Multimode optical fiber mode detection method and system based on metasurface

Abstract

The invention belongs to the technical field of mode recognition, and discloses a metasurface-based multimode optical fiber mode detection method and system. Complex amplitude distributions of a plurality of linear polarization modes transmitted by a multi-mode optical fiber are respectively used as inputs of a plurality of detection channels on a metasurface, and intensity distributions of emergent target areas corresponding to the plurality of linear polarization modes transmitted by the multi-mode optical fiber are respectively used as outputs of the plurality of detection channels on the metasurface. Determining structure parameters of the metasurface according to the working wavelength and input and output of a plurality of detection channels of the metasurface; a plurality of linear polarization modes transmitted by the multimode optical fiber are converted into circular polarization states, then are incident to the metasurface, are subjected to phase modulation by the metasurface, and are respectively diffracted to a plurality of emergent target areas which have the same diffraction angle and are at equal intervals on the same detection plane in the far-field transmission space of the metasurface, so that the detection and identification of the incident mode are realized. According to the invention, the design complexity of multimode optical fiber mode detection is reduced, and the integration level and the accuracy of the device are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of pattern recognition, and more specifically, relates to a multimode fiber mode detection method and system based on a metasurface. Background Art

[0002] Multimode fiber mode detection technology is one of the research hotspots in the field of optical fiber communication technology. Most traditional communication systems use optoelectronic devices such as couplers, phase plates, and photonic crystals to perform technical operations of multiplexing and demultiplexing various transmission modes in multimode fibers, so as to achieve the purpose of mode detection. However, due to the fixed and single functions of such devices and the high compatibility with various principles and devices related to traditional optical fiber communication systems, their design complexity is relatively high, the device integration level is relatively low, and the recognition accuracy is relatively low. Summary of the Invention

[0003] The present invention provides a multimode fiber mode detection method and system based on a metasurface, which solves the problems of relatively high design complexity, relatively low device integration level, and relatively low detection and recognition accuracy in multimode fiber mode detection in the prior art.

[0004] The present invention provides a multimode fiber mode detection method based on a metasurface, including the following steps:

[0005] Taking the complex amplitude distributions of multiple linearly polarized modes transmitted by the multimode fiber as the inputs of multiple detection channels on the metasurface respectively; taking the intensity distributions of the outgoing target regions corresponding to the multiple linearly polarized modes transmitted by the multimode fiber as the outputs of the multiple detection channels on the metasurface respectively;

[0006] Determining the structural parameters of the metasurface according to the working wavelength, and the inputs and outputs of the multiple detection channels of the metasurface;

[0007] The multiple linearly polarized modes transmitted by the multimode fiber are incident on the metasurface after being converted into circular polarization states. After being phase-modulated by the metasurface, they are diffracted to multiple outgoing target regions with the same diffraction angle and equally spaced on the same detection plane in the far-field transmission space of the metasurface, so as to realize the detection and recognition of the incident modes.

[0008] Preferably, the metasurface is composed of several periodically arranged nano-unit structures, and each nano-unit structure includes a substrate and nano-bricks arranged on the working surface of the substrate. The geometric sizes of all the nano-bricks included in the metasurface are the same; determining the structural parameters of the metasurface includes: determining the period of the nano-unit structure, and the length, width, height, and rotation angle of the nano-bricks.

[0009] Preferably, the working wavelength of the metasurface is set to be the working wavelength of the multimode fiber, and the period of the nano-unit structure, as well as the length, width, and height of the nano-bricks, are determined by electromagnetic simulation, so that the metasurface has the function of a half-wave plate; the phase distribution of the metasurface is determined according to the inputs and outputs of multiple detection channels of the metasurface, and the steering angles of several nano-bricks included in the metasurface are determined based on the phase distribution.

[0010] Preferably, the inputs and outputs of the detection channels satisfy the following mapping relationship:

[0011] Out={FFT[Inexp(i×Phi)]} 2

[0012] In the formula, OUT represents the output, In represents the input, FFT represents the Fourier transform, i represents the imaginary unit, and Phi represents the phase distribution of the metasurface;

[0013] Based on the mapping relationship, the phase distribution of the metasurface is determined by performing multiple iterative calculations using the simulated annealing algorithm. Preferably, the following relationship is satisfied between the phase distribution of the metasurface and the steering angle of the nano-bricks:

[0014] Phi=2θ

[0015] In the formula, Phi represents the phase distribution of the metasurface, and θ represents the steering angle of the nano-bricks.

