Graded refractive index high bandwidth multimode optical fiber

Abstract

The invention discloses a high-bandwidth multimode optical fiber with a gradient refractive index. The multimode optical fiber sequentially comprises a core layer I, a core layer II, an inner cladding layer I, an inner cladding layer II, a sunken cladding layer, a transition cladding layer and an outer cladding layer from inside to outside in the radial direction of the multimode optical fiber, the refractive index profile of the core layer I and the core layer II is continuously and gradually changed, the refractive index profile of the inner cladding I is continuously and gradually changed, and the refractive index profile is a straight line or a curve. By improving the control opening degree of the core layer II GeCl4 mass flow controller, the defect of flow fluctuation or poor flow control consistency caused by multi-loop flow control is overcome, and the defect of poor valve opening degree repeatability under the low opening degree of the mass flow controller is improved; accurate control of the refractive index profile of the multi-mode optical fiber prepared under single dopant control flow control is achieved, the batch preparation consistency of the multi-mode optical fiber is high, and the bandwidth characteristic is excellent. The refractive index profile of the core layer II is adjusted more accurately, the DMD performance of the 20-23 [mu] m area is optimized, and the effective mode bandwidth EMB of the prepared multimode optical fiber at the wavelength of 953 nm is remarkably improved.

Description

Technical Field

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

[0002] Compared with traditional electrical communication, optical fiber communication has the advantages of huge transmission bandwidth, extremely low transmission loss, low cost and high fidelity. As the transmission medium of optical fiber communication, the characteristics of optical fiber have a very important influence on the transmission of optical signals. Using VCSEL lasers with lower power consumption, multimode optical fiber connection solutions with obvious advantages in energy consumption, carbon emissions and cost have been widely used in medium and short-distance optical fiber network scenarios such as large data centers, local area networks, storage networks, board connections, and chip connections. Most application scenarios of multimode optical fiber are generally relatively narrow cabinets, wiring boxes and other integrated systems, and the optical fiber will be subjected to a small bending radius. When conventional multimode optical fiber is bent with a small bending radius, the higher-order modes transmitted near the edge of the fiber core are easily leaked out, resulting in signal loss. With the continuous growth of user demand for network speed and network capacity, high-performance transmission networks have put forward higher requirements for the performance of multimode optical fiber. Among them, the effective mode bandwidth index (EMB) and the additional bending loss index are often the two performance indicators that application manufacturers focus on.

[0003] An effective method to optimize the additional loss of optical fiber bending is generally to add a depressed cladding in the low refractive index area to the optical fiber cladding to limit the leakage of high-order modes. Generally, better bending performance is obtained by optimizing the depth and width of the depressed cladding and adjusting the distance between the depressed cladding and the optical fiber core. In theory, a wider depressed cladding width, a deeper depressed cladding, and adjusting the depressed cladding to the appropriate distance from the core layer can increase the bending resistance of the optical fiber.

[0004] In order to improve the bending resistance of traditional high-bandwidth multimode optical fibers, the core layer is generally co-doped with Ge / F or co-doped with Ge / P / F, the inner cladding is doped with F or co-doped with Ge / F, and the depressed cladding is prepared by deep F doping. In optical fibers with this structure, a large viscosity difference is formed between the optical fiber core layer, the inner cladding and the depressed cladding. During the process of drawing the prepared optical fiber preform into an optical fiber, the optical fiber core layer and the inner cladding are easily affected by the drawing tension and the thermal stress caused by the viscosity difference, resulting in the distortion of the refractive index profile at the optical fiber core-cladding interface, which has an adverse effect on the higher-order modes transmitted near the edge of the core, resulting in a decrease in the bending resistance, and at the same time, it will also deteriorate the DMD performance and cause a decrease in the effective mode bandwidth of the optical fiber.

[0005] The method of optimizing the effective mode bandwidth (EMB) of optical fiber is generally to prepare the process by accurately adjusting the refractive index profile of the core layer. In theory, the smoother the refractive index profile is, the smaller the deviation from the ideal power index refractive index profile is, and the higher the effective mode bandwidth (EMB) of the prepared optical fiber is. Engineering technicians generally use GeCl4 flow compensation to accurately correct the deviation of the refractive index profile from the ideal power index profile caused by the change of gas phase chemical reaction conditions as the thickness of the deposited layer gradually increases.

