Graded-index high-bandwidth multimode fiber

Abstract

A graded-index, high-bandwidth multimode optical fiber comprises, radially outward, a core layer I, a core layer II, an inner cladding layer I, an inner cladding layer II, a depressed cladding layer, a transition cladding layer, and an outer cladding layer. The refractive index profiles of core layers I and II exhibit a continuous gradient, while the refractive index profile of inner cladding I also exhibits a continuous gradient, with either a straight line or a curved line. By increasing the control aperture of the GeCl4 mass flow controller in core layer II, this method avoids the flow fluctuations and poor flow control consistency associated with multi-loop flow control. This also improves the poor valve opening repeatability of the mass flow controller at low openings, enabling precise control of the refractive index profile of the multimode optical fiber produced using a single dopant flow control mechanism. This results in high batch consistency and excellent bandwidth characteristics for the produced multimode optical fiber. By more precisely adjusting the refractive index profile of core layer II, the DMD performance in the 20-23 μm region is optimized, significantly improving the effective mode bandwidth (EMB) of the produced multimode optical fiber at a wavelength of 953 nm.

Description

Technical Field

[0001] The patent of this invention belongs to the field of optical fiber communication technology and relates to a graded refractive index high-bandwidth multimode optical fiber. Background Art

[0002] Compared to traditional electrical communications, fiber-optic communications offer advantages such as vast transmission bandwidth, extremely low transmission loss, low cost, and high fidelity. As the transmission medium for fiber-optic communications, the properties of optical fiber significantly impact the transmission of optical signals. Multimode fiber connectivity solutions, utilizing lower-power VCSEL lasers, offer significant advantages in terms of energy consumption, carbon emissions, and cost. They are widely used in medium- and short-distance fiber networks, such as large data centers, local area networks, storage networks, and board-to-board and chip-to-chip interconnects. Multimode fiber is primarily used in relatively narrow integrated systems, such as cabinets and distribution boxes, where it is subject to relatively small bend radii. When bending conventional multimode fiber within a small bend radius, higher-order modes propagating near the core edge are easily leaked, resulting in signal loss. With increasing user demand for network speed and capacity, high-performance transmission networks are placing higher demands on the performance of multimode fiber. Effective mode bandwidth (EMB) and bend-added loss are two key performance indicators that application vendors focus on.

[0003] An effective method for optimizing optical fiber bending loss is to add a depressed cladding layer (low-refractive-index region) to the fiber cladding to limit the leakage of higher-order modes. This is achieved by optimizing the depth and width of the depressed cladding and adjusting its distance from the fiber core. In theory, a wider depressed cladding width, a deeper depressed cladding, and an appropriate distance from the depressed cladding to the core can improve the fiber's bending resistance.

[0004] To improve the bending resistance of traditional high-bandwidth multimode optical fibers, the core layer is generally co-doped with Ge / F or Ge / P / F, the inner cladding is doped with F or Ge / F co-doped, and the depressed cladding is produced using a deep F-doping process. This structure creates a significant viscosity difference between the core, inner cladding, and depressed cladding. During the drawing process of the prepared optical fiber preform, the core and inner cladding are easily affected by the drawing tension and thermal stress caused by the viscosity difference. This causes the refractive index profile at the core-cladding interface to distort, adversely affecting higher-order modes propagating near the core edge, resulting in a decrease in bending resistance. This also worsens the DMD performance and reduces the effective modal bandwidth of the fiber.

[0005] Optimizing the effective modal bandwidth (EMB) of optical fibers is typically accomplished by precisely adjusting the refractive index profile of the core layer during fabrication. Theoretically, the smoother the refractive index profile and the smaller the deviation from the ideal power-index refractive index profile, the higher the effective modal bandwidth (EMB) of the resulting fiber. Engineers typically use GeCl4 flow compensation to precisely correct for deviations from the ideal power-index profile caused by changes in gas-phase chemical reaction conditions as the deposited layer thickness increases.

[0006] The GeCl4 flow rate is generally precisely controlled by a mass flow controller. In engineering practice, mass flow controllers are limited by the instability of low-opening flow, especially for GeCl4 vapor, a large molecular gas. In practice, large fluctuations and poor repeatability often occur within the 0-5% or even 0-10% opening range. This poses a great challenge to GeCl4 flow compensation, making it difficult to achieve effective and precise compensation of the refractive index profile. In particular, it has a significant impact on the refractive index distribution in the 20-25μm region of the optical fiber corresponding to the low GeCl4 flow opening range.

