A high-coupling-ratio high-mode-purity few-mode fiber multiplexer

Abstract

This invention proposes a few-mode fiber multiplexer with high coupling ratio and high mode purity, comprising a broadband mode selection coupler fabricated by polishing. The broadband mode selection coupler couples a few-mode fiber and a single-mode fiber. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=b*sqrt(exp(-c*(r / d)^e)), where b∈[0.004,0.008], c∈[0.02,0.10], d∈[8,16], and e∈[6,14]. In the mode selection coupler, the remaining thickness after polishing is reduced by using graded-index few-mode fiber. The remaining thickness x from the cladding to the core edge after polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x=R-r-a), ranging from 0.5 to 7.0 μm. This invention achieves a mode coupling efficiency greater than 80%, a mode purity greater than 88%, and a crosstalk between the output target mode and non-target modes less than -22.6dB, exhibiting characteristics of high coupling efficiency, low mode crosstalk, high mode purity, and high bandwidth.

Description

Technical Field

[0001] This invention relates to a mode selection coupler (MSC) in the field of optical fiber communication, and more particularly to an optical fiber coupling method using the mode selection coupler, as well as a mode multiplexer having a cascaded structure of multiple mode selection couplers. Background Technology

[0002] For ease of understanding, the technical terms involved in this invention are as follows:

[0003] SMF: Single Mode Fiber.

[0004] FMF: Few Mode Fiber.

[0005] MSC: Mode Selective Coupler.

[0006] MDM: Mode Division Multiplexing.

[0007] SDM: Space Division Multiplexing.

[0008] QAM: Quadrature Amplitude Modulation.

[0009] QPSK: Quadrature Phase Shift Keying.

[0010] MIMO: Multiple-Input Multiple-Output.

[0011] The Shannon limit, or Shannon capacity, is the theoretical maximum information transmission rate of a communication channel for a specific noise level. The well-known Shannon theorem is given by the formula: C = B × log₂(1 + S / N), where C is the achievable link speed (channel capacity), B is the link bandwidth, S is the average signal power, N is the average noise power, and the signal-to-noise ratio (S / N) is usually expressed in decibels (dB), where dB = 10 × lg(S / N).

[0012] From the perspective of optical fiber communication capacity development trends, based on the widely adopted wavelength division multiplexing (WDM) and various advanced modulation technologies (QAM, QPSK, etc.), existing optical fiber communication systems have approached and will soon reach the Shannon limit of standard single-mode fiber (SMF) transmission capacity. 5G (5th generation mobile communication) uses low-frequency bands, primarily improving channel capacity through multiple-input multiple-output (MIMO) and non-orthogonal multiplexing (NOMA) technologies. In the 6G stage, because 5G technology is approaching the Shannon limit, the focus will be on increasing channel bandwidth, and the core of channel bandwidth lies in the "information-carrying medium—optical fiber material." With the explosive growth of data transmission in 5G, 6G faces many new transmission challenges, making it imperative to overcome the transmission capacity bottleneck.

[0013] Few-mode fiber is a type of optical fiber capable of transmitting several different spatial modes in parallel. Generally, the capacity of few-mode fiber is proportional to the number of modes. Few-mode fiber can be used in spatial division multiplexing (SDM) technology, which allows for simultaneous transmission of information in several modes, thereby further expanding the transmission capacity of optical fibers. In mode-multiplexed communication using few-mode fiber, the mode multiplexer is a key component. However, in practice, the mode selection coupler of the mode multiplexer encounters problems when exciting modes in few-mode fiber, such as low coupling ratio, unsatisfactory mode purity, high loss, excessive crosstalk between the target mode and non-target modes, and the multiplexed modes lacking the ability to support high communication capacity. Furthermore, the mode selection coupler is difficult to manufacture and its performance is unstable, necessitating solutions to these technical challenges.