[0016] Preferably, the multimode fiber mode detection method based on the metasurface further includes: determining the materials of the substrate and the nano-bricks according to the working wavelength and the target amplitude transmittance.

[0017] Preferably, the complex amplitude distribution of the linearly polarized mode is obtained in the following manner: based on the transmission characteristics and characteristic equation of the linearly polarized mode, a mode distribution grid is divided, and the amplitude distribution of the linearly polarized mode is calculated; based on the distribution characteristics of the linearly polarized mode, the amplitude distribution of the linearly polarized mode is superimposed with its corresponding phase distribution to obtain the complex amplitude distribution of the linearly polarized mode.

[0018] Preferably, the position coordinates of the outgoing target area on the detection plane satisfy the following relational formula:

[0019]

[0020] where α is the diffraction angle, D is the distance between the metasurface and the detection plane, and (x0, y0) are the position coordinates of the outgoing target area on the detection plane;

[0021] According to the relationship satisfied by the position coordinates of the outgoing target region on the detection plane, a circular region is selected on the detection plane, and a plurality of regions are equally spaced in the circular region as the outgoing target regions of multiple transmission modes of the multimode optical fiber.

[0022] Preferably, the modes transmitted by the multimode optical fiber include LP 01 , LP 11a , LP 11b , LP 21a , LP 21b , LP 31a , LP 31b and LP 02 .

[0023] On the other hand, the present invention provides a multimode optical fiber mode detection system based on a metasurface, including a laser, a multimode optical fiber, a conversion device, a metasurface, and a detection screen arranged in sequence along the optical path; the laser is used to provide a light source with a working wavelength, the multimode optical fiber supports the parallel and independent transmission of multiple linearly polarized modes, and the conversion device is used to convert the linearly polarized mode into a circular polarization state; the detection screen serves as a detection plane in the far-field transmission space of the metasurface for detecting the incident mode energy; the multimode optical fiber mode detection system based on the metasurface is used to perform the steps in the above-mentioned multimode optical fiber mode detection method based on the metasurface.

[0024] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0025] The present invention takes the complex amplitude distributions of multiple linearly polarized modes transmitted by the multimode optical fiber as the inputs of multiple detection channels on the metasurface, and takes the intensity distributions of the outgoing target regions corresponding to the multiple linearly polarized modes transmitted by the multimode optical fiber as the outputs of multiple detection channels on the metasurface; according to the working wavelength, as well as the inputs and outputs of multiple detection channels of the metasurface, the structural parameters of the metasurface are determined; after the multiple linearly polarized modes transmitted by the multimode optical fiber are converted into circular polarization states and incident on the metasurface, after being phase-modulated by the metasurface, they are respectively diffracted to multiple outgoing target regions with the same diffraction angle and equally spaced on the same detection plane in the far-field transmission space of the metasurface, realizing the detection and identification of the incident mode. That is, the present invention combines the metasurface device and the fiber mode multiplexing communication technology, makes multiple transmission modes in the multimode optical fiber incident on the metasurface, and realizes the separation and identification of multiple transmission modes in the multimode optical fiber by detecting the energy in different regions on the same far-field plane in the transmission space, achieving the detection purpose.

[0026] The present invention reduces the design complexity of multimode fiber mode detection technology. By utilizing the geometric phase modulation of the metasurface and the Fourier transform principle, independent phase modulation of multiple detection channels and independent partition mapping of multiple transmission modes are achieved, promoting the expandability and transferability of the design of fiber optic communication systems.

[0027] The present invention improves the device integration of multimode fiber mode detection technology. By using a monolithic metasurface with a single nano-unit structure at the nanoscale, independent phase modulation of multiple detection channels and independent partition mapping of multiple transmission modes are realized, promoting the miniaturization and integration of devices in fiber optic communication systems.

[0028] The present invention improves the functional accuracy of multimode fiber mode detection technology. By optimizing the metasurface structure parameters using electromagnetic simulation software and optimizing the metasurface phase distribution using the simulated annealing algorithm, independent phase modulation of multiple detection channels and independent partition mapping of multiple transmission modes are achieved, promoting high efficiency and low crosstalk of the functions of fiber optic communication systems.