[0007] The existing technology also uses a multi-channel gas flow controller to solve the problem of poor repeatability and large flow fluctuations in the low opening range of the larger flow meter at the edge of the core layer by adding a lower-range gas flow control branch. The process is prepared by automatically opening the second branch flow controller after reaching the target flow. This method still cannot solve the problem of large flow fluctuations and poor repeatability in the low opening range of the low-range mass flow controller. Moreover, since more branches are introduced to participate in the control, when the second branch is opened, the switching of the new branch will cause instantaneous fluctuations in the total gas flow and pressure. It is very easy for burrs and other rough points to appear on the refractive index profile of the core layer, resulting in a decrease in DMD performance, which in turn affects the effective mode bandwidth. At the same time, the actual flow consistency calibration of different branch flow controllers is also a difficult problem to overcome. Summary of the Invention

[0008] The technical problem to be solved by the patent of this invention is to provide a graded-refractive-index high-bandwidth multimode optical fiber. The prepared multimode optical fiber preform has high batch consistency. The drawn graded-refractive-index structure optical fiber has excellent bending resistance and high bandwidth performance at a wavelength of 850nm. Furthermore, by more accurately adjusting the refractive index profile of the core layer II, the DMD performance in the 20~23um region is optimized. The effective mode bandwidth EMB of the prepared graded-refractive-index structure optical fiber at a wavelength of 953nm also has a significant improvement.

[0009] To solve the above technical problems, the technical solution adopted by the patent of the present invention is: a graded-index high-bandwidth multimode optical fiber, which includes, from the inside to the outside along the radial direction of the multimode optical fiber, a core layer I, a core layer II, an inner cladding layer I, an inner cladding layer II, a depressed cladding layer, a transition cladding layer and an outer cladding layer; the refractive index profile of the core layer I and the core layer II has a power index distribution; the radial refractive index n(r) of the graded-index high-bandwidth multimode optical fiber is expressed as:

[0010] ;

[0011] in is the relative refractive index difference, , is the central refractive index of the optical fiber core, is the refractive index of pure quartz glass, is the refractive index of the optical fiber cladding, is the radius of the optical fiber core layer, and α is the power exponent parameter of the refractive index profile distribution.

[0012] The refractive index profiles of the core layer I and the core layer II are continuously gradient, and the refractive index profiles conform to the power index distribution. The distribution power index α is 1.92-2.2, and the maximum relative refractive index difference between the core layer center and the outer cladding is It is 0.9~1.2%.

[0013] The core II radius 24.5~27.5um, core layer I radius and Difference - 2~5um.

[0014] The width w1 of the inner cladding I is 0~1um, the refractive index profile is continuously gradient, the refractive index profile is a straight line or a curve, the width w2 of the inner cladding II is 1~1.5um, and the relative refractive index difference between the inner cladding II and the outer cladding is It is -0.2~0.2%.

[0015] The width W3 of the depressed cladding is 3 to 8 μm, and the relative refractive index difference with the outer cladding is The outer cladding is made of pure quartz glass, and the cladding radius It is 61~64um.

[0016] The core layer I is SiO2 glass co-doped with Cl, F, and Ge. The core layer I performs flow compensation correction on the gas dopant GeCl4 participating in the deposition reaction to optimize its DMD performance.

[0017] The core layer II is SiO2 glass co-doped with Cl, F and Ge, and the opening of the flow controller of the gas dopant GeCl4 participating in the deposition reaction of the core layer II is 5% to 15%.

[0018] The opening of the GeCl4 flow controller during the preparation of the core layer II is constant; the opening of the GeCl4 flow controller during the preparation of the core layer II increases with the number of deposited layers of the core layer II; the flow rate of the Freon gas dopant in the core layer II is It decreases with the number of sedimentary layers and conforms to the following function distribution:

[0019] ; (1)

[0020] Where, is the initial flow rate of Freon in core layer II, is the flow rate at the boundary between core layer II and core layer I (i.e. the end of core layer II deposition), For the Number of layers Freon flow rate, is the total number of deposited layers of core layer II, is the flow index, The value range is 1~3.

[0021] The refractive index profile parameter deviation ∆α of the core layer II and the core layer I is less than 0.02; the refractive index profile distribution parameter of the core layer II is Less than or equal to the refractive index profile distribution parameter of core layer I ; The core layer II performs flow compensation correction on the dopant Freon gas flow to optimize the DMD performance of the core layer II.