[0014] To address this, Chinese scientists have conducted extensive research. Chinese patent CN107942443A proposes a low-loss, low-crosstalk graded refractive index distribution three-mode multiplexer. However, this multiplexer is limited to three-mode fiber, has fewer operating modes and lower information transmission capacity, and suffers from high mode crosstalk, making it unsuitable for high-capacity transmission applications.

[0015] Chinese patent CN112505829A proposes a mode selection coupler design method, but this method can only excite a single mode and cannot achieve the reuse of multiple modes, thus failing to meet application requirements.

[0016] Korean patent KR102080301 proposes a mode multiplexer comprising multiple cascaded mode selection couplers, but does not provide parameters for the mode selection couplers and optical fibers used to achieve mode multiplexing performance. Furthermore, the quartz substrate slotted mode selection coupler is relatively complex to manufacture, making it inconvenient for practical fabrication and application.

[0017] In summary, existing invention patents cannot adequately address the technical requirements of few-mode fiber mode multiplexers in the field of high-capacity mode multiplexing optical fiber communication systems. To address the critical need for the practical application of mode multiplexing technology in next-generation optical fiber communication systems, this invention proposes a few-mode fiber multiplexer with high coupling ratio and high mode purity. Summary of the Invention

[0018] The purpose of this invention is to solve the mode multiplexing problem in few-mode optical fibers by using LP (Low-mode) multiplexing technology in single-mode optical fibers. 01 The energy of the mode is transferred to the higher-order modes in the few-mode fiber to excite the few-mode fiber mode, thereby enabling the transmission of multiple modes in the few-mode fiber and realizing mode multiplexing to increase the information transmission capacity.

[0019] The technical solution of this invention:

[0020] A few-mode fiber multiplexer with high coupling ratio and high mode purity includes a broadband mode-selective coupler fabricated by polishing. The broadband mode-selective coupler couples a few-mode fiber and a single-mode fiber. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=b*sqrt(exp(-c*(r / d)^e)), where b∈[0.004,0.008], c∈[0.02,0.10], d∈[8,16], and e∈[6,14]. The cross-section of the few-mode fiber in the broadband mode-selective coupler is polished into a D-shape. Compared to step-index fiber, the graded refractive index distribution of the few-mode fiber used for mode multiplexing has a more concentrated mode field distribution, allowing for a greater polishing depth with the same core size, thus achieving higher coupling efficiency.

[0021] In a few-mode fiber mode multiplexer, the effective refractive index of the single-mode fiber core at 1550 nm is between 1.44445 and 1.45375; the core radius of the few-mode fiber used for mode multiplexing is 8.0-15.0 μm; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), ranging from 0.5 to 7.0 μm. Multiple broadband mode selection couplers are connected in series, with the output of one coupler connected to the input of the next coupler.

[0022] Each mode selection coupler has a coupling length of 2000-7000μm; this mode multiplexer can achieve stable transmission of 6 different modes in the C-band, LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12The taper diameter of the six single-mode fibers is between 70-125 μm; the coupling ratio of all six modes is higher than 80%, the maximum loss is no more than 1.2 dB, the mode purity is greater than 88%, and at 1550 nm, the crosstalk between the target mode and the non-target mode at the few-mode end of the mode selection coupler is less than -22.6 dB.

[0023] The effective refractive index of the single-mode fiber used in the mode selection coupler at a wavelength of 1550nm is between 1.44445 and 1.45375. The mode selection coupler uses graded-index few-mode fiber to reduce the remaining thickness of the fiber after polishing. The remaining thickness x from the cladding to the core edge after polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), which ranges from 0.5 to 7.0 μm.

[0024] The core radius of the few-mode fiber used in the mode selection coupler is 8.0-15.0 μm; at 1530-1565 nm, LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The cone diameters corresponding to these six single-mode fibers are 125μm, 107μm, 86μm, 125μm, 101μm and 76μm, respectively.

[0025] This mode multiplexer can achieve stable transmission with six different mode outputs in the C-band (1530nm-1565nm). LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The taper diameter of the six single-mode fibers is between 70 and 125 μm; the coupling ratio of all six modes is higher than 80%, the maximum loss does not exceed 1.2 dB, the mode purity is greater than 88%, and at 1550 nm, the crosstalk between the target mode and the non-target mode at the output of the few-mode end of the mode selection coupler is less than -22.6 dB.