[0029] In summary, the present invention can solve the problems of relatively high design complexity, relatively low device integration, and relatively low functional accuracy in multimode fiber mode detection in the prior art.

[0030] The present invention has rich research value and broad application prospects in the fields of fiber optic communication, pattern recognition, object classification, encryption and decryption of information, and multiplexing and demultiplexing of fiber modes. Description of the Drawings

[0031] Figure 1 It is a schematic diagram of the nano-unit structure in a multimode fiber mode detection method based on a metasurface provided in Embodiment 1 of the present invention;

[0032] Figure 2 It is a complex amplitude distribution diagram of the energy optical field of various fiber modes in a multimode fiber mode detection method based on a metasurface provided in Embodiment 1 of the present invention;

[0033] Figure 3 It is an intensity distribution diagram of the outgoing target region of various fiber modes in a multimode fiber mode detection method based on a metasurface provided in Embodiment 1 of the present invention;

[0034] Figure 4 It is a curve diagram of the amplitude transmittance and phase modulation amount of the nano-unit structure in a multimode fiber mode detection method based on a metasurface provided in Embodiment 1 of the present invention;

[0035] Figure 5 It is a phase distribution diagram of the metasurface in a multimode fiber mode detection method based on a metasurface provided in Embodiment 1 of the present invention;

[0036] Figure 6Schematic diagram of the overall structure of the metasurface in a multimode fiber mode detection method based on metasurface provided in Embodiment 1 of the present invention;

[0037] Figure 7 Schematic diagram of the working principle of a multimode fiber mode detection system based on metasurface provided in Embodiment 2 of the present invention;

[0038] Figure 8 Schematic diagram of the architecture of a multimode fiber mode detection system based on metasurface provided in Embodiment 2 of the present invention.

[0039] Wherein, 1 - laser, 2 - multimode fiber, 3 - collimating lens, 4 - polarizer, 5 - quarter-wave plate, 6 - metasurface, 7 - detection screen. Detailed implementation manners

[0040] In order to better understand the above technical solutions, the above technical solutions will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners.

[0041] Embodiment 1:

[0042] Embodiment 1 provides a multimode fiber mode detection method based on metasurface, including the following steps:

[0043] The complex amplitude distributions of multiple linearly polarized modes transmitted by the multimode fiber are respectively used as the inputs of multiple detection channels on the metasurface; the intensity distributions of the output target regions corresponding to the multiple linearly polarized modes transmitted by the multimode fiber are respectively used as the outputs of the multiple detection channels on the metasurface;

[0044] According to the working wavelength, and the inputs and outputs of the multiple detection channels of the metasurface, determine the structural parameters of the metasurface;

[0045] The multiple linearly polarized modes transmitted by the multimode fiber are converted into circular polarization states and then incident on the metasurface. After being phase-modulated by the metasurface, they are respectively diffracted to multiple output target regions with the same diffraction angle and equally spaced on the same detection plane in the far-field transmission space of the metasurface, realizing the detection and identification of the incident modes.

[0046] Wherein, the metasurface is composed of several periodically arranged nano-unit structures, and each nano-unit structure includes a substrate and nano-bricks arranged on the working surface of the substrate. The geometric sizes of all the nano-bricks included in the metasurface are the same; determining the structural parameters of the metasurface includes: determining the period of the nano-unit structure, and the length, width, height and turning angle of the nano-bricks. That is, the metasurface includes several nano-bricks with the same geometric size and different turning angles.

[0047] For example, the substrate is divided into a plurality of periodically arranged square units with the same size, and a cuboid nanobrick is arranged on the working surface of each square unit. The geometric schematic diagram of the nano-unit structure is as shown in Figure 1 shown. Let the side length of the square unit be the period CS of the nano-unit structure; let the geometric dimensions of the nanobrick include the length L, width W, and height H of the nanobrick; establish a coordinate system XOY on the working surface of the substrate, the X-axis and the Y-axis are respectively parallel to two groups of sides of the working surface of the substrate, the long side direction of the nanobrick represents the major axis, the short side direction of the nanobrick represents the minor axis, and let the angle between the major axis of the nanobrick and the X-axis be the rotation angle θ of the nanobrick.

