A low-loss multimode optical fiber and its fabrication method

Abstract

This invention discloses a low-loss multimode optical fiber and its fabrication method, belonging to the field of optical fiber technology. The optical fiber comprises, radially from the inside out, a graded-index core layer, an inner cladding layer, and an outer cladding layer. The inner cladding layer includes an aluminum-doped silica diffusion barrier layer, and the outer cladding layer is a sodium-doped silica glass layer. The residual compressive stress at the center of the core layer is -30 MPa to -45 MPa, and the hypothetical temperature is 1200°C to 1260°C. The fabrication method includes: depositing a core rod containing the barrier layer using PCVD; soaking synthetic quartz powder in sodium acetate followed by APS spraying to form a sodium-doped outer cladding layer; drawing the fiber and then forcibly cooling it with helium. This invention reconstructs the viscosity gradient of the core-cladding layer and the residual stress field of the fiber drawing layer by slightly doping the outer cladding layer with sodium. This reduces the hypothetical temperature of the core layer without changing its composition or introducing sodium into the core layer, while completely avoiding hydrogen sensitivity and concentration fluctuation scattering. It exhibits good process compatibility and is suitable for data centers and short-wavelength optical communication systems.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, specifically relating to a low-loss multimode optical fiber and its fabrication method. Background Technology

[0002] Multimode optical fiber is widely used in data centers, local area networks, and short-distance optical interconnects. Its transmission loss is mainly caused by Rayleigh scattering and absorption by non-bridged oxygen hole center defects. The intensity of Rayleigh scattering is positively correlated with the hypothetical temperature of the glass. Existing methods to reduce loss mainly involve optimizing the doping composition of the core layer or inner cladding and improving the drawing and annealing process. However, these methods can easily affect the fiber dispersion, bending performance, or reduce production efficiency. While directly doping the core layer with alkali metals can reduce the viscosity of the glass, it can introduce problems such as concentration fluctuation scattering, increased hydrogen sensitivity, and redshift of the ultraviolet absorption edge. At the same time, existing cladding doping methods are only used to adjust optical or processing performance and are difficult to indirectly control the hypothetical temperature and defect level of the core layer without changing the core layer composition. Summary of the Invention

[0003] To address the problems existing in the prior art, this invention provides a low-loss multimode optical fiber and its fabrication method, which reduces transmission loss without changing the core composition, while retaining the original optical and mechanical properties of the multimode optical fiber.

[0004] The technical solution adopted in this invention is as follows: In a first aspect, the present invention provides a low-loss multimode optical fiber, which includes a graded refractive index core layer, an inner cladding layer and an outer cladding layer in a radial direction from the inside to the outside, wherein the graded refractive index core layer is a graded refractive index distribution core layer made of silica-based glass. The inner cladding layer includes an aluminum-doped silicon dioxide diffusion barrier layer, and the outer cladding layer is a sodium-doped silicon dioxide glass layer, wherein the sodium doping concentration in the outer cladding layer is 10 ppm to 80 ppm by weight.

[0005] In conjunction with the first aspect, the present invention provides a first embodiment of the first aspect, wherein the gradient refractive index core layer is GeO2-doped silica glass, the refractive index profile is distributed in a power-law manner, the power-law exponent α is 1.9-2.2, and the relative refractive index difference Δ1 between the core layer center and pure quartz is 0.9%-1.2%; The low-loss multimode fiber has a 50 / 125μm structure and operates at wavelengths of 850nm, 953nm, or 1300nm.

[0006] In conjunction with the first aspect, the present invention provides a second embodiment of the first aspect, wherein the total thickness of the inner cladding layer is 12.5 μm, and it is composed of an inner layer of pure silicon dioxide and an outer layer of aluminum-doped silicon dioxide diffusion barrier layer, wherein the thickness of the aluminum-doped silicon dioxide diffusion barrier layer is 2.5 μm, and the aluminum doping concentration is 500 ppm-1000 ppm.

[0007] In conjunction with the first aspect, the present invention provides a third embodiment of the first aspect, wherein the sodium doping concentration in the outer cladding layer is 10ppm-40ppm by weight; the sodium concentration in the graded refractive index core layer is less than 0.1ppm; and the peak sodium concentration at the interface between the graded refractive index core layer and the inner cladding layer is 1ppm-5ppm.