[0026] At wavelengths of 1530nm-1565nm, LP 01 The coupling ratio of the mode coupler ranges from 94.7% to 98.4%, LP 11 The coupling ratio of the mode coupler ranges from 99.1% to 99.9%, LP 21 The coupling ratio of the mode coupler ranges from 87.6% to 95.1%, LP 02 The coupling ratio of the mode coupler ranges from 81.3% to 91.4%, LP 31The coupling ratio of the mode coupler ranges from 89.9% to 93.55%, LP 12 The coupling ratio of the mode coupler ranges from 89.3% to 99.2%.

[0027] LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The mode crosstalk of the mode selection coupler is less than -18.2dB, -23.4dB, -24.2dB, -18.5dB, -18.6dB and -19.7dB, respectively.

[0028] At 1530-1565nm, LP 01 The loss range of the mode coupler is 0.51-0.86 dB, LP 11 The loss range of the mode coupler is 0.73-0.99 dB, LP 21 The loss range of the mode coupler is 0.46-0.94 dB, LP 02 The loss range of the mode coupler is 0.37-1.05 dB, LP 31 The loss range of the mode coupler is 0.60-1.08 dB, LP 12 The loss range of the mode coupler is 0.44-1.19 dB, LP 11 LP 21 LP 02 LP 31 and LP 12 The model purities were 88.1%, 88.0%, 99.3%, 96.2%, and 88.7%, respectively.

[0029] Compared with the prior art, the advantages of the present invention are as follows:

[0030] (1) Compared to step-index fibers, graded-index few-mode fibers used for mode multiplexing have a more concentrated mode field distribution, allowing for a greater polishing depth and higher coupling efficiency with the same core size. This invention reduces the remaining thickness of the fiber after polishing by using graded-index fibers in the 1530-1565nm range, achieving a mode coupling efficiency higher than 80%, mode purity greater than 88%, and crosstalk between the output target mode and non-target modes less than -22.6dB. It exhibits high coupling efficiency, low mode crosstalk, high mode purity, and high bandwidth.

[0031] (2) The present invention designs a method for manufacturing mode selection couplers. Compared with the method of grinding the quartz substrate groove, it only requires polishing the optical fiber itself, which greatly shortens the device cycle and has the advantages of simple manufacturing process and high manufacturing efficiency. Compared with the fused taper method, it does not damage the fiber core structure and has higher output end mode purity.

[0032] (3) The present invention realizes a cascaded mode multiplexer for six-mode optical fibers, which can freely select the required number of modes according to specific circumstances, and supports multiplexing of six modes, which has excellent flexibility and can improve the capacity of optical fiber communication. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0034] The above-described features and advantages of the present invention will be better understood after reading the following detailed description of embodiments of the present disclosure in conjunction with the accompanying drawings. In the drawings, components are not necessarily drawn to scale, and components having similar related characteristics or features may have the same or similar reference numerals.

[0035] Figure 1 This is a schematic diagram of the end face structure of the polished optical fiber in this invention. The remaining cladding thickness is defined as the distance between the polished surface of the optical fiber and the edge of the fiber core.

[0036] Figure 2 This is a schematic diagram of the cascaded structure in Implementation Example 1 of this invention. The six-mode fiber three-mode multiplexer consists of three mode selection couplers, one six-mode few-mode fiber, and three single-mode fibers. Single-mode fibers with different parameters are injected into the LP. 01 After mode selection, it is coupled to a six-mode few-mode fiber in a mode selection coupler to excite a specified mode in the six-mode few-mode fiber, thereby realizing LP. 01 LP 11 and LP 21 These three modes are reused.