[0022] The SiCl4 flow rate at the boundary of the core layer I and the SiCl4 flow rate at the final deposition value of the core layer I continuously and linearly decrease with the number of deposition reaction layers; the dopant Freon flow rate at the boundary of the core layer I and the Freon flow rate at the final deposition value of the core layer II Same, core layer I dopant Freon flow It varies with the number of sedimentary layers and conforms to the following functional distribution:

[0023] , (2)

[0024] Where, is the Freon flow rate at the center of core layer I, For the Layer Freon flow, is the total number of deposited layers of core layer I, is the flow index, The value range is 1~1.5.

[0025] The main beneficial effects of this invention are mainly reflected in:

[0026] By improving the control opening of the core layer II GeCl4 mass flow controller, the flow fluctuation or poor flow control consistency caused by multi-loop flow control is avoided, and the poor repeatability of the valve opening at low opening of the mass flow controller is improved. The precise control of the refractive index profile of the prepared multimode optical fiber under the control of a single dopant flow is achieved. The prepared multimode optical fiber has high batch consistency and excellent bandwidth characteristics. The prepared multimode optical fiber has excellent effective mode bandwidth (EMB) performance at a wavelength of 850nm.

[0027] By dividing the optical fiber core layer into two parts, core layer I and core layer II, the refractive index profile of core layer II is adjusted more accurately, and the DMD performance in the 20-23um region is optimized. Compared with the existing technology, the effective mode bandwidth EMB of the prepared multimode optical fiber at a wavelength of 953nm has a more significant improvement.

[0028] By improving the concentration of each dopant in the core layer II, the viscosity of the optical fiber core edge is improved, the viscosity matching between the optical fiber core and cladding is more reasonable, and the design reduces the impact on the optical fiber core during the optical fiber drawing process. Through the optimized transition cladding structure and sunken cladding structure, the prepared multimode optical fiber not only has a low attenuation coefficient, but also has good bending insensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Figure 1 Schematic diagram of the refractive index profile of the multimode optical fiber of the present invention.

[0031] Figure 2 This is a graph showing the relationship between the flow rate of Freon F in the core layer II and the number of deposition layers.

[0032] Figure 3 This is a graph showing the relationship between the flow rate of Freon F in the core layer I and the number of deposition layers.

[0033] Figure 4 This is the time delay diagram of the multimode optical fiber at 850nm wavelength of the present invention.

[0034] Figure 5 This is a comparison diagram of the time delay of the multimode optical fiber of the present invention at 850nm and 953nm wavelengths. DETAILED DESCRIPTION

[0035] like Figures 1 to 5 In this paper, a graded-index high-bandwidth multimode optical fiber is used.

[0036] Example 1,

[0037] The graded-index high-bandwidth multimode optical fiber mentioned in this application refers to a 50 / 125um structured multimode optical fiber used for high-speed, medium- and short-distance optical fiber networks. It consists of two parts: the core layer and the cladding. For a schematic diagram of the refractive index profile of the optical fiber, please refer to Figure 1 .

[0038] The refractive index profile of the optical fiber core is parabolic, and the radial refractive index n(r) conforms to the power exponential distribution, which is expressed as:

[0039] ;

[0040] Where R is the radius of the fiber core, is the refractive index of the optical fiber core, Cladding refractive index.

[0041] The relative refractive index difference mentioned in this application refers to the deviation calculated using the following formula, generally recorded as a percentage:

[0042] ;

[0043] in is the refractive index of pure quartz glass, therefore, for the It represents the relative refractive index difference between the optical fiber core and pure quartz glass. The calculation methods for other layers are similar.

[0044] The numerical aperture NA mentioned in this embodiment refers to the maximum light receiving angle (radian) of the prepared multimode optical fiber, and is calculated using the following formula:

[0045] ;

[0046] Where k is the correction coefficient, and the k value range is 0.91~0.97.

[0047] The optimal refractive index distribution parameters mentioned in this embodiment are , refers to the optimal refractive index profile distribution parameter with the minimum intermodal dispersion and the maximum multimode fiber bandwidth at a certain operating wavelength, and the expression is:

[0048] ;(3)

[0049] in It is a function related to the operating wavelength and the composition of the optical fiber glass material.