[0008] In conjunction with the first aspect, the present invention provides a fourth embodiment of the first aspect, wherein the residual stress in the central region of the gradient refractive index core layer is a compressive stress of -30MPa to -45MPa, and the hypothetical temperature at the center of the core layer is 1200°C to 1260°C.

[0009] In a second aspect, the present invention provides a preparation method for preparing the low-loss multimode optical fiber described in any one of the above-mentioned methods, comprising the following steps: S1. A plasma chemical vapor deposition process is used to sequentially deposit an aluminum-doped silicon dioxide diffusion barrier layer, a pure silicon dioxide inner cladding layer, and a GeO2-doped gradient refractive index core layer on the inner wall of the substrate tube. After melting and stretching, a core rod is obtained. S2. High-purity silica powder is mixed and soaked with sodium acetate aqueous solution, dried, and then calcined at 900°C-1000°C to obtain sodium-doped silica powder. The sodium-doped silica powder is deposited on the outer surface of the core rod using an external plasma spraying process to form a sodium-doped outer cladding layer, thereby obtaining an optical fiber preform. S3. The optical fiber preform is drawn into fibers at 1950°C-2050°C, and then forced to cool with helium gas at a cooling rate ≥10. 4 °C / s.

[0010] It is worth noting that in the PCVD process, the above deposition sequence proceeds from the inner wall of the substrate tube outwards. After fusion shrinkage, the optical fiber radially consists of the core layer, the pure silicon dioxide inner layer, the aluminum-doped diffusion barrier layer, and the outer cladding layer, from the inside out.

[0011] In conjunction with the second aspect, the present invention provides a first embodiment of the second aspect, wherein in step S1, the parameters of the plasma chemical vapor deposition process are: microwave power 3kW-4kW, substrate tube internal pressure 1mbar-1.5mbar, deposition temperature 1000°C-1200°C, and number of deposition layers 2000-2500.

[0012] In conjunction with the second aspect, the present invention provides a second embodiment of the second aspect, wherein in step S2, the soaking time is 12-24 hours, the calcination time is 8-12 hours, and the parameters of the external plasma spraying process are: plasma power 35kW-45kW, powder feeding rate 8g / min-12g / min, mandrel rotation speed 40rpm-50rpm, and reciprocating movement speed 3mm / s-4mm / s; and the radial deposition thickness of the sodium-doped outer coating is 3mm-5mm.

[0013] In conjunction with the second aspect, the present invention provides a third embodiment of the second aspect, wherein in step S3, the wire drawing speed is 400m / min-600m / min and the wire drawing tension is 40g-60g; preferably, the wire drawing speed is 500m / min-550m / min and the wire drawing tension is 45g-55g.

[0014] In conjunction with the second aspect, the present invention provides a fourth embodiment of the second aspect, wherein in step S2, the high-purity silica powder is synthetic quartz powder, the initial aluminum residue is not greater than 0.1 ppm, and the initial sodium residue is not greater than 0.1 ppm; the sodium doping concentration in the outer coating is controlled to be 10 ppm-80 ppm by adjusting the concentration of the sodium acetate aqueous solution.

[0015] It should be noted that the purpose of the helium forced cooling after drawing described in this invention is to rapidly reduce the temperature of the outer cladding and core-cladding interface region to below approximately 900°C, the temperature threshold for significant diffusion of sodium ions in silica glass, within an extremely short time and length after drawing. This effectively confines sodium elements to the outer cladding and interface region, preventing them from entering the core transport region. The reduction in the hypothetical core temperature is not directly caused by this rapid cooling process itself, but rather stems from the core-cladding viscosity gradient and residual stress field reconstructed due to the decrease in outer cladding viscosity during the high-temperature section of drawing (1950°C–2050°C).

[0016] This stress field applies continuous compressive stress to the core layer at approximately 1100°C–1200°C in the glass transition region, suppressing excessive thermal relaxation of the core layer network structure and allowing it to form a more ordered glass network structure before rapid cooling. Therefore, rapid cooling and hypothetical temperature reduction are independent effects at different temperature ranges and stages in terms of physical mechanism.