[0037] Figure 3 This is a schematic diagram of the cascaded structure in Embodiment 2 of this invention. The five-mode multiplexer of the six-mode fiber consists of five mode selection couplers, one six-mode few-mode fiber, and five single-mode fibers. Single-mode fibers with different parameters are injected into the LP. 01 After mode selection, it is coupled to a six-mode few-mode fiber in a mode selection coupler to excite a specified mode in the six-mode few-mode fiber, thereby realizing LP. 01 LP 11 LP21 LP 02 and LP 31 These 5 patterns are reused.

[0038] Figure 4 This is a schematic diagram of the cascaded structure in Embodiment 3 of this invention. The six-mode fiber four-mode multiplexer consists of six mode selection couplers, one six-mode few-mode fiber, and six single-mode fibers. Single-mode fibers with different parameters are injected into the LP. 01 After mode selection, it is coupled to a six-mode few-mode fiber in a mode selection coupler to excite a specified mode in the six-mode few-mode fiber, thereby realizing LP. 01 LP 11 LP 21 LP 02 LP 31 and LP 12 These 6 modes are reused.

[0039] Figure 5 This invention demonstrates the normalized refractive index and field strength distribution of graded refractive index and step refractive index few-mode fibers, illustrating how graded refractive index few-mode fibers can achieve a smaller cladding residual thickness to achieve higher coupling efficiency. Detailed Implementation

[0040] See appendix Figure 2 , Figure 3 and Figure 4As shown, this invention provides a few-mode fiber mode multiplexer based on a mode selection coupler. The mode multiplexer structure includes a broadband mode selection coupler fabricated by side-polished fiber bonding, a few-mode fiber for mode multiplexing, and a single-mode fiber for mode multiplexing. Compared to step-index fiber, the graded-index few-mode fiber used in this mode multiplexer has a more concentrated mode field distribution, achieving a greater polishing depth and higher coupling efficiency with the same core size. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=b*sqrt(exp(-c*(r / d)^e)), where b∈[0.004,0.008], c∈[0.02,0.10], d∈[8,16], and e∈[6,14]. The core radius of the few-mode fiber used for mode multiplexing in the few-mode mode multiplexer is 8.0-15.0 μm; the effective refractive index of the single-mode fiber core at 1550 nm is between 1.44445 and 1.45375; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), ranging from 0.5 to 7.0 μm; the core radius of the single-mode fiber is 0.5-3.0 μm; the coupling length of each mode selection coupler is 2000-7000 μm; this mode multiplexer can achieve stable transmission of 6 different modes in the C-band (1530-1565 nm), LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The taper diameters of the six single-mode fibers are 125μm, 107μm, 86μm, 125μm, 101μm and 76μm, respectively. The coupling ratios of the six modes are all higher than 80%, the maximum loss is no more than 1.2dB, the mode purity is greater than 88%, and at 1550nm, the crosstalk between the target mode and the non-target mode at the few-mode end of the mode selection coupler is less than -22.6dB.

[0041] Implementation Example 1:

[0042] This invention provides a three-mode few-mode fiber mode multiplexer based on a mode-selective coupler. The multiplexer structure includes a broadband mode-selective coupler fabricated by polishing, a few-mode fiber for mode multiplexing, and a single-mode fiber for mode multiplexing. Compared to step-index fiber, the graded-index few-mode fiber used in this multiplexer has a more concentrated mode field distribution, allowing for a greater polishing depth with the same core size, thus achieving higher coupling efficiency. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=0.0048*sqrt(exp(-0.07*(r / 11)^4)). The core radius of the few-mode fiber used for mode multiplexing in the tri-mode mode multiplexer is 8.25 μm; the effective refractive indices of the single-mode fiber at 1550 nm are 1.45365, 1.45206, and 1.44947, respectively; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), which are 0.74 μm, 2.03 μm, and 4.98 μm, respectively; the core radii of the single-mode fiber are 2.480 μm, 2.105 μm, and 1.71 μm, respectively; the coupling lengths of the mode couplers are 2300 μm, 2048 μm, and 6945 μm, respectively; this mode multiplexer can achieve LP in the C-band (1530 nm-1565 nm). 01 LP 11 and LP 21 These three different output modes achieve stable transmission, LP 01 LP 11 and LP 21 The taper diameters of these three single-mode fibers are 125 μm, 106 μm, and 86 μm, respectively; the coupling ratios of these three modes are 96.7%, 99.5%, and 93.2%, respectively, and at 1550 nm, the few-mode output of the mode selection coupler is LP. 01 LP 11 and LP 21 The crosstalk to the non-target mode for these three modes is less than -23.0dB, less than -23.6dB, and less than -24.2dB, respectively; the losses are 0.51dB, 0.79dB, and 0.75dB, respectively.