[0006] The flow rate of GeCl4 is generally precisely controlled by a mass flow controller. In engineering practice, the mass flow controller is limited by the instability of low-opening flow, especially GeCl4 vapor, which is a large molecular gas. In practice, it is easy to have large fluctuations and poor repeatability in the opening range of 0~5% or even 0~10%. This brings great trouble to the compensation of GeCl4 flow, and it is difficult to achieve effective and precise compensation of the refractive index profile, especially for the refractive index distribution of the 20~25um area of ​​the optical fiber corresponding to the low opening range of GeCl4 flow.

[0007] The prior art also uses a multi-channel gas flow controller to solve the problem of poor repetition accuracy and large flow fluctuation 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, and automatically opens the second branch flow controller after reaching the target flow. This method still cannot solve the problem of large flow fluctuation and poor repetition accuracy in the low opening range of the low-range mass flow controller, and because 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, which is very easy to cause burrs and other rough points 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 that is difficult to overcome. Summary of the invention

[0008] The technical problem to be solved by the patent of the present invention is to provide a gradient refractive index high-bandwidth multimode optical fiber. The prepared multimode optical fiber preform has high batch consistency, and the drawn gradient 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, and the effective mode bandwidth EMB of the prepared gradient refractive index structure optical fiber at a wavelength of 953nm also has a significant improvement.

[0009] In order to solve the above technical problems, the technical solution adopted by the patent of the present invention is: a gradient refractive index high-bandwidth multimode optical fiber, which 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 in sequence from inside to outside along the radial direction of the multimode optical fiber; the refractive index profile power index distribution of core layer I and core layer II; the radial refractive index n (r) of the gradient refractive index high-bandwidth multimode optical fiber is expressed as: ; 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 exponential parameter of the refractive index profile distribution.

[0010] 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 layer is It is 0.9~1.2%.

[0011] Core II radius 24.5~27.5um, core layer I radius and Difference - 2~5um.

[0012] The width w1 of the inner cladding I is 0-1 um, the refractive index profile is a continuous gradient, the refractive index profile is a straight line or a curve, the width w2 of the inner cladding II is 1-1.5 um, and the relative refractive index difference between the inner cladding II and the outer cladding is It is -0.2~0.2%.

[0013] The width W3 of the depressed cladding is 3-8um, 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.

[0014] 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.

[0015] The core layer II is SiO2 glass co-doped with Cl, F and Ge, and the opening degree of the flow controller of the gas dopant GeCl4 participating in the deposition reaction of the core layer II is 5%-15%.

[0016] The opening of the GeCl4 flow controller is constant during the preparation of the core layer II; the opening of the GeCl4 flow controller increases with the number of deposited layers of the core layer II during the preparation of the core layer II; the flow rate of the Freon gas dopant in the core layer II It decreases with the number of sedimentary layers and conforms to the following function distribution: ; (1) In the formula, 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 deposition of core layer II), 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.

[0017] 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 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.

[0018] 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 function distribution: , (2) In the formula, 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.

[0019] The main beneficial effects of the present invention are mainly reflected in: By improving the control opening of the core layer II GeCl4 mass flow controller, the flow fluctuation or poor consistency of flow control caused by multi-loop flow control is avoided, the poor repeatability of the valve opening under low opening of the mass flow controller is improved, and the precise control of the refractive index profile of the prepared multimode optical fiber under the control of a single dopant control 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.

[0020] 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 area 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. By improving the concentration of each dopant in the core layer II, the viscosity of the edge of the optical fiber core layer is improved, the viscosity matching between the optical fiber core and cladding is more reasonable, and the impact on the optical fiber core layer during the optical fiber drawing process is reduced from the design perspective. 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

[0021] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0022] Figure 1 Schematic diagram of the refractive index profile of the multimode optical fiber of the present invention.

[0023] 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 deposited layers.

[0024] 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 of the present invention.

[0025] Figure 4 This is the time delay diagram of the multimode optical fiber at 850nm wavelength of the present invention.