[0017] The beneficial effects of this invention are as follows: This invention confines sodium to the outer cladding layer and the core-cladding interface region by performing trace sodium doping on the outer cladding layer and setting a diffusion barrier layer in the inner cladding layer, thereby preventing sodium from entering the core transport region. This invention utilizes the viscosity adjustment of the outer cladding layer to reconstruct the residual stress field during the wire drawing and cooling process, thereby promoting the ordering of the core glass structure and reducing the hypothetical temperature of the core layer. This invention suppresses the formation of non-bridged oxygen hole center defects in optical fibers, reducing absorption loss in the corresponding wavelength band. This invention maintains the original composition and refractive index profile of the core layer, preserving the mode bandwidth and bending performance of the multimode fiber. This invention is highly compatible with existing multimode optical fiber fabrication processes and can achieve stable mass production without significantly altering the main process. Attached Figure Description

[0018] Figure 1 A cross-sectional schematic diagram of the refractive index in an embodiment of the present invention; Figure 2 This is a schematic diagram comparing the absorption peaks of the undoped Comparative Example 1 and the Sample 1 of Example 1 doped with 10 ppm Na in the embodiments of the present invention. Detailed Implementation

[0019] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application. Example 1:

[0022] This embodiment provides a low-loss multimode optical fiber and its fabrication method. Figure 1 This is a schematic diagram of the refractive index profile of the low-loss multimode optical fiber described in an embodiment of the present invention. (Refer to...) Figure 1 The optical fiber consists of a gradient refractive index core, an inner cladding, and an outer cladding in the radial direction from the inside to the outside. It adopts a standard 50 / 125μm structure and operates at wavelengths of 850nm, 953nm, and 1300nm.

[0023] The specific structural parameters are as follows: The gradient refractive index core layer is GeO2-doped silica glass with a power-law refractive index profile. The power-law index α is 2.0. The relative refractive index difference Δ1 between the core center and pure quartz is 1.0%. The core diameter is 50 μm, which means the core radius is 25 μm.

[0024] The inner cladding has a total thickness of 12.5 μm, consisting of a 10 μm thick pure silicon dioxide inner layer and a 2.5 μm thick aluminum-doped silicon dioxide diffusion barrier layer with an aluminum doping concentration of 750 ppm. The outer cladding is sodium-doped silicon dioxide glass with an outer diameter of 125 μm.

[0025] The preparation method includes the following steps: S1. Preparation of a core rod containing a barrier layer: The barrier layer is sequentially deposited on the inner wall of a high-purity quartz substrate tube using plasma chemical vapor deposition. Specifically: using AlCl3 as a precursor, controlling the AlCl3 flow rate at 0.75 sccm, and depositing an aluminum-doped silicon dioxide diffusion barrier layer under the conditions of deposition temperature 1100°C, microwave power 3.5 kW, and substrate tube internal pressure 1.2 mbar; Subsequently, under the same gas pressure and power, the process was switched to depositing a pure silicon dioxide inner cladding layer using pure SiCl4. Finally, by controlling the GeCl4 flow rate using a gradient, a GeO2-doped gradient refractive index core layer was deposited, with a total of 2300 layers deposited to ensure that the refractive index profile accuracy error was ≤±0.01%. After deposition, the core was melt-shrunk and stretched to obtain a 20mm diameter mandrel.

[0026] S2. Preparation of sodium-doped outer cladding preform: Deposition of sodium-doped outer cladding using external plasma spraying process; Specifically: Synthetic quartz powder was selected as the outer coating substrate. The initial aluminum residue of the powder was ≤0.1ppm and the initial sodium residue was ≤0.1ppm. The synthetic quartz powder was immersed in sodium acetate aqueous solution. The solution concentration was linearly calibrated according to the target sodium doping concentration of 40ppm. The immersion time was 18 hours, during which the solution was continuously stirred to ensure uniform adsorption of sodium ions.

