave simulation is performed to obtain the simulation propagation results;

[0040] The propagation of the optical field is obtained by using the phase shift between angular spectra as the transfer function;

[0041] The phase shift between the angular spectra is expressed as:

[0042]

[0043] And it satisfies:

[0044]

[0045] Where, A0(f x ,f y A(f,0) represents the spectrum of the z=0 plane or the spectrum of the fiber optic output end face; x ,f y ,z) is the spectrum of the propagation plane.

[0046] A second aspect of this disclosure provides a coupling device including an optical fiber, an on-chip waveguide, and a metasurface disposed between the optical fiber and the on-chip waveguide, wherein the metasurface is configured according to the metasurface design method for coupling an optical fiber and an on-chip waveguide as described in any of the preceding claims.

[0047] Optionally, the metasurface is formed directly on the end face of the optical fiber.

[0048] Optionally, it includes a superlens; the superlens includes a substrate and a metasurface disposed on the substrate, the superlens being disposed on the end face of the optical fiber.

[0049] Optionally, there is no relative rotation angle between the nanostructures of the metasurface.

[0050] Optionally, the nanostructure is a columnar body with a rectangular cross-section.

[0051] Optionally, the nanostructures are arranged to form densely packed structural units; the structural units are squares or regular hexagons.

[0052] Optionally, the coupling device operates at a wavelength of 1550 nm; and the metasurface therein has a nanostructure with at least the following parameters:

[0053] Length 205nm to 545nm, width 95nm to 460nm, transmittance 0.914 to 0.968, phase 0.1 to 0.94.

[0054] Optionally, the phase distribution of the metasurface satisfies:

[0055]

[0056] in, Let λ be the metasurface phase, λ be the operating wavelength, and f be the focal length.

[0057] or

[0058]

[0059]

[0060] in, θ(x,y) is the metasurface focusing phase, λ is the metasurface vortex phase, f is the working wavelength, l is the focal length, l is the angular quantum number, and θ is the spatial azimuth angle.

[0061] The technical solutions in this application embodiment can achieve at least the following beneficial effects:

[0062] By combining optical fibers with metasurfaces, and designing metasurfaces on the fiber endfaces, the coupling of first-order modes of the fiber to on-chip single-mode waveguides can be achieved through flexible wavefront manipulation of the metasurfaces. In particular, this enables the realization of LP... 11 -TE0 mode coupling. Metasurfaces are two-dimensional planar structures with subwavelength characteristic dimensions and a thickness of only about 100 nanometers. Therefore, coupling devices based on them are small in size, easy to integrate and miniaturize into systems, and can be made into independent coupling devices or directly formed on the end face of optical fibers.

[0063] Furthermore, due to the characteristics of metasurfaces, which allow for high-precision arbitrary control of the optical field with multiple degrees of freedom, coupling metasurfaces can be combined with other optical functions and are compatible with CMOS processes. They are simple to fabricate and easy to prepare. Attached Figure Description

[0064] The accompanying drawings are provided to further understand this application and are incorporated in and form a part of this specification. The drawings illustrate embodiments of this application and, together with the following description, serve to explain the principles of this application.

[0065] Figure 1 This is a flowchart of the design method steps in the embodiment;

[0066] Figure 2 This is a flowchart illustrating the method for screening nanostructures in the embodiments;

[0067] Figure 3 In this embodiment, the fiber first-order linear polarization (LP) 11 Schematic diagram of amplitude and phase distribution after mode normalization;

[0068] Figure 4 These are schematic diagrams of two optional nanostructures in the embodiments;

[0069] Figure 5 This is a schematic diagram of two optional structural unit arrangements in the embodiment;

[0070] Figure 6 These are schematic diagrams of the spectral response, transmittance response, and phase response of the nanostructure used in the embodiments, as well as the transmittance and phase coverage of the 8th-order nanostructure (pillar) used in the embodiments.

[0071] Figure 7 The phase distribution diagram is shown for the scheme of imparting a π phase difference between the left and right halves of the metasurface in the embodiment.