[0050] The delay mentioned in this embodiment refers to the differential mode (group) delay T (DMD), and the expression between it and the radial coordinate of the optical fiber is:

[0051] (4)

[0052] in, is the multimode fiber group refractive index at the operating wavelength, is the relative radial coordinate of the optical fiber Refractive index profile distribution parameters at .

[0053] The flow compensation mentioned in this application refers to the flow compensation of the optical fiber preform according to the refractive index profile of the prepared optical fiber preform. The ideal refractive index profile deviation of the distribution parameters is adjusted to the corresponding dopant (GeCl4 or F) flow rate, which is calculated as follows:

[0054] (5)

[0055] in is the relative radial coordinate The deviation adjustment amount of the dopant at is the relative radial coordinate The flow rate of the dopant at is the relative radial coordinate The relative refractive index difference, is the relative radial coordinate The refractive index profile of the optical fiber preform at The relative refractive index difference of the ideal refractive index profile under the distribution parameters.

[0056] Example 2,

[0057] A graded-index high-bandwidth multimode optical fiber, wherein the multimode optical fiber is provided with core layer I, core layer II, inner cladding layer I, inner cladding layer II, depressed cladding layer, transition cladding layer and outer cladding layer in sequence from the inside to the outside along the radial direction of the multimode optical fiber. Figure 1 As shown, the core layer I and the core layer II are glass core layers co-doped with GeO2 and F, and the relative refractive index difference between the core center and the outer cladding is 0.97%, core layer I radius 22.6um, core layer II radius The inner cladding layer I is a glass transition layer co-doped with GeO2 and F, and the relative refractive index difference of the inner cladding layer I is The initial refractive index difference is the same as that of the core layer II, which is -0.045% in this embodiment, and the width w1 is 0.5um; the inner cladding layer II is an F-doped transition layer, and the relative refractive index difference of the inner cladding layer II is is 0.003%, and the width w2 is 1.3um; the sunken cladding is an F-doped glass cladding, and its relative refractive index difference The transition cladding and outer cladding are made of pure quartz glass, and the cladding radius Rc is 62.3 um.

[0058] The radial refractive index n(r) of the core layer I and the core layer II conforms to a power exponential distribution, and the distribution power exponent α is 2.05.

[0059] Example 3,

[0060] A microwave plasma vapor deposition process is used to first deposit a certain thickness of pure silicon transition cladding on the inner wall of the substrate tube, then deposit a certain thickness of F-doped depressed cladding, and then deposit inner cladding II and inner cladding I. The thickness of inner cladding II is equivalent to the thickness of optical fiber cladding II, which is 1.3um, and the thickness of inner cladding I is equivalent to the thickness of optical fiber cladding II, which is 0.5um. After the deposition of inner cladding I is completed, a core layer II of a certain thickness is deposited.

[0061] During the deposition of core layer II, the GeCl4 flow rate is kept constant at 10%. Decreases with the number of sedimentary layers (see Figure 2 S2), and conforms to the following function distribution:

[0062] , ;

[0063] The initial deposition opening of F flowmeter 76%, the core layer II deposition is completed is 30%, is 1.6.

[0064] After the core layer I is deposited, during the deposition of the core layer II, the GeCl4 flow rate starts at 10% and increases with the number of layers deposited in the core layer I. Dopant Freon gas flow rate Decreases with the number of sedimentary layers (see Figure 3 As shown), and conforms to the following function distribution:

[0065] , ;

[0066] The initial deposition opening of the F flow controller 30%, the core layer I is finished depositing F flow controller opening is 5%, is 1.

[0067] In the above technical solution, a multimode optical fiber preform is obtained and subjected to RIT drawing. The obtained optical fiber is subjected to a numerical aperture test (PK2314), and the NA is 0.196, which meets the requirements of 50 / 125um multimode optical fiber.

[0068] The differential mode delay test PK2550 was performed on the obtained optical fiber winding 1 reel, see Figure 4According to the DMD calculation template, the effective mode bandwidth EMB at 850nm wavelength is 5835.97MHz.km, the full injection bandwidth at 850nm wavelength is 4815.87MHz.km, and the full injection bandwidth at 1300nm wavelength is 842.01 MHz.km. The maximum delay value DMD(Inner) = 0.047 ps / m, DMD(Outer) = 0.085 ps / m, and DMD(Interval) = 0.054 ps / m, meeting the technical indicator requirements of OM4 fiber.