[0027] After removing the powder, it was dried in an oven at 120°C for 12 hours, and then transferred to a high-temperature furnace and calcined at 950°C for 10 hours to completely remove moisture and organic components. Thermogravimetric analysis verified that the residual carbonate content of the calcined powder was below the detection limit (<0.01wt%), and there was no carbon residue.

[0028] The sodium-doped powder was deposited onto the outer surface of the core rod using a plasma spray gun. The plasma power was 40 kW, the powder feed rate was 10 g / min, the core rod rotation speed was 45 rpm, and the reciprocating speed was 3.5 mm / s. The deposition time was controlled to achieve a radial thickness of 4 mm for the outer cladding, resulting in a complete optical fiber preform with a diameter of approximately 28 mm.

[0029] S3. Controllable fiber drawing and rapid cooling: High-precision fiber drawing tower is used for fiber drawing.

[0030] Specifically: the fiber drawing temperature was controlled at 2000°C, the drawing speed at 520 m / min, and the drawing tension at 50 g, ensuring that the fiber diameter uniformity was ≤ ±0.5 μm. Immediately after drawing, the fiber was rapidly cooled using a helium forced cooling tube; the measured cooling rate was 1.2 × 10⁻⁶. 4 °C / s.

[0031] Structural and performance verification: The radial distribution of sodium in the cross-section of the optical fiber was quantitatively analyzed by secondary ion mass spectrometry. The results are shown in Table 1 below. Table 1 Middle of outer layer 15~30 38.5 Inner side of outer layer 30~35 12.3 outer side of the barrier layer 35~37.5 3.5 (peak value) Inside the barrier layer 37.5~40 0.8 Pure silica inner cladding 40~50 <0.1 Core region 50~62.5 (center) <0.1 (below the detection limit) The data above show that the sodium diffusion depth is 1.8 μm, which is less than the barrier layer thickness of 2.5 μm, and no sodium was detected in the core layer region. The peak sodium concentration at the interface between the graded refractive index core layer and the inner cladding is 3.5 ppm, which meets the requirement of 1 ppm to 5 ppm; the sodium concentration in the graded refractive index core layer is less than 0.1 ppm.

[0032] The viscosity of the outer cladding glass in this embodiment within the drawing temperature range was tested using a rotating high-temperature viscometer. At 2000°C, the logarithmic viscosity (logη) of the sodium-doped outer cladding glass in this embodiment was 4.2, while the logarithmic viscosity (logη) of the undoped comparative outer cladding glass was 4.6. This demonstrates that sodium doping in the outer cladding does indeed reduce the viscosity of the glass in the high-temperature region, providing a physical basis for the reconstruction of the viscosity gradient of the core-cladding layer.

[0033] The radial residual stress distribution of the optical fiber was tested using the refractive index near-field scanning method. The residual stress in the central region of the graded refractive index core layer was a compressive stress of -38 MPa, which meets the requirement of a range of -30 MPa to -45 MPa.

[0034] The hypothetical temperature at the core layer was determined using micro Raman spectroscopy. The temperature was measured at a depth of 520 cm⁻¹. -1 The full width at half maximum (FWHM) of the Si-O-Si bending vibration peak yields a hypothetical core temperature of 1215°C, which meets the range requirement of 1200°C to 1260°C. This is 105°C lower than the undoped reference (Comparative Example 1, 1320°C, mentioned later).

[0035] The attenuation spectrum of the optical fiber in the 600nm~1700nm band was tested using the truncation method. The effective mode bandwidth (EMB) was tested using a PK2500 system, and the additional loss after two turns around a 7.5mm radius fiber was measured using a bending loss meter. The optical performance results are shown in Table 2 below: Table 2 Attenuation coefficient 850nm 2.14 dB / km Attenuation coefficient 1300nm 0.54dB / km NBOHC absorption 650nm 0.02dB / km Effective mode bandwidth 850nm EMB 5860MHz·km Bending additional loss 850nm, 7.5mm radius × 2 circles 0.15dB Reliability and mechanical performance testing: The sample was aged in a hydrogen atmosphere at 85°C and 1 atm for 168 hours, and the change in the OH absorption peak at 1383 nm was measured, with an increase of 0.03 dB / km. After aging for an extended period of 1000 hours, the increase was 0.05 dB / km. The tensile strength was measured using an optical fiber strength tester, with an average value of 5.2 GPa.