[0043] Implementation Example 2:

[0044] This invention provides a five-mode mode multiplexer based on a mode-selective coupler. The multiplexer structure includes a broadband mode-selective coupler fabricated by polishing, a few-mode fiber for mode multiplexing, and a single-mode fiber for mode multiplexing. Compared to a step-index fiber, the graded-index few-mode fiber used in this multiplexer has a more concentrated mode field distribution, allowing for a greater polishing depth with the same core size, thus achieving higher coupling efficiency. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=0.006*sqrt(exp(-0.05*(r / 14)^8)). The core radius of the few-mode fiber used for mode multiplexing in the five-mode multiplexer is 9.0 μm; the effective refractive indices of the single-mode fiber at 1550 nm are 1.45365, 1.45206, 1.44947, 1.44795, and 1.44636, respectively; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), which are 0.77 μm, 1.97 μm, and 5.12, respectively. The fiber diameters are 3.02μm and 5.05μm; the core radii of the single-mode fibers are 2.480μm, 2.105μm, 1.710μm, 4.100μm and 3.323μm, respectively; the coupling lengths of the mode couplers are 2350μm, 2038μm, 6951μm, 4425μm and 2658μm, respectively; this mode multiplexer can achieve stable transmission of five different modes in the C-band (1530nm-1565nm), LP 01 LP 11 LP 21 LP 02 and LP 31 The taper diameters of the five single-mode fibers are 125 μm, 106 μm, 86 μm, 125 μm, and 101 μm, respectively; the coupling ratios of these three modes are 97.8%, 99.2%, 94.3%, 90.6%, and 89.5%, respectively, and at 1550 nm, the few-mode output of the mode selection coupler is LP. 01 LP 11 LP 21 LP 02 and LP 31 The crosstalk to the non-target mode for these five modes is less than -22.7dB, less than -23.2dB, less than -24.0dB, less than -23.5dB, and less than -22.6dB, respectively; and the losses are 0.82dB, 0.94dB, 0.84dB, 0.95dB, and 1.05dB, respectively.

[0045] Implementation Example 3:

[0046] This invention provides a five-mode mode multiplexer based on a mode-selective coupler. The multiplexer structure includes a broadband mode-selective coupler fabricated by polishing, a few-mode fiber for mode multiplexing, and a single-mode fiber for mode multiplexing. Compared to a step-index fiber, the graded-index few-mode fiber used in this multiplexer has a more concentrated mode field distribution, allowing for a greater polishing depth with the same core size, thus achieving higher coupling efficiency. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=0.0072*sqrt(exp(-0.08*(r / 12)^6)). The core radius of the few-mode fiber used for mode multiplexing in the five-mode multiplexer is 12.0 μm; the effective refractive indices of the single-mode fiber at 1550 nm are 1.45365, 1.45206, 1.44947, 1.44795, 1.44636, and 1.44455, respectively; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), which are 0.76 μm, 1.91 μm, 5.05 μm, and 5.5 μm, respectively. The fiber diameters are 8μm, 2.98μm, and 5.95μm; the core radii of the single-mode fibers are 2.480μm, 2.105μm, 1.710μm, 4.100μm, 3.323μm, and 2.491μm, respectively; the coupling lengths of the mode couplers are 2349μm, 2080μm, 6950μm, 4400μm, 2637μm, and 2207μm, respectively; this mode multiplexer can achieve stable transmission of six different modes in the C-band (1530nm-1565nm), LP. 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The taper diameters of the six single-mode fibers are 125 μm, 106 μm, 86 μm, 125 μm, 101 μm, and 76 μm, respectively; the coupling ratios of the four modes are 97.4%, 99.1%, 95.0%, 91.2%, 90.3%, and 98.1%, respectively, and at 1550 nm, the few-mode output of the mode selection coupler is LP. 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The crosstalk to the non-target mode for these six modes is less than -23.1dB, less than 23.5dB, less than -24.0dB, less than 23.8dB, less than -22.9dB, and less than -24.5dB, respectively; the losses are 0.62dB, 0.78dB, 0.92dB, 0.98dB, 0.79dB, and 1.07dB, respectively.