[0069] The obtained optical fiber was wound twice with a radius of 7.5 mm and subjected to bending additional loss test using OTDR8800. The additional loss at 850 nm wavelength was 0.026 dB, and the additional loss at 1300 nm wavelength was 0.323 dB, which was consistent with the bending insensitive characteristic.

[0070] The radial refractive index n(r) of the core layer I conforms to the power index distribution. is 2.05; the radial refractive index n(r) of the core layer II conforms to the power exponential distribution, and the power exponent of the distribution is is 2.04.

[0071] Example 4,

[0072] A microwave plasma vapor deposition process is used to first deposit a certain thickness of pure silicon transition cladding on the inner wall of the substrate tube, then deposit a certain thickness of F-doped depressed cladding, and then deposit inner cladding II and inner cladding I. The thickness of inner cladding II is equivalent to the thickness of optical fiber cladding II, which is 1.3um, and the thickness of inner cladding I is equivalent to the thickness of optical fiber cladding II, which is 0.5um. After the deposition of inner cladding I is completed, a core layer II of a certain thickness is deposited.

[0073] According to this technical solution, during the deposition of the core layer II, the GeCl4 flow opening starts at 6%, and gradually increases with the increase of the core layer II deposition layer to 10% at the end of the core layer II.

[0074] During the deposition of the core layer II, the dopant Freon gas flow rate Decreases with the number of sedimentary layers (see Figure 2 S1), and conforms to the following function distribution:

[0075] , ;

[0076] The initial deposition opening of F flowmeter 76%, the core layer II deposition is completed is 30%, is 1.

[0077] According to this technical solution, after the core layer I is deposited, during the deposition of the core layer II, the GeCl4 flow rate is initially opened at 10%, and increases with the number of layers deposited in the core layer I. Dopant Freon gas flow rate Decreases with the number of sedimentary layers (see Figure 3 As shown), and conforms to the following function distribution:

[0078] , ;

[0079] The initial deposition opening of the F flow controller 30%, the core layer I is finished depositing F flow controller opening 5%, is 1.

[0080] The multimode optical fiber preform obtained by the implemented technical solution was subjected to RIT drawing, and the obtained optical fiber was subjected to numerical aperture test (PK2314), NA=0.194, which meets the requirements of 50 / 125um multimode optical fiber;

[0081] The obtained optical fiber winding 1 reel was tested for differential mode delay using PK2550, see Figure 5 According to the DMD calculation template, the effective modal bandwidth (EMB) at 850nm is 5613.87MHz.km, the full injection bandwidth at 850nm is 4537.12MHz.km, and the full injection bandwidth at 1300nm is 635.04MHz.km. The maximum delay values ​​DMD(Inner) are 0.066ps / m, DMD(Outer) are 0.109ps / m, and DMD(Interval) is 0.057ps / m. The effective modal bandwidth (EMB) at 953nm is 3143.96MHz.km, the full over-injection bandwidth (OMBc) at 953nm is 4292.1MHz.km, the maximum delay values ​​DMD(Inner) are 0.169ps / m, DMD(Outer) are 0.214ps / m, and DMD(Interval) are 0.147ps / m. These meet the technical requirements of OM5 fiber.

[0082] The obtained optical fiber was wound twice with a radius of 7.5 mm and subjected to bending additional loss test using OTDR8800. The additional loss at 850 nm wavelength was 0.028 dB, and the additional loss at 1300 nm wavelength was 0.334 dB, which met the bending insensitive characteristic.

[0083] The DMD delay diagram of the obtained multimode optical fiber clearly shows that due to the more precise adjustment of core layer II, the delay at 18~23μm in the radial direction of the optical fiber is accelerated, as shown in Formula 4, which significantly improves the effective mode bandwidth (EMB) at a wavelength of 953nm, giving it high bandwidth characteristics over a wider wavelength range.

[0084] In the above technical solution, multiple embodiments are used to solve in detail the problem of reduced effective mode bandwidth (EMB) caused by poor consistency in the refractive index profile distribution at the edge of the core layer of high-bandwidth multimode optical fiber in the existing technology. The batch production consistency of high-bandwidth multimode optical fiber is high, which significantly improves the output ratio of high-bandwidth multimode optical fiber.

[0085] By performing segmented control on the fiber core layer, the delay in the 20~23um region of the fiber core can be more accurately controlled, thereby beneficially improving the effective mode bandwidth of the multimode optical fiber at longer wavelengths. The prepared multimode optical fiber has an effective mode bandwidth of ≥4700MHz•km at a wavelength of 850nm, and an effective mode bandwidth of ≥2470MHz•km at a wavelength of 953nm.