[0036] The sample was aged in an environment of 85°C / 85%RH for 1000 hours, and the decay change at 850nm was 0.02dB / km.

[0037] After 100 cycles of temperature cycling from -40°C to +85°C, the attenuation at 850nm was 0.01dB / km.

[0038] It should be noted that the absolute reduction in attenuation coefficient at 850nm in this embodiment compared to the undoped Comparative Example 1 is approximately 0.07 dB / km, and the relative reduction is approximately 3%. This reduction is limited because the 850nm band is already close to the theoretical limit of Rayleigh scattering in standard 50 / 125μm multimode fiber, and the space for further reduction is limited by the intrinsic Rayleigh scattering coefficient of the quartz glass.

[0039] The core of this embodiment lies in compressing the Rayleigh scattering component to near its theoretical limit by lowering the hypothetical core temperature, while completely eliminating NBOHC defect absorption and ensuring that the mode bandwidth and bending performance do not deteriorate. Further reduction of the 850nm attenuation would require improvements in raw material purity and core dehydration processes, which are beyond the scope of this invention.

[0040] Mechanism verification: The NBOHC defect concentration was quantitatively determined using electron paramagnetic resonance spectroscopy. In this embodiment, the NBOHC defect concentration of the sample was 8.3 × 10⁻⁶. 13 spins / cm 3 To distinguish the contributions of the hypothetical temperature decrease and the interfacial sodium chemical interaction, a horizontal comparison is made with the following comparative examples, as shown in Table 3 below: Table 3 Comparative Example 1 (undoped) follows. 1320 0 0 <![CDATA[2.1×10 15 ]]> 0.02 Comparative Example 4 (with a barrier layer but no sodium) follows. 1315 0 0 <![CDATA[2.0×10 15 ]]> 0.02 Comparative Example 2 (core layer doped with 40 ppm Na) is described later. 1230 N / A 40 <![CDATA[9.5×10 13 ]]> 1.2 This embodiment 1215 3.5 <0.1 <![CDATA[8.3×10 13 ]]> 0.03 Analysis of the above data shows that, compared with Comparative Example 1, Comparative Example 4, which only added a barrier layer without adding sodium, showed almost no change in NBOHC concentration and hypothetical temperature, proving that the barrier layer itself does not have the function of reducing hypothetical temperature or inhibiting NBOHC.

[0041] While Comparative Example 2, which directly dopes the core layer with sodium, reduces NBOHC, it dramatically increases hydrogen sensitivity. This embodiment, without introducing sodium into the core layer, achieves an NBOHC suppression effect similar to that of direct sodium doping, while completely avoiding the hydrogen sensitivity problem.

[0042] Figure 2This is a schematic diagram comparing the absorption attenuation of Comparative Example 1 and this embodiment near the 650nm band.

[0043] As shown in the table below, Comparative Example 1 exhibits a significant NBOHC absorption peak at 650 nm, with an attenuation value of 0.41 dB / km; while this embodiment almost eliminates this absorption peak in the same wavelength band, with an attenuation value of only 0.02 dB / km, a reduction of over 95%. See Table 4 below for details: Table 4 600 0.38 0.02 620 0.4 0.02 640 0.41 0.02 650 0.41 0.02 660 0.4 0.02 680 0.38 0.02 Example 2:

[0044] The only difference between this embodiment and Example 1 is that the concentration of the sodium acetate aqueous solution is adjusted in step S2 to make the sodium doping concentration in the outer coating layer 10 ppm (by weight). All other preparation process parameters are exactly the same as in Example 1.

[0045] The same test method as in Example 1 was used for verification, and the results are as follows: The radial distribution of sodium was tested using SIMS. The sodium diffusion depth was 1.5 μm, the sodium concentration in the core region was less than 0.1 ppm, and the peak sodium concentration at the interface between the graded refractive index core and the inner cladding was 1.2 ppm.

[0046] Using the RNF method, the residual stress in the central region of the graded refractive index core layer is a compressive stress of -32 MPa.

[0047] The core was tested using micro Raman spectroscopy, with the hypothetical temperature at the center of the core layer set at 1260°C.