[0072] Figure 8 The phase distribution diagram shows the scheme for imparting metasurface vortex phase in the embodiment;

[0073] Figure 9 for Figure 7A schematic diagram of the simulated light field propagation of the superlens corresponding to the Chinese scheme;

[0074] Figure 10 for Figure 8 A schematic diagram of the simulated light field propagation of the superlens corresponding to the Chinese scheme;

[0075] Figure 11 This is a schematic diagram of the waveguide end mode field intensity distribution in the embodiment;

[0076] Figure 12 The figure shows the simulation results of full-wave simulation using FDTD Solutions in the example. Detailed Implementation

[0077] The present application will now be described more fully below with reference to the accompanying drawings, in which various embodiments are illustrated. However, the present application may be implemented in many different ways and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present application will be exhaustive and complete, and will fully convey the scope of the present application to those skilled in the art. The same reference numerals denote the same parts throughout the drawings. Furthermore, in the drawings, the thickness, proportions, and dimensions of parts are enlarged for clarity.

[0078] The terminology used herein is for descriptive purposes only and is not intended to be limiting. Unless the context clearly indicates otherwise, the terms “a,” “an,” “the,” and “at least one” as used herein are not intended to limit the quantity but are intended to include both singular and plural forms. For example, unless the context clearly indicates otherwise, “a component” has the same meaning as “at least one component.” “At least one” should not be construed as limited to the quantity “a.” “Or” means “and / or.” The term “and / or” includes any and all combinations of one or more of the associated listed items.

[0079] Unless otherwise specified, all terms used herein, including technical and scientific terms, shall have the same meaning as commonly understood by one of ordinary skill in the art. Terms defined in commonly used dictionaries shall be interpreted as having the same meaning as in the relevant technical context, and shall not be construed as having a formal meaning in an idealized or overly formal sense unless expressly defined in the specification.

[0080] The meaning of “includes” or “contains” specifies a nature, quantity, step, operation, component, part, or combination thereof, but does not exclude other natures, quantities, steps, operations, components, parts, or combinations thereof.

[0081] This document describes embodiments with reference to cross-sectional views as idealized implementations. Thus, variations in shape relative to the illustrations are anticipated as a result of, for example, manufacturing techniques and / or tolerances. Therefore, the embodiments described herein should not be construed as limited to the specific shapes of the regions shown herein, but should include deviations in shape due to, for example, manufacturing processes. For example, regions shown or described as flat may typically have rough and / or non-linear characteristics. Furthermore, acute angles shown may be rounded. Therefore, the regions shown in the figures are schematic in nature, and their shapes are not intended to show precise shapes of the regions and are not intended to limit the scope of the claims.

[0082] In the following description, exemplary embodiments according to this application will be described with reference to the accompanying drawings.

[0083] To address the issues of large size in existing fiber-on-chip waveguides, which hinders miniaturization, and the inability to achieve LP in a single step... 11 To address the issue of -TE0 mode coupling, this disclosure provides a coupler based on the fiber endface and a design method for a metasurface used for coupling fiber to on-chip waveguide. The core idea is to combine fiber with a metasurface, utilizing the functional metasurface to flexibly modulate the emitted light from the fiber using multiple degrees of freedom. This method offers advantages such as small size, simple structure, and ease of fabrication, and can be used in fiber-to-fiber, fiber-to-chip, and fiber-to-free-space scenarios, with the potential for wide application in optical communication systems.

[0084] like Figure 1 As shown, this disclosure first provides a design method for a metasurface for coupling optical fiber and on-chip waveguide. This method first sets a metasurface between the optical fiber end face and the on-chip waveguide. It should be understood that the metasurface can be a standalone superlens or a metasurface fabricated / constructed on the end face of the optical fiber or the on-chip waveguide. The metasurface allows for wavefront modulation of the emitted light field of the optical fiber to couple the optical fiber to the on-chip waveguide. The key point is that the metasurface is designed based on the following steps:

[0085] Step S1: Determine the mode characteristics of the optical fiber and the on-chip waveguide;

[0086] Step S2: Based on the transmittance and transmission phase response of the metasurface for the working band, screen nanostructures for constructing the metasurface.

[0087] Step S3: Based on the mode characteristics of the optical fiber, obtain the phase distribution of the metasurface, thereby determining the arrangement of the nanostructures of the metasurface;

[0088] Step S4: Perform light field propagation simulation on the metasurface.