[0026] Figure 5 This is a comparison diagram of the wavelength delay of 850nm and 953nm for the multimode optical fiber of the present invention. DETAILED DESCRIPTION

[0027] like Figure 1~Figure 5 In this paper, a graded-index high-bandwidth multimode optical fiber is proposed. Embodiment 1, The graded-index high-bandwidth multimode optical fiber mentioned in this application refers to a 50 / 125um structure multimode optical fiber used for high-speed medium- and short-distance optical fiber networks, including a core layer and a cladding layer. For a schematic diagram of the refractive index profile of the optical fiber, please refer to Figure 1 .

[0028] The refractive index profile of the optical fiber core is a parabola, and the radial refractive index n(r) conforms to the power exponential distribution, expressed as: ; Where R is the radius of the fiber core, is the central refractive index of the optical fiber core, Cladding refractive index.

[0029] The relative refractive index difference mentioned in this application refers to the deviation calculated using the following formula, which is generally recorded as a percentage value: ; 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.

[0030] The numerical aperture NA mentioned in this embodiment refers to the maximum light receiving angle (radian) of the prepared multimode optical fiber, which is calculated using the following formula: ; Where k is the correction coefficient, and the k value range is 0.91~0.97.

[0031] The optimal refractive index distribution parameters mentioned in this embodiment are , refers to the optimal refractive index profile distribution parameter with the minimum inter-modal dispersion and the maximum multimode fiber bandwidth at a certain working wavelength, and the expression is: ; (3) in It is a function related to the working wavelength and the composition of the optical fiber glass material.

[0032] The delay mentioned in this embodiment refers to the differential mode (group) delay T (DMD), and the expression between the delay and the radial coordinate of the optical fiber is: (4) 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 .

[0033] The flow compensation mentioned in this application refers to the flow compensation according to the refractive index profile of the prepared optical fiber preform rod and the 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: (5) in is the relative radial coordinate The deviation adjustment of the dopant at is the relative radial coordinate The flow rate of 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.

[0034] Embodiment 2, A multimode optical fiber with a gradient refractive index and high bandwidth, wherein the multimode optical fiber is provided with 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 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 layer center and the outer cladding layer 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 is -0.59%, and the width w3 is 4.9um; the transition cladding and the outer cladding are pure quartz glass, and the cladding radius Rc is 62.3um.

[0035] 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.

[0036] Embodiment 3, A microwave plasma vapor deposition process is used to first deposit a pure silicon transition cladding of a certain thickness on the inner wall of the substrate tube, then deposit a F-doped depressed cladding of a certain thickness, 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.

[0037] During the deposition of core layer II, the GeCl4 flow rate was kept constant at 10%. Decreases with the number of sedimentary layers (see Figure 2 S2), and conforms to the following function distribution: , ; The initial deposition opening of F flowmeter 76%, the core layer II ends deposition opening is 30%, is 1.6.

[0038] 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: , ; Where F flow controller initial deposition opening 30%, the core layer I ends deposition F flow controller opening is 5%, is 1.

[0039] In the above technical solution, a multimode optical fiber preform is obtained for RIT drawing, and a numerical aperture test (PK2314) is performed on the obtained optical fiber, and NA=0.196, which is in line with 50 / 125um multimode optical fiber; The obtained optical fiber winding 1 disk was subjected to differential mode delay test PK2550, see Figure 4 According 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, 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, DMD(Interval) = 0.054 ps / m, which meets the technical index requirements of OM4 optical fiber.

[0040] The obtained optical fiber was wound twice with a radius of 7.5 mm and subjected to a 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.

[0041] The radial refractive index n(r) of the core layer I conforms to the power exponential distribution, and the distribution power exponent is is 2.05; the radial refractive index n(r) of the core layer II conforms to the power exponential distribution, and the distribution power exponent is 2.04.

[0042] Embodiment 4, A microwave plasma vapor deposition process is used to first deposit a pure silicon transition cladding of a certain thickness on the inner wall of the substrate tube, then deposit a F-doped depressed cladding of a certain thickness, 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.

[0043] According to the technical solution, during the deposition of the core layer II, the initial opening value of the GeCl4 flow rate is 6%, which gradually increases with the increase of the deposited layers of the core layer II to 10% at the end of the core layer II.