[0048] Optical performance test results: attenuation of 2.15dB / km at 850nm, attenuation of 0.55dB / km at 1300nm, absorption of 0.03dB / km at 650nm, EMB of 5820MHz·km at 850nm, and bending-related loss of 0.16dB at 850nm.

[0049] After 168 hours of hydrogen aging, the increase in OH absorption at 1383 nm was 0.03 dB / km; the average tensile strength was 5.1 GPa. Example 3:

[0050] The only difference between this embodiment and Example 1 is that the concentration of the sodium acetate aqueous solution is adjusted in step S2 to make the sodium doping concentration in the outer coating layer 80 ppm (by weight). All other preparation process parameters are exactly the same as in Example 1.

[0051] The same test method as in Example 1 was used for verification, and the results are as follows: The radial distribution of sodium was measured using SIMS. The sodium diffusion depth was 2.0 μm, the sodium concentration in the core region was less than 0.1 ppm, and the peak sodium concentration at the interface between the graded refractive index core and the inner cladding was 4.8 ppm.

[0052] Using the RNF method, the residual stress in the central region of the graded refractive index core is a compressive stress of -42 MPa.

[0053] The core was tested using micro Raman spectroscopy, with the hypothetical temperature at the center of the core layer set at 1205°C.

[0054] Optical performance test results: attenuation of 2.16dB / km at 850nm, attenuation of 0.56dB / km at 1300nm, absorption of 0.04dB / km at 650nm, EMB of 5790MHz·km at 850nm, and bending-related loss of 0.17dB at 850nm.

[0055] After 168 hours of hydrogen aging, the increase in OH absorption at 1383 nm was 0.04 dB / km; the average tensile strength was 5.2 GPa. Example 4:

[0056] This embodiment is used to verify the process window for drawing temperature and cooling rate. Based on the 40ppm sodium doping scheme of Example 1, the drawing temperature and cooling rate were varied to test the relationship between process stability and fiber performance.

[0057] Variation 4a (drawing temperature 1950°C): The drawing temperature was adjusted to 1950°C, the drawing speed to 500 m / min, the drawing tension to 48 g, and the cooling rate was maintained at 1.2 × 10⁻⁶. 4 °C / s. The measured fiber diameter uniformity was ±0.6μm, and the continuous drawing breakage rate was 0.9%. Optical performance: attenuation of 2.15dB / km at 850nm, absorption of 0.02dB / km at 650nm, and EMB of 5820MHz·km at 850nm.

[0058] Variation 4b (drawing temperature 2050°C): The drawing temperature was adjusted to 2050°C, the drawing speed to 540 m / min, the drawing tension to 52 g, and the cooling rate was maintained at 1.2 × 10⁻⁶. 4 °C / s. The measured fiber diameter uniformity was ±0.5μm, and the continuous drawing breakage rate was 0.5%. Optical performance: attenuation of 2.14dB / km at 850nm, absorption of 0.02dB / km at 650nm, and EMB of 5850MHz·km at 850nm.

[0059] Variation 4c (cooling rate 5 × 10) 3°C / s): Maintain the wire drawing temperature at 2000°C, and adjust the cooling rate to 5×10 by reducing the flow rate of the helium cooling pipe. 3 °C / s. SIMS testing showed that the sodium diffusion depth increased to 3.2 μm, and the sodium concentration in the core region was detected to be 0.5 ppm. Optical performance: 850 nm attenuation 2.19 dB / km, 650 nm absorption 0.07 dB / km, 850 nm EMB 5430 MHz·km. After 168 hours of hydrogen aging, the increase in OH absorption at 1383 nm was 0.25 dB / km. These results indicate that a cooling rate below 10 °C / s... 4 At °C / s, sodium diffusion control fails, and fiber performance deteriorates significantly.

[0060] Variation 4d (cooling rate 2×10) 4 °C / s): Maintain the drawing temperature at 2000°C, and adjust the cooling rate to 2×10 by increasing the helium flow rate. 4 °C / s. Using SIMS testing, the sodium diffusion depth was reduced to 1.2 μm, and the sodium concentration in the core region was below 0.1 ppm. Optical performance: attenuation at 850 nm 2.14 dB / km, absorption at 650 nm 0.02 dB / km, 850 nm EMB 5860 MHz·km.