[0089] Explanatoryly, the various superlenses / metasurfaces described in the embodiments and alternative embodiments of this application include the following features:

[0090] The metasurface is a subwavelength artificial nanostructure film, typically implemented as a superlens as described in the embodiments of this disclosure. The amplitude, phase, and polarization of incident light can be modulated by nanostructure units disposed thereon. It should be noted that a nanostructure can be understood as a subwavelength structure containing all-dielectric or plasma particles capable of causing phase abrupt changes, while a nanostructure unit is a structural unit centered on each nanostructure obtained by dividing the superlens. In the superlens, nanostructures are periodically arranged on the substrate, with each period's nanostructures forming a superstructure unit. The superstructure unit is a close-packed pattern, such as a regular square, regular hexagon, etc. Each period contains a set of nanostructures, and the vertices and / or centers of the superstructure unit may, for example, contain nanostructures. When the superstructure unit is a regular hexagon, at least one nanostructure is disposed at each vertex and center position of the hexagon. Alternatively, when it is a square, at least one nanostructure is disposed at each vertex and center position of the square. Ideally, the superstructure unit should be a nanostructure with hexagonal vertices and a central arrangement, or a nanostructure with square vertices and a central arrangement. It should be understood that in actual products, due to the limitations of the superlens shape, there may be missing nanostructures at the edges of the superlens, preventing it from satisfying the complete hexagonal / square shape requirement. Specifically, for example... Figure 5 As shown, the superstructure unit is composed of nanostructures arranged in a regular pattern, and several superstructure units are arranged in an array to form a metasurface structure.

[0091] like Figure 5 The left part shows an embodiment in which the superstructure unit includes a central nanostructure and six peripheral nanostructures equidistant from it. The peripheral nanostructures are evenly distributed along the circumference to form a regular hexagon, which can also be understood as multiple nanostructures forming an equilateral triangle combined with each other.

[0092] like Figure 5 The right side shows one embodiment where the superstructure unit comprises a central nanostructure and four peripheral nanostructures equidistant from it, forming a square.

[0093] The superstructure unit and its dense stack / array form can also be a circumferentially arranged sector, including a sector with two arc-shaped sides, or a sector with one arc-shaped side.

[0094] For the sake of simplicity and clarity, the examples are attached. Figure 5 The right figure in the diagram only shows the nanostructure set at the center of the superstructure unit. It should be understood that nanostructures should also be set at the vertices / intersections of the square outline in the diagram.

[0095] The nanostructures in metasurfaces / metalenses can be polarization-dependent structures, such as nanofins and nanoelliptical cylinders, which impose a geometric phase on the incident light; or they can be polarization-independent structures, such as nanocylinders and nanoprismatic structures, which impose a propagation phase on the incident light. In the embodiments of this disclosure, the nanostructures are in the form of... Figure 4 As shown, a polarization-independent structure is preferred.

[0096] The spaces between the nanostructures can be filled with air or other materials that are transparent or translucent in the operating wavelength range. According to embodiments of this disclosure, the absolute value of the difference between the refractive index of the filling material and the refractive index of the nanostructure should be greater than or equal to 0.5.

[0097] Specifically, the modal characteristics of the aforementioned optical fibers include the amplitude and phase distribution in the first-order linear polarization mode. The modal characteristics of the on-chip waveguide include the amplitude and phase distribution in the single-mode waveguide mode. Regarding the modal characteristics of optical fibers, the electromagnetic field distribution of all linear polarization modes in weakly guided fibers can be obtained from fiber mode theory. Figure 3 Give the first-order linearly polarized LP of the optical fiber 11 The amplitude and phase distribution of the mode. (From...) Figure 3 It can be seen that LP 11 It is a two-lobed distribution with a phase difference of π between the left and right sides. In contrast, the single-mode waveguide mode (TE0 mode) has a uniform phase distribution. Therefore, without any mode-changing device, LP... 11 The mode cannot be directly coupled into the on-chip waveguide for transmission.

[0098] In view of this, the design method provided in this disclosure requires the design of metasurfaces based on the aforementioned optical fiber mode characteristics. It should be noted that the optical fiber mode characteristics require low-loss coupling. The first step involves the geometric design of the nanostructure, i.e., determining the material, geometric parameters, shape, and arrangement of the nanostructure. Specifically, this includes:

[0099] The operating wavelength is determined, for example, to be 1550 nm; and the material is determined, for example, to be silicon. This is determined by subwavelength conditions and the emission phase of the periodic grating. The lattice constant and the height of the nanopillars are determined; it should be understood that the lattice constant determined therein determines the arrangement period of the nanostructures.