[0086] The glass viscosity of the core edge and inner cladding is further optimized, and the structure of the inner cladding is optimized, which reduces the impact of viscosity difference on optical fiber performance. This makes the multimode optical fiber have good bending insensitivity. The additional bending loss under 7.5mm radius and 2 turns is <0.2dB / km@850nm and <0.5dB / km@1300nm.

[0087] The above embodiments are only preferred technical solutions of the present invention and should not be considered as limitations of the present invention. The embodiments and features in the embodiments of this application may be combined arbitrarily unless they conflict. The scope of protection of the present invention shall be the technical solutions described in the claims, including equivalent replacement solutions of the technical features in the technical solutions described in the claims. In other words, equivalent replacement improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A graded-index, high-bandwidth multimode optical fiber, characterized by: From the inside to the outside along the radial direction of the multimode optical fiber, it includes core layer I, core layer II, inner cladding layer I, inner cladding layer II, depressed cladding layer, transition cladding layer and outer cladding layer. The refractive index profile power index distribution of core layer I and core layer II is as follows: The radial refractive index n(r) of the graded-index high-bandwidth multimode optical fiber is expressed as: ； in is the relative refractive index difference, , is the refractive index of the optical fiber core, is the refractive index of pure quartz glass, is the refractive index of the optical fiber cladding, is the fiber core radius, α is the refractive index profile power index parameter; The refractive index profile distribution power index parameter deviation between the core layer II and the core layer I α is less than 0.02; the refractive index profile distribution power index parameter of core layer II Less than or equal to the refractive index profile distribution power index parameter of core layer I ; The core layer II performs flow compensation correction on the dopant Freon gas flow to optimize the DMD performance of the core layer II.

2. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The refractive index profiles of the core layer I and the core layer II are continuously gradient, and the refractive index profiles conform to the power index distribution. The power index parameter α of the refractive index profile distribution is 1.92-2.2, and the maximum relative refractive index difference between the core layer center and the outer cladding is It is 0.9~1.2%.

3. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The core II radius 24.5~27.5um, core layer I radius and The difference R2-R1 is 2~5um.

4. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The width w1 of the inner cladding I is 0~1um, and its refractive index profile is continuously gradient; the width w2 of the inner cladding II is 1~1.5um, and the relative refractive index difference between the inner cladding II and the outer cladding is It is -0.2~0.2%.

5. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The width W3 of the depressed cladding is 3 to 8 μm, and the relative refractive index difference with the outer cladding is -0.5%~-0.75%; the outer cladding is pure quartz glass, the cladding radius It is 61~64um.

6. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The core layer I is SiO2 glass co-doped with Cl, F, and Ge. The core layer I performs flow compensation correction on the gas dopant GeCl4 participating in the deposition reaction to optimize its DMD performance.

7. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The core layer II is SiO2 glass co-doped with Cl, F and Ge, and the opening of the flow controller of the gas dopant GeCl4 participating in the deposition reaction of the core layer II is 5% to 15%.

8. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The opening of the GeCl4 flow controller is constant during the preparation of the core layer II, or the opening of the GeCl4 flow controller increases with the number of core layer II deposition layers during the preparation of the core layer II; the flow rate of the Freon gas dopant of the core layer II is It decreases with the number of sedimentary layers and conforms to the following function distribution: ；（1） Where, is the initial flow rate of Freon in core layer II, is the boundary flow between core layer II and core layer I, For the Number of layers Freon flow rate, is the total number of deposited layers of core layer II, is the flow index, The value range is 1~3.

9. The graded-index, high-bandwidth multimode optical fiber according to claim 1, wherein: The SiCl4 flow rate at the boundary of the core layer I and the SiCl4 flow rate at the final deposition value of the core layer I continuously and linearly decrease with the number of deposition reaction layers; the dopant Freon flow rate at the boundary of the core layer I and the Freon flow rate at the final deposition value of the core layer II Same, core layer I dopant Freon flow It varies with the number of sedimentary layers and conforms to the following functional distribution: , （2） Where, is the Freon flow rate at the center of core layer I, For the Layer Freon flow, is the boundary flow between core layer II and core layer I, is the total number of deposited layers of core layer I, is the flow index, The value range is 1~1.5.

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