[0048] In Embodiment 1, aiming at the characteristic of a large number of transmission modes of multimode optical fibers, the number of detection channels of the corresponding constructed metasurface is also large. The mapping relationship between incidence and emergence can be realized through a single-layer single-structure metasurface, which has the advantage of a large information capacity. And in Embodiment 1, a plurality of equidistant regions with the same diffraction angle in space are taken as the emergence target regions, and the design complexity of multimode optical fiber mode detection is further reduced by controlling variables.

[0049] The following further describes several main aspects of the multimode optical fiber mode detection method based on the metasurface provided by the present invention.

[0050] (1) Set the input of the detection channel.

[0051] The present invention respectively takes the complex amplitude distributions of a plurality of linearly polarized modes transmitted by the multimode optical fiber as the inputs of a plurality of detection channels on the metasurface.

[0052] Calculate the incident energy optical field of the linearly polarized mode transmitted by the multimode optical fiber, that is, the complex amplitude distribution of the transverse optical field of the mode. Set the complex amplitude distribution of each energy optical field as the input distribution of each detection channel on the metasurface to realize multimode multiplexing of the transverse spatial degree of freedom.

[0053] Specifically, the transmission characteristics of the linearly polarized mode satisfy the following relational expression:

[0054] V 2 =U 2 +W 2 (1)

[0055] where V is the normalized cut-off frequency of each mode in the optical fiber, U is the normalized radial constant of each mode, and W is the attenuation constant of each mode.

[0056] The characteristic equation of the linearly polarized mode is:

[0057]

[0058] Among them, l is the logarithm of the maximum value of the amplitude distribution of each mode optical field in the circumferential direction, and at the same time is the number of times of the periodic transformation of the optical field phase distribution, reflecting the distribution law of the mode optical field; U and W are the normalized cut-off frequency and attenuation constant of each mode respectively, and J l (U) is the first kind of Bessel function of order l of U, and J l-1 (U) is the first kind of Bessel function of order l-1 of U, and K l (W) is the modified Bessel function of order l of W, and K l-1 (W) is the modified Bessel function of order l-1 of W.

[0059] Based on the transmission characteristic parameters and characteristic equations of each linearly polarized mode, divide the mode distribution grid and calculate the optical field amplitude distribution In of each mode amp ; Based on the distribution characteristic parameters of each linearly polarized mode, perform the superposition of the phase distribution In amp on the amplitude distribution In of each mode phi to obtain the complex amplitude distribution In of each transmission mode.

[0060] For example, select a multimode optical fiber that can support the transmission of 8 modes (LP 01 mode, LP 11a mode, LP 11b mode, LP 21a mode, LP 21b mode, LP 31a mode, LP 31b mode and LP 02 mode). The amplitude and phase distributions of these 8 modes are as Figure 2 shown. Based on the orthogonality of the transmission of each mode in the multimode optical fiber, the metasurface can realize independent phase modulation of the above 8 modes, generate independent detection channels corresponding to each mode, and therefore set the complex amplitude distribution of each mode as the input distribution of each detection channel on the metasurface.

[0061] (2) Set the output of the detection channel.

[0062] The present invention takes the intensity distributions of the outgoing target regions corresponding to the multiple linearly polarized modes transmitted by the multimode optical fiber as the outputs of the multiple detection channels on the metasurface respectively.

[0063] Set the outgoing target regions of the linearly polarized modes transmitted by the multimode optical fiber, that is, the equally spaced regions at the same diffraction angle on the detection plane in the far-field transmission space of the metasurface, and set the intensity distribution of each target region as the output distribution of each detection channel on the metasurface to realize the separation and routing of multiple modes in the transverse spatial degree of freedom.

[0064] Among them, based on the calculation of the spatial diffraction angle and regional interval, the distance of the detection plane in the far-field transmission space of the metasurface is determined to be D, and the size is A×B pixels. The diffraction angle of the target area is α, and the size is a×b pixels, and the intensity distribution Out of each target area is obtained.

[0065] For example, it is set that the distance D between the detection plane and the metasurface is 5 cm, and the size is 200×200 pixels. It is set that the diffraction angle α of the target area is 30°, and the size is 10×10 pixels. Its position on the detection plane satisfies the following relational formula:

[0066]

[0067] Among them, x0 and y0 are the position coordinates of the target area on the detection plane.