[0100] Within a certain parameter range, the spectral response of the periodic grating is calculated using numerical methods such as rigorous coupled-wave analysis (RCWA) or finite-difference time-domain method (FDTD). This involves scanning the parameters of the unit structure until a parameter range that satisfies the requirements of high transmittance and 2π phase coverage is found.

[0101] This patent employs a rectangular nanostructure to achieve wavefront modulation of incident light, such as... Figure 4 As shown in the left figure.

[0102] Based on the metasurface characteristics discussed above, the embodiments can use a regular hexagonal or square nanostructure arrangement, preferably a regular hexagonal arrangement, because the regular hexagonal arrangement is more compact and has better periodicity.

[0103] The selected nanostructure parameters meet the requirements of high transmittance and 2π full-phase coverage at a wavelength of 1550 nm, such as... Figure 6 As shown in Table 1, blank areas in the transmittance and phase diagrams represent data points where the structural size exceeds that of a single hexagonal lattice. Eighth-order discrete nanounits were selected from these, and their structural parameters are shown in Table 1.

[0104] Nanopillars length / nm Width / nm Transmission rate Phase 1 460 200 0.966 0.1 2 525 200 0.914 0.18 3 545 235 0.934 0.303 4 495 280 0.967 0.4 5 260 95 0.97 0.555 6 205 460 0.97 0.7 7 255 360 0.967 0.8 8 270 450 0.968 0.94

[0105] Table 1. Parameters of the 8th-order nanopillar unit structure

[0106] In the specific implementation process, such as Figure 2 As shown, the following steps can be followed in sequence:

[0107] Step S21: Determine the working wavelength and the material of the nanostructure;

[0108] Step S22: Determine the lattice constant and the height of the nanopillars. The height of the nanopillars must satisfy the subwavelength condition of the operating wavelength, and be determined based on the output phase of the periodic grating.

[0109]

[0110] Determine the lattice constant;

[0111] Step S23: Calculate the spectral response of the periodic grating to obtain the range of nanostructure parameters that meet the requirements of transmittance and phase response.

[0112] The phase response requirement is to satisfy the 2π phase coverage requirement.

[0113] Step S24: Discretize the obtained nanostructure parameter range to obtain the geometric dimensions, transmittance and phase parameters corresponding to the multi-order nanostructure.

[0114] Among them, the geometric dimensions, transmittance and phase parameters corresponding to the 8th order nanopillars were obtained.

[0115] One of the problems that this embodiment needs to solve is the mode field mismatch problem in fiber-on-chip waveguide coupling. This is because the characteristic dimensions of optical fibers are in the range of tens of micrometers, while the characteristic dimensions of on-chip waveguides are only a few tenths of a micrometer. The difference in characteristic dimensions is significant. Therefore, to achieve fiber-on-chip waveguide coupling, two methods can be used: reducing the mode field size of the fiber's output field and increasing the mode size at the waveguide end, thereby increasing the mode field matching degree η. The coupling efficiency is measured by the mode matching degree η. Both methods modulate the phase of the fiber's output light through the phase distribution of nanostructures on the metasurface. Therefore, the phase distribution of the metasurface needs to be designed. The aforementioned η is given by formula Eq-1:

[0116]

[0117] In the formula, E1(x,y) is the complex amplitude of the fiber output field, and E2(x,y) is the complex amplitude of the chip edge waveguide mode.

[0118] As can be seen from formula Eq-1, to achieve LP 11 Low-loss coupling of the -TE0 mode requires not only matching mode amplitudes but also matching phases. Based on this, this patent designs the spatial phase of the metasurface using two methods.

[0119] Method 1. Based on the phase distribution that achieves focusing, assign a phase difference between the left and right halves of π, as given by formula Eq-2:

[0120]

[0121] In the formula, Let f be the metasurface phase, λ be the operating wavelength, and f be the focal length.

[0122] Method 2. Add a vortex phase to the phase used for focusing, as given by formula Eq-3:

[0123]

[0124] in, θ(x,y) is the metasurface focusing phase, λ is the metasurface vortex phase, f is the working wavelength, l is the focal length, l is the angular quantum number, and θ is the spatial azimuth angle.

[0125] In a specific implementation of the embodiment, in step S23 above, the spectral response of the periodic grating is calculated using rigorous coupled-wave analysis or the finite-difference time-domain method. Also, in step S3, the phase distribution of the nanostructure capable of increasing the mode field matching degree of the fiber-on-chip waveguide is obtained.