[0044] During the deposition of 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: , ; The initial deposition opening of F flowmeter 76%, the core layer II ends deposition opening is 30%, is 1.

[0045] According to the 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 deposited layers of 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: , ; Where F flow controller initial deposition opening 30%, the core layer I ends deposition F flow controller opening is 5%, is 1.

[0046] 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 is in line with 50 / 125um multimode optical fiber; The obtained optical fiber winding 1 disk was subjected to differential mode delay test PK2550, see Figure 5According to the DMD calculation template, the effective mode bandwidth EMB at 850nm wavelength is 5613.87MHz.km, the full injection bandwidth at 850nm wavelength is 4537.12MHz.km, the full injection bandwidth at 1300nm wavelength is 635.04 MHz. km, the maximum delay value DMD(Inner)=0.066ps / m, DMD(Outer)=0.109ps / m, DMD(Interval) = 0.057ps / m; the effective mode bandwidth (EMB) at 953nm wavelength is 3143.96MHz.km, the full over-injection bandwidth (OMBc) at 953nm wavelength is 4292.1MHz.km, the maximum delay value DMD(Inner)=0.169ps / m, DMD(Outer)=0.214ps / m, DMD(Interval) = 0.147ps / m; meet the technical index requirements of OM5 optical fiber; The obtained optical fiber was wound twice with a radius of 7.5 mm and subjected to a 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 was consistent with the bending insensitive characteristic.

[0047] It can be clearly seen from the DMD delay diagram of the obtained multimode optical fiber that due to the more precise adjustment of the core layer II, the delay at 18~23um 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.

[0048] In the above technical solution, a plurality of embodiments are used to solve in detail the problem of reduced effective mode bandwidth (EMB) caused by poor consistency of refractive index profile distribution at the edge of the core layer of high multimode optical fiber in the prior art. The batch production consistency of high-bandwidth multimode optical fiber is high, which significantly improves the output ratio of high-bandwidth multimode optical fiber.

[0049] By performing segmented control on the optical fiber core layer, the delay in the 20~23um region of the 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.

[0050] The glass viscosity of the core edge and inner cladding is further optimized, the structure of the inner cladding is optimized, and the influence of viscosity difference on the optical fiber performance is reduced, so that the multimode optical fiber has good bending insensitivity. The additional bending loss is <0.2dB / km@850nm and <0.5dB / km@1300nm when winding 2 times with a radius of 7.5mm.

[0051] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations on the present invention. The embodiments and features in the embodiments in this application can be combined with each other in any way without conflict. The protection scope of the present invention should be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A graded-index high-bandwidth multimode optical fiber, characterized by: The multimode optical fiber 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 from inside to outside in radial direction; the refractive index profile power index distribution of core layer I and core layer II; the radial refractive index n(r) of graded-index high-bandwidth multimode optical fiber is expressed as: ； 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 exponential parameter of the refractive index profile distribution.

2. The gradient-index high-bandwidth multimode optical fiber according to claim 1, characterized in that: 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 layer is It is 0.9~1.2%.

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

4. The gradient-index high-bandwidth multimode optical fiber according to claim 1, characterized in that: The width w1 of the inner cladding I is 0-1 um, the refractive index profile is a continuous gradient, the refractive index profile is a straight line or a curve, the width w2 of the inner cladding II is 1-1.5 um, and the relative refractive index difference between the inner cladding II and the outer cladding is It is -0.2~0.2%.

5. The gradient-index high-bandwidth multimode optical fiber according to claim 1, characterized in that: The width W3 of the depressed cladding is 3-8um, 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.

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

8. The gradient-index high-bandwidth multimode optical fiber according to claim 1, characterized in that: The opening of the GeCl4 flow controller is constant during the preparation of the core layer II; the opening of the GeCl4 flow controller increases with the number of deposited layers of the core layer II during the preparation of the core layer II; the flow rate of the Freon gas dopant in the core layer II It decreases with the number of sedimentary layers and conforms to the following function distribution: ；（1） In the formula, 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 deposition of core layer II), 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 gradient-index high-bandwidth multimode optical fiber according to claim 1, characterized in that: 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 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.

10. The graded-index high-bandwidth multimode optical fiber according to claim 9, characterized in that: 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 function distribution: , （2） In the formula, 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.

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