[0068] Based on the above diffraction angle formula, the relationship between x0 and y0 is determined, and then a circle with a certain radius is selected. Multiple target areas are equally spaced on the circle. For example, 8 target areas are equally spaced on the circle as the target mapping areas of 8 transmission modes in the multimode optical fiber. Therefore, the intensity distribution Out of each target area is set as the output distribution of each detection channel on the metasurface, as Figure 3 shown.

[0069] (3) Determine the structural parameters and arrangement of the metasurface to obtain the finally designed metasurface.

[0070] (3.1) In the present invention, the working wavelength of the metasurface is set as the working wavelength of the multimode optical fiber, and the period of the nano-unit structure, as well as the length, width, and height of the nano-brick, are determined by electromagnetic simulation, so that the metasurface has the function of a half-wave plate.

[0071] In addition, in the present invention, the materials of the substrate and the nano-brick can also be determined according to the working wavelength and the target amplitude transmittance.

[0072] Specifically, the geometric dimension parameters of the nano-unit structure are scanned by the electromagnetic simulation software COMSOL Multiphysics, and the anisotropy of the nano-unit structure is adjusted to convert most of the incident light into reverse polarized light with a phase modulation amount, thereby improving the polarization conversion efficiency of the metasurface.

[0073] For example, set the operating wavelength of the metasurface to 1550 nm, the operating wavelength of the multimode fiber, set the height H of the nanobrick to 1000 nm, scan the period CS of the nano-unit structure, as well as the length L and width W of the nanobrick, adjust the anisotropy of the nano-unit structure, calculate the amplitude transmission and phase modulation data of the nano-unit structure, convert most of the incident light into reverse polarized light with a phase modulation amount, and be able to cover a phase modulation range of 0 to 2π. Finally, determine a set of optimized geometric structure parameters of the nano-unit structure: CS = 900 nm, L = 680 nm, W = 260 nm, H = 1000 nm, and the transformation curves of its amplitude transmittance and phase modulation amount with respect to the steering angle of the nanobrick are as shown in Figure 4 shown. The materials of the substrate and the nanobrick disposed on the working surface of the substrate are both crystalline silicon.

[0074] (3.2) The present invention determines the phase distribution of the metasurface according to the inputs and outputs of multiple detection channels of the metasurface, and determines the steering angles of several nanobricks included in the metasurface based on the phase distribution.

[0075] That is, optimize the phase distribution of the metasurface, and realize the one-to-one mapping correspondence between the incident light field of the fiber mode and the output target on each detection channel through the metasurface, so as to realize the multiplexing of multiple mode detection channels labeled with the incident mode and the output light spot in the transverse spatial degree of freedom.

[0076] Specifically, the mapping relationship between the incident light field (i.e., input) of the fiber mode on each detection channel of the metasurface and the output target (i.e., output) can be expressed as:

[0077] Out = {FFT[Inexp(i×Phi)]} 2 (4)

[0078] In the formula, OUT represents the output, In represents the input, FFT represents the Fourier transform, i represents the imaginary unit, and Phi represents the phase distribution of the metasurface.

[0079] For example, 8 transmission modes of the multimode fiber respectively correspond to 8 detection channels of the metasurface, the complex amplitude distribution In of each transmission mode corresponds to the input of each detection channel, and the target region intensity distribution Out of each transmission mode corresponds to the output of each detection channel.

[0080] Based on the above mapping relation formula (that is, based on the relationship between the inputs and outputs of multiple detection channels of the metasurface), use the simulated annealing algorithm to perform multiple iterative calculations on the phase distribution Phi of the metasurface to realize the one-to-one mapping relationship between each transmission mode and its target region. The finally optimized phase distribution of the metasurface is as shown in Figure 5 shown.

[0081] Then, based on the phase distribution of the metasurface, the steering angles of several nanobricks at corresponding positions are calculated, and accordingly, the nano-unit structures are arranged orderly to obtain the finally designed metasurface for realizing multimode fiber mode detection.

[0082] Among them, based on the geometric phase modulation principle of the metasurface, there is a certain linear relationship between the phase distribution Phi of the metasurface and the steering angle θ of the nanobricks. Therefore, according to the phase distribution of the metasurface, the steering angle arrangement modes of several nano-unit structures can be obtained.