[0126] In step S4, the light field propagation simulation of the metasurface is performed, and the light field propagation is calculated using angular spectrum.

[0127] The propagation of light waves in free space can be described by Fresnel diffraction, Fraunhofer diffraction, and plane wave angular spectrum theory. Among them, plane wave angular spectrum theory discusses the propagation of light waves in the frequency domain, and any two-dimensional light field distribution can be considered as a linear superposition of plane light waves of various spatial frequencies.

[0128] Assuming the wave vector of the plane wave is k, and the angles it makes with the x, y, and z axes are α, β, and γ, respectively, according to the definition of spatial frequency—that is, the number of times the light field signal repeats per unit distance—the spatial frequencies of the x, y, and z axes are expressed as follows:

[0129]

[0130] because

[0131] cos 2 α+cos 2 β+cos 2 γ = 1,

[0132] It can be obtained

[0133]

[0134] Therefore, the spatial frequency in the z-direction can be expressed as:

[0135]

[0136] Let a plane wave propagating in any direction be represented as:

[0137] U(x,y,z)=Ae ik(x cosα+y cosβ) ,

[0138] in,

[0139]

[0140] Therefore, a plane wave U propagating in any direction can be represented as:

[0141]

[0142] Let a(x,y) be the light field function of a certain xy plane. Its inverse Fourier transform yields:

[0143]

[0144] Among them, A(f) x ,f y The spectrum of a(x,y) is called the light field. The above equation shows that the light field can be considered as a spectrum composed of basis functions. According to different weights A(f) x ,f y The linear superposition of the light field. Conversely, by performing a Fourier transform on the light field, its spectrum can be obtained, i.e.:

[0145]

[0146] Let the light field of the initial z = 0 plane be represented as U(x,y,0), and its spectrum be represented as A0(f x ,f y The light field of the observed plane is represented as U(x,y,z), and its spectrum is represented as A(f,0). x ,f y ,z), can be obtained from the Helmholtz equation:

[0147]

[0148] in,

[0149] The phase shift between two angular spectra is called the system's transfer function. Therefore, to obtain the light field function of the observation plane, we only need to consider A(f) x ,f y The inverse Fourier transform of z can be performed.

[0150] Where, A0(f x ,f y A(f,0) represents the spectrum of the z=0 plane, specifically the spectrum of the fiber optic output end face; x ,f y ,z) is the spectrum of the propagation plane.

[0151] For example, after discretizing the ideal phase in multiple orders, full-wave simulation can be performed using commercial software FDTD Solutions to obtain the simulated propagation results of the metasurface. The FDTD simulation results are as follows: Figure 12 As shown.

[0152] Among them, such as Figure 9 The figure shown is a schematic diagram of the simulated light field propagation of the superlens corresponding to Method 1 above;

[0153] like Figure 10 The figure shown is a schematic diagram of the simulated light field propagation of the superlens corresponding to Method 2 above;

[0154] like Figure 11 The figure shows the waveguide-end mode field intensity distribution after coupling. It can be seen that TE1-TE0 coupling between the fiber and the on-chip waveguide has been achieved. Specifically, the waveguide-end optical field amplitude can be used as the target optical field to preset the focal spot field distribution, and then the phase modulation value of the metasurface can be designed to match the focused spot with the waveguide-end mode field. The metric is the mode matching degree, expressed by formula Eq-1.

[0155] In this application embodiment, a coupling device for an optical fiber and an on-chip waveguide is also provided, which includes an optical fiber, an on-chip waveguide, and a metasurface disposed between the optical fiber and the on-chip waveguide, wherein the metasurface is constructed according to the design method of the foregoing embodiments and any of the optional embodiments thereof.

[0156] In a preferred embodiment, the metasurface is formed directly on the fiber end face. Typically, the end face of the fiber core can be etched directly to form a nanostructure that meets the phase requirements. Preferably, an additional protective layer can be applied.

[0157] In another implementation, the metasurface is formed directly on the end face of the on-chip waveguide, particularly the side face of the semiconductor element.

[0158] In a preferred embodiment, the metasurface is formed as a separate superlens, which includes a substrate and a metasurface disposed on the substrate. The superlens is disposed on the end face of the optical fiber. The substrate needs to have high transmittance to the operating wavelength band. Furthermore, the substrate material can be the same as or different from the optical fiber core.