[0061] Based on the above variations, it can be seen that the wire drawing temperature is in the range of 1950°C to 2050°C, and the cooling rate is ≥10. 4 At °C / s, optimal optical performance and sodium diffusion control can be achieved while ensuring process stability.

[0062] Comparative Example 1 The only difference between this comparative example and Example 1 is that undoped synthetic quartz powder (without sodium acetate soaking treatment) is used in step S2, and the outer coating does not contain sodium. All other preparation process parameters are exactly the same as in Example 1.

[0063] The same test method as in Example 1 was used for verification, and the results are as follows: The residual stress at the core center was measured using the RNF method, which was a compressive stress of -5 MPa.

[0064] The core was tested using micro Raman spectroscopy, with the hypothetical temperature at the center being 1320°C.

[0065] Optical performance test results: attenuation at 850nm is 2.21dB / km, attenuation at 1300nm is 0.62dB / km, absorption at 650nm is 0.41dB / km, EMB at 850nm is 5605MHz·km, and bending-related loss at 850nm is 0.16dB.

[0066] Electron paramagnetic resonance spectroscopy was used to determine the NBOHC defect concentration to be 2.1 × 10⁻⁶. 15 spins / cm 3 .

[0067] Comparative Example 2 This comparative example is used to illustrate the side effects of directly doping the core layer with sodium. The difference from Example 1 is that: in step S1, when depositing the gradient refractive index core layer, sodium acetate vapor is introduced as a sodium source to make the sodium doping concentration in the core layer reach 40 ppm (by weight); in step S2, undoped quartz powder is used to prepare the outer cladding layer.

[0068] The same test method as in Example 1 was used for verification, and the results are as follows: The hypothetical temperature at the core layer is 1230°C, the attenuation at 850nm is 2.17dB / km, the absorption at 650nm is 0.05dB / km, the EMB at 850nm is 5210MHz·km, and the additional bending loss at 850nm is 0.21dB.

[0069] The sample was aged in an 85°C, 1 atm hydrogen environment for 168 hours, and the increase in the 1383nm OH absorption peak was as high as 1.20dB / km.

[0070] Electron paramagnetic resonance spectroscopy was used to determine the NBOHC defect concentration to be 9.5 × 10⁻⁶. 13 spins / cm 3 .

[0071] This comparative example shows that although direct sodium doping in the core layer can reduce the hypothetical temperature and NBOHC concentration, it will severely degrade hydrogen sensitivity and mode bandwidth, making it unsuitable for industrial application.

[0072] Comparative Example 3 This comparative example illustrates the consequences of not having a diffusion barrier layer. The only difference from Example 1 is that no aluminum-doped silicon dioxide diffusion barrier layer is deposited in step S1; the inner cladding is entirely pure silicon dioxide, and the total thickness remains 12.5 μm. All other fabrication process parameters are identical to those in Example 1.

[0073] SIMS testing showed that the sodium diffusion depth was 5.5 μm and the sodium concentration in the core region was 2.1 ppm.

[0074] Optical performance test results: attenuation at 850nm is 2.19dB / km, absorption at 650nm is 0.07dB / km, EMB at 850nm is 5430MHz·km, and bending loss at 850nm is 0.18dB.

[0075] The sample was aged in an 85°C, 1 atm hydrogen atmosphere for 168 hours, and the increase in the 1383 nm OH absorption peak was 0.85 dB / km.

[0076] This comparative example shows that in the absence of a diffusion barrier layer, sodium diffuses into the core layer in large quantities, leading to deterioration of optical performance and increased hydrogen sensitivity.

[0077] Comparative Example 4 This comparative example illustrates that the diffusion barrier layer itself does not have the function of reducing losses. The only difference from Example 1 is that the aluminum-doped silicon dioxide diffusion barrier layer is retained in step S1, but the outer cladding layer is prepared using undoped quartz powder in step S2.

[0078] The residual stress at the core center was measured using the RNF method and was found to be -8 MPa compressive stress.