[0047] The main performance indicators of the above implementation example pattern multiplexer are shown in Table 1.

[0048] Table 1 Main performance indicators of the fiber optic waveguide in the implementation example.

[0049]

[0050]

[0051] This invention proposes a few-mode multiplexer for a graded-index fiber mode selection coupler. The multiplexer structure includes 1-6 broadband mode selection couplers fabricated using a polishing method, a graded-index six-mode few-mode fiber for mode multiplexing, and 1-6 single-mode fibers for mode multiplexing. Compared to step-index fibers, the graded-index few-mode fiber used for mode multiplexing has a more concentrated mode field distribution, allowing for a greater polishing depth with the same core size, thus achieving higher coupling efficiency. The core-cladding refractive index difference Δ of the few-mode fiber satisfies the formula Δ=b*sqrt(exp(-c*(r / d)^e)), where b∈[0.004,0.008], c∈[0.02,0.10], d∈[8,16], and e∈[6,14]. The single-mode fiber used in the mode selection coupler has an effective refractive index of 1.44445-1.45375 at a wavelength of 1550nm. The core radius of the few-mode fiber used for mode multiplexing in the mode selection coupler is 8.0-15.0μm. The remaining thickness x of the cladding to the core edge after polishing in the mode selection coupler is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a (x = Rra), ranging from 0.5-7.0μm. The coupling length of each mode selection coupler is 2000-7000μm. The diameter of the taper region corresponding to the single-mode fiber is 70.0-125.0μm. The mode multiplexer of the present invention has the characteristics of simple manufacturing process, short manufacturing cycle, low loss, high coupling efficiency and high mode purity. It can reduce the remaining thickness after polishing by using the higher refractive index profile and field strength distribution consistency in graded refractive index few-mode fiber, and efficiently achieve selective excitation of target modes to complete the function of mode conversion and selection. It is suitable for high-capacity mode multiplexing optical fiber communication systems.

[0052] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0053] The prior description of this disclosure is provided to enable any person skilled in the art to make or use it. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles defined herein can be applied to other variations without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not intended to be limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0054] The above description is merely a preferred example of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application shall be included within the scope of protection of this application.

Claims

1. A high-coupling-ratio, high-mode-purity few-mode fiber multiplexer comprising a broadband mode-selective coupler fabricated by a polishing method, characterized in that: The broadband mode selection coupler couples few-mode fiber and single-mode fiber, utilizing the core-cladding refractive index difference of the few-mode fiber. Satisfy the formula =b*sqrt(exp(-c*(r / d)^e)), where bϵ[0.004,0.008], cϵ[0.02,0.10], dϵ[8,16], eϵ[6,14], and the cross-section of the few-mode fiber in the broadband mode selection coupler is ground into a D shape.

2. The few-mode fiber multiplexer of high coupling ratio and high mode purity according to claim 1, characterized in that: In a few-mode fiber mode multiplexer, the effective refractive index of the single-mode fiber core at 1550 nm is between 1.44445 and 1.45375; the core radius of the few-mode fiber used for mode multiplexing is 8.0-15.0 μm; the remaining thickness x from the cladding to the core edge after fiber polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a, x = Rra, and ranges from 0.5 to 7.0 μm.