[0083] Specifically, the linear relationship between the phase distribution Phi of the metasurface and the steering angle θ of the nanobricks can be expressed as:

[0084] Phi = 2θ (5)

[0085] Based on the above formula, the metasurface can realize continuous phase modulation of the incident circularly polarized light: when left-handed circularly polarized light is incident, the outgoing light includes right-handed circularly polarized light with a phase delay of 2θ; when right-handed circularly polarized light is incident, the outgoing light includes left-handed circularly polarized light with a phase delay of -2θ. Therefore, based on the phase distribution of the metasurface, the steering angle arrangement modes of several nano-unit structures on the metasurface are obtained, and the overall structure of the metasurface is constructed as Figure 6 shown.

[0086] (4) Use the metasurface to realize multimode fiber mode detection.

[0087] After obtaining the designed metasurface, the multiple linearly polarized modes transmitted by the multimode fiber are converted into circular polarization states and then incident on the metasurface. After being phase-modulated by the metasurface, they are respectively diffracted into multiple outgoing target regions with the same diffraction angle and equally spaced on the same detection plane in the far-field transmission space of the metasurface, realizing the detection and identification of the incident modes. That is, the energy of the corresponding incident modes is detected in different outgoing target regions on the detection plane in the far-field transmission space, realizing the separation and identification of the incident modes.

[0088] Correspondingly, the present invention can also specifically build a multimode fiber mode detection system based on the metasurface to realize the function of multimode fiber mode detection, which will be described below with Embodiment 2.

[0089] Embodiment 2:

[0090] Embodiment 2 provides a multimode fiber mode detection system based on the metasurface, see Figure 7 and Figure 8, including a laser 1, a multimode optical fiber 2, a conversion device, a metasurface 6, and a detection screen 7 arranged in sequence along the optical path; the laser 1 is used to provide a light source with a working wavelength, the multimode optical fiber 2 supports the parallel and independent transmission of multiple linearly polarized modes (LP modes), and the conversion device is used to convert the linearly polarized mode into a circular polarization state; the detection screen 7 serves as a detection plane in the far-field transmission space of the metasurface, and is used to detect the incident mode energy.

[0091] Among them, the conversion device can adopt a polarizer 4 and a quarter-wave plate 5 arranged in sequence along the optical path, that is, after using the polarizer 4 and the quarter-wave plate 5 to convert each linearly polarized mode into a circular polarization state, it is incident on the metasurface 6, and after being phase-modulated by the metasurface 6, it diffracts to the target regions of each mode.

[0092] In addition, a collimating lens 3 can be arranged between the multimode optical fiber 2 and the polarizer 4. The laser 1 can adopt an infrared laser.

[0093] The multimode optical fiber mode detection system provided in Embodiment 2 is used to execute the steps in the multimode optical fiber mode detection method based on the metasurface described in Embodiment 1.

[0094] The function of the metasurface 6 in Embodiment 2 can be understood with reference to the description in Embodiment 1.

[0095] For example, it is set that the working wavelengths of the metasurface 6 and the multimode optical fiber 2 are both 1550 nm, and the laser 1 is used to provide a light source with a wavelength of 1550 nm. The 8 modes in the multimode optical fiber 2 are independently and parallelly transmitted in the optical fiber, and are emitted from the coupling port of the multimode optical fiber 2 to the collimating lens 3 to be converted into a collimated mode field, and are converted into a circular polarization state by the polarizer 4 and the quarter-wave plate 5, and are incident on the metasurface 6. After being phase-modulated by the metasurface 6, it diffracts to the respective outgoing target regions, and the corresponding incident mode energy is detected in different outgoing target regions on the detection screen 7 in the far-field transmission space, realizing the separation and identification of the incident modes and realizing the multimode optical fiber mode detection.

[0096] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A multimode optical fiber mode detection method based on a metasurface, characterized in that: The following steps are involved: The complex amplitude distributions of multiple linear polarization modes transmitted by the multimode optical fiber are used as inputs of multiple detection channels on the metasurface respectively; Using the intensity distributions of the emission target area corresponding to the multiple linear polarization modes transmitted by the multimode optical fiber as the outputs of the multiple detection channels on the metasurface respectively; Determining structural parameters of the metasurface according to the operating wavelength and the input and output of the multiple detection channels of the metasurface; The multiple linear polarization modes transmitted by the multimode optical fiber are converted into circular polarization states and then incident on the metasurface. After being phase modulated by the metasurface, they are diffracted to multiple exit target areas with the same diffraction angle and equal spacing on the same detection plane in the far-field transmission space of the metasurface, thereby realizing detection and identification of the incident mode.