[0079] The hypothetical temperature at the core center was determined using micro Raman spectroscopy, with a value of 1315°C.

[0080] Optical performance test results: attenuation at 850nm is 2.20dB / km, absorption at 650nm is 0.39dB / km, EMB at 850nm is 5620MHz·km, and bending loss at 850nm is 0.16dB.

[0081] Electron paramagnetic resonance spectroscopy was used to determine the NBOHC defect concentration to be 2.0 × 10⁻⁶. 15 spins / cm 3 .

[0082] This comparative example shows that setting an aluminum-doped diffusion barrier layer alone, without combining it with sodium-doped outer cladding, has almost no effect on the hypothetical core temperature and NBOHC defects.

[0083] Batch consistency verification Using the fabrication process of Example 1, three independent batches were continuously produced for verification. Each batch produced 100km of optical fiber, and the batch consistency of attenuation coefficient at 850nm, absorption at 650nm, and hypothetical core temperature was tested. The results are shown in Table 5 below: Table 5 Batch 1 2.14 0.012 0.02 0.003 1216 4 Batch 2 2.15 0.014 0.02 0.004 1218 5 Batch 3 2.13 0.011 0.03 0.005 1214 3 The standard deviation of the 850nm attenuation coefficient for three independent batches is ≤0.015dB / km, and the standard deviation of the hypothetical core temperature is ≤5°C, proving that the process of this invention has good repeatability and batch consistency.

[0084] Comparative Example 5 This comparative example uses the industry-standard core layer chlorine doping loss reduction process: during the core layer deposition in step S1, Cl2 is introduced as a dopant to make the core layer chlorine concentration 500ppm, and other process parameters are exactly the same as in Example 1.

[0085] Test results: The hypothetical temperature at the core layer is 1270°C, the attenuation at 850nm is 2.18dB / km, the absorption at 650nm is 0.38dB / km, and the EMB at 850nm is 5520MHz·km.

[0086] After 168 hours of hydrogen aging, the increase in OH absorption at 1383 nm was 0.12 dB / km.

[0087] The wire drawing speed needs to be reduced to 300m / min to ensure sufficient annealing, which reduces production efficiency by 40%.

[0088] Conclusion: While existing chlorine doping processes can reduce some attenuation, they cannot suppress 650nm NBOHC absorption and worsen mode bandwidth and hydrogen sensitivity, while significantly reducing production efficiency. The optical fiber in this embodiment outperforms existing technologies in all four dimensions: attenuation, bandwidth, reliability, and production efficiency.

[0089] Comparative Example 6 This comparative example uses an extended drawing and annealing process: after drawing in step S3, a 10-meter-long annealing furnace is added, the annealing temperature is 1100°C, the drawing speed is reduced to 200m / min, and other process parameters are exactly the same as those of Comparative Example 1 (undoped).

[0090] Test results: The hypothetical temperature at the core is 1260°C, with a decay of 2.19 dB / km at 850 nm and an absorption of 0.35 dB / km at 650 nm.

[0091] Production efficiency is reduced by 60%, and the additional loss due to fiber bending increases to 0.25dB (7.5mm radius × 2 turns), which fails to meet the requirement of bending insensitivity.

[0092] Conclusion: Extended annealing process has limited loss reduction effect and seriously sacrifices production efficiency and bending performance. The optical fiber in this embodiment achieves better loss reduction effect without changing production efficiency.

[0093] Based on the test data from the above embodiments and comparative examples, the present invention achieves the following technical effects by performing trace sodium doping on the outer cladding layer and combining it with an inner cladding diffusion barrier layer and a rapid cooling process: Without altering the core layer's chemical composition and without sodium entering the core layer (core layer sodium concentration <0.1ppm), the hypothetical core layer temperature was successfully reduced by more than 100°C, thereby reducing the 850nm attenuation from 2.21dB / km to 2.14dB / km. The attenuation at 850nm in Example 1 was better than that in Comparative Example 2, proving that the indirect control path actually achieved better optical performance than the direct core-doped layer, an effect that was unexpected. The NBOHC absorption at 650 nm decreased from 0.41 dB / km to 0.02 dB / km, a reduction of more than 95%. This inhibition effect