[0159] According to embodiments of this application, the nanostructure can be formed from at least one of the following materials: titanium oxide, silicon oxide, silicon nitride, gallium nitride, gallium phosphide, aluminum oxide, hydrogenated amorphous silicon, etc. For example, when the target wavelength is visible light, the material of the nanostructure includes one or more of silicon nitride, titanium oxide, gallium nitride, gallium phosphide, and hydrogenated amorphous silicon; when the target wavelength is near-infrared light, the material of the nanostructure includes one or more of silicon nitride, titanium oxide, gallium nitride, gallium phosphide, hydrogenated amorphous silicon, amorphous silicon, and crystalline silicon; when the target wavelength is far-infrared light, the material of the nanostructure includes one or more of crystalline silicon, crystalline germanium, zinc sulfide, and zinc selenide; when the target wavelength is ultraviolet light, the material of the nanostructure includes hafnium oxide.

[0160] The aforementioned superlens can be mounted with its nanostructured surface facing the fiber core, or it can be mounted in the opposite direction.

[0161] In a preferred embodiment, the nanostructure of the metasurface is a polarization-independent structure. Further, the nanostructure is a columnar body with a circular or square cross-section.

[0162] In a preferred embodiment, the nanostructures are arranged to form densely packed structural units; the structural units are squares or regular hexagons.

[0163] In a preferred embodiment, the coupling device operates at a wavelength of 1550 nm; and the metasurface therein has a nanostructure with at least the following parameters:

[0164] Serial Number length / nm Width / nm Transmission rate Phase (π) 1 460 200 0.966 0.1 2 525 200 0.914 0.18 3 545 235 0.934 0.303 4 495 280 0.967 0.4 5 260 95 0.97 0.555 6 205 460 0.97 0.7 7 255 360 0.967 0.8 8 270 450 0.968 0.94 .

[0165] In a preferred embodiment, the phase distribution of the metasurface satisfies:

[0166]

[0167] in, Let λ be the metasurface phase, λ be the operating wavelength, and f be the focal length.

[0168] or

[0169]

[0170]

[0171] in, θ(x,y) is the metasurface focusing phase, λ is the metasurface vortex phase, f is the working wavelength, l is the focal length, l is the angular quantum number, and θ is the spatial azimuth angle.

[0172] In summary, the design method provided in this application, or the coupling device based on the same technical concept, combines optical fiber with a metasurface. A metasurface is designed on the end face of the optical fiber, and the coupling between the first-order mode of the optical fiber and the on-chip single-mode waveguide is achieved through flexible wavefront modulation of the metasurface, without the need for an additional mode converter. The metasurface is a two-dimensional planar structure with subwavelength characteristic dimensions and a thickness of only a few hundred nanometers, thus its small size facilitates system integration and miniaturization.

[0173] Furthermore, due to their ability to arbitrarily control the light field with high precision and multiple degrees of freedom, metasurfaces can achieve functions comparable to traditional optical devices, and are lighter, thinner, more compact, and less expensive than traditional devices. Moreover, metasurfaces are compatible with CMOS processes, are simple to fabricate, and easy to manufacture.

[0174] The above are merely specific embodiments of this application, but the protection scope of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the embodiments of this application should be included within the protection scope of this application. Therefore, the protection scope of this application should be determined by the scope of the claims.

Claims

1. A design method for a metasurface used for coupling optical fiber and on-chip waveguide, characterized in that, The metasurface is disposed between the fiber end face and the on-chip waveguide for wavefront modulation of the emitted light field of the fiber, thereby achieving mode conversion and coupling between the fiber and the on-chip waveguide. The design method includes the following steps: Step S1: Determine the mode characteristics of the optical fiber and the on-chip waveguide, wherein the mode characteristics of the optical fiber include the amplitude and phase distribution in the first-order linear polarization mode of the optical fiber, and the mode characteristics of the on-chip waveguide include the amplitude and phase distribution in the single-mode waveguide mode. Step S2: Based on the transmittance and transmission phase response of the metasurface for the working band, screen the nanostructures that the metasurface can include. Step S3: Based on the mode characteristics of the optical fiber, with the design goal of reducing the mode field size of the output field of the optical fiber or increasing the mode size at the waveguide end, calculate the phase distribution of the metasurface used to improve the mode field matching degree of the optical fiber and the on-chip waveguide, and then determine the arrangement of the nanostructures of the metasurface.

2. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 1, characterized in that, The design steps for the metasurface also include: Step S4: Perform light field propagation simulation on the metasurface formed in step S3.

3. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 1 or 2, characterized in that, Step S2 specifically includes: Step S21: Determine the material of the nanostructure based on the operating wavelength; Step S22, according to ； Determine the height d of the nanopillar, where φ is the emission phase and n is the refractive index of the material. The wavelength of the operating band; Determine the arrangement period of the nanostructure, wherein the arrangement period satisfies the subwavelength condition; Step S23: Calculate the spectral response of the periodic grating to obtain the range of nanostructure parameters that meet the requirements of transmittance and transmission phase response.

4. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 3, characterized in that, The nanostructure must meet the 2π phase coverage requirement.

5. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 3, characterized in that, Step S2 further includes: Step S24: Discretize the obtained nanostructure parameter range to obtain the geometric dimensions, transmittance and phase parameters corresponding to the multi-order nanostructure.

6. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 5, characterized in that, The geometric dimensions, transmittance, and phase parameters of the 8th-order nanopillars were obtained.

7. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 3, characterized in that, In step S23, the spectral response of the periodic grating is calculated by rigorous coupled-wave analysis or the finite-difference time-domain method.

8. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 1, characterized in that, In step S3, a phase distribution of the metasurface capable of increasing the mode field matching degree of the fiber-on-chip waveguide is obtained, wherein the mode field matching degree satisfies: ； in, E1(x,y) represents the mode field matching degree, E2(x,y) represents the complex amplitude of the fiber output field, and E2(x,y) represents the complex amplitude of the chip edge waveguide mode.

9. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 8, characterized in that, In step S3, obtaining the phase distribution of the metasurface includes, based on the focused phase, assigning a phase difference of π in the positive and negative x-axis directions of the metasurface, wherein the phase distribution satisfies: ； in, For metasurface phase, For the operating wavelength, It is the focal length.

10. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 8, characterized in that, In step S3, obtaining the phase distribution of the metasurface includes adding a vortex phase to the focused phase, wherein the phase distribution satisfies: ； in, For metasurface focusing phase, For metasurface vortex phase, For the operating wavelength, Let l be the focal length and l be the angular quantum number. It is the spatial azimuth angle.

11. The design method for a metasurface for coupling optical fiber and on-chip waveguide according to claim 2, characterized in that, In step S4, after multi-order discretization of the phase distribution, a full-wave simulation is performed to obtain the simulation propagation results; and... The propagation of the optical field is obtained by using the phase shift between angular spectra as the transfer function; The phase shift between the angular spectra is expressed as: ； And it satisfies: ； in, This refers to the spectrum of the z=0 plane or the spectrum of the fiber optic output end face. The spectrum of the propagation plane.

12. A coupling device, characterized in that, The device includes an optical fiber, an on-chip waveguide, and a metasurface disposed between the optical fiber and the on-chip waveguide, wherein the metasurface is constructed according to the design method of the metasurface for coupling optical fiber and on-chip waveguide according to any one of claims 1 to 11.

13. The coupling device according to claim 12, characterized in that, The metasurface is formed directly on the end face of the optical fiber.

14. The coupling device according to claim 12, characterized in that, It includes a superlens; the superlens includes a substrate and a metasurface disposed on the substrate, the superlens being disposed on the end face of the optical fiber.

15. The coupling device according to any one of claims 12 to 14, characterized in that, There is no relative rotation angle between the nanostructures of the metasurface.

16. The coupling device according to claim 15, characterized in that, The nanostructure is a columnar body with a rectangular cross-section.

17. The coupling device according to any one of claims 12 to 14, characterized in that, The nanostructures are arranged to form densely packed structural units; the structural units are squares or regular hexagons.

18. The coupling device according to claim 16, characterized in that, The coupling device operates at a wavelength of 1550 nm; and the metasurface therein has a nanostructure with at least the following parameters: Length from 205 nm to 545 nm, width from 95 nm to 460 nm, transmittance from 0.914 to 0.968, phase from 0.1π rad to 0.94π rad.

19. The coupling device according to claim 12, characterized in that, The phase distribution of the metasurface satisfies: ； in, For metasurface phase, For the operating wavelength, Focal length; or ； in, For metasurface focusing phase, For metasurface vortex phase, For the operating wavelength, Let l be the focal length and l be the angular quantum number. It is the spatial azimuth angle.

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