3. The few-mode fiber multiplexer of high coupling ratio and high mode purity according to claim 2, characterized in that: Multiple broadband mode selection couplers are connected in series, and the output of the mode selection coupler is connected to the input of the next mode selection coupler via a few-mode fiber.

4. The few-mode fiber multiplexer of high coupling ratio and high mode purity according to claim 3, characterized in that: Each mode selection coupler has a coupling length of 2000-7000 μm; this few-mode fiber multiplexer can achieve stable transmission of 6 different modes in the C-band, LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The taper diameter of the six single-mode fibers ranges from 70 to 125 μm; the coupling ratio of all six modes is higher than 80%, the maximum loss does not exceed 1.2 dB, the mode purity is greater than 88%, and at 1550 nm, the crosstalk between the target mode and the non-target mode at the output of the few-mode end of the mode selection coupler is less than -22.6 dB.

5. The few-mode fiber multiplexer with high coupling ratio and high mode purity according to claim 3, characterized in that: In the mode selection coupler, the remaining thickness of the fiber after polishing is reduced by using graded refractive index few-mode fiber. The remaining thickness x from the cladding to the core edge after polishing is defined as the difference between the fiber cladding radius R, the core radius r, and the polished thickness a, x = Rra, and the range is 0.5-7.0 μm.

6. The high-coupling-ratio, high-mode-purity few-mode fiber multiplexer of claim 4, wherein The core radius of the few-mode fiber used in the mode selection coupler is 8.0-15.0 μm; at 1530-1565 nm, LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The cone diameters corresponding to these six single-mode optical fibers are 125 μm, 107 μm, 86 μm, 125 μm, 101 μm, and 76 μm, respectively.

7. The high-coupling-ratio, high-mode-purity few-mode fiber multiplexer of claim 4, wherein: This few-mode fiber multiplexer can achieve stable transmission with six different mode outputs in the C-band (1530nm-1565nm). 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The taper diameter of the six single-mode fibers is between 70 and 125 μm; the coupling ratio of all six modes is higher than 80%, the maximum loss does not exceed 1.2 dB, the mode purity is greater than 88%, and at 1550 nm, the crosstalk between the target mode and the non-target mode at the output of the few-mode end of the mode selection coupler is less than -22.6 dB.

8. The high-coupling-ratio, high-mode-purity few-mode fiber multiplexer of claim 4, wherein: At wavelengths of 1530nm-1565nm, LP 01 The coupling ratio of the mode coupler ranges from 94.7% to 98.4%, LP 11 The coupling ratio of the mode coupler ranges from 99.1% to 99.9%, LP. 21 The coupling ratio of the mode coupler ranges from 87.6% to 95.1%, LP 02 The coupling ratio of the mode coupler ranges from 81.3% to 91.4%, LP 31 The coupling ratio of the mode coupler ranges from 89.9% to 93.55%, LP 12 The coupling ratio of the mode coupler ranges from 89.3% to 99.2%.

9. The high-coupling-ratio, high-mode-purity few-mode fiber multiplexer of claim 4, wherein: LP 01 LP 11 LP 21 LP 02 LP 31 and LP 12 The mode crosstalk of the mode selection coupler is less than -18.2dB, -23.4dB, -24.2dB, -18.5dB, -18.6dB and -19.7dB, respectively.

10. The high coupling ratio and high mode purity few-mode fiber multiplexer according to claim 4: at 1530-1565 nm, LP 01 The loss range of the mode coupler is 0.51-0.86 dB, LP. 11 The loss range of the mode coupler is 0.73-0.99 dB, LP. 21 The loss range of the mode coupler is 0.46-0.94 dB, LP. 02 The loss range of the mode coupler is 0.37-1.05 dB, LP. 31 The loss range of the mode coupler is 0.60-1.08 dB, LP. 12 The loss range of the mode coupler is 0.44–1.19 dB, LP 11 LP 21 LP 02 LP 31 and LP 12 The purity of the models were 88.1%, 88.0%, 99.3%, 96.2% and 88.7%, respectively.

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