2. The method for detecting multimode optical fiber modes based on a metasurface according to claim 1, characterized in that: The super surface is composed of a plurality of periodically arranged nano unit structures, each of which includes a substrate and a nano brick arranged on a working surface of the substrate, and all the nano bricks included in the super surface have the same geometric size; Determining the structural parameters of the metasurface includes: determining the period of the nanounit structure, and the length, width, height and turning angle of the nanobrick.

3. The method for detecting multimode optical fiber modes based on a metasurface according to claim 2, characterized in that: The working wavelength of the metasurface is set to the working wavelength of the multimode optical fiber, and the period of the nanounit structure, as well as the length, width and height of the nanobrick are determined by electromagnetic simulation, so that the metasurface has the function of a half-wave plate; the phase distribution of the metasurface is determined according to the input and output of multiple detection channels of the metasurface, and the steering angles of several nanobricks contained in the metasurface are determined based on the phase distribution.

4. The method for detecting multimode optical fiber modes based on a metasurface according to claim 3, characterized in that: The input and output of the detection channel satisfy the following mapping relationship: Out={FFT[Inexp(i×Phi)]} 2 Where OUT represents output, In represents input, FFT represents Fourier transform, i represents imaginary unit, and Phi represents the phase distribution of the metasurface; Based on the mapping relationship, a simulated annealing algorithm is used to perform multiple iterative calculations to determine the phase distribution of the metasurface.

5. The method for detecting multimode optical fiber modes based on a metasurface according to claim 3, characterized in that: The phase distribution of the metasurface and the steering angle of the nanobrick satisfy the following relationship: Phi=2θ Where Phi represents the phase distribution of the metasurface and θ represents the steering angle of the nanobrick.

6. The method for detecting multimode optical fiber modes based on a metasurface according to claim 2, characterized in that: Also includes: The materials of the substrate and the nanobricks are determined according to the operating wavelength and the target amplitude transmittance.

7. The method for detecting multimode optical fiber modes based on a metasurface according to claim 1, characterized in that: The complex amplitude distribution of the linear polarization mode is obtained in the following manner: based on the transmission characteristics and characteristic equation of the linear polarization mode, the mode distribution grid is divided and the amplitude distribution of the linear polarization mode is calculated; based on the distribution characteristics of the linear polarization mode, the amplitude distribution of the linear polarization mode is superimposed on its corresponding phase distribution to obtain the complex amplitude distribution of the linear polarization mode.

8. The method for detecting multimode optical fiber modes based on a metasurface according to claim 1, characterized in that: The position coordinates of the emission target area on the detection plane satisfy the following relationship: Where α is the diffraction angle, D is the distance between the metasurface and the detection plane, and (x0, y0) is the position coordinate of the exit target area on the detection plane; According to the relationship satisfied by the position coordinates of the emission target area on the detection plane, a circular area is selected on the detection plane, and multiple areas are selected at equal intervals on the circular area as the emission target areas of multiple transmission modes of the multimode optical fiber.

9. The method for detecting multimode optical fiber modes based on a metasurface according to claim 1, characterized in that: The multimode fiber transmission modes include LP 01 LP 11a LP 11b LP 21a LP 21b LP 31a LP 31b and LP 02 .

10. A multimode optical fiber mode detection system based on a metasurface, characterized in that: It includes a laser, a multimode optical fiber, a conversion device, a metasurface and a detection screen which are sequentially arranged along an optical path; the laser is used to provide a light source having an operating wavelength, the multimode optical fiber supports parallel and independent transmission of multiple linear polarization modes, and the conversion device is used to convert a linear polarization mode into a circular polarization state; the detection screen serves as a detection plane in the far-field transmission space of the metasurface, and is used to detect incident mode energy; the metasurface-based multimode optical fiber mode detection system is used to execute the steps in the metasurface-based multimode optical fiber mode detection method as described in any one of claims 1 to 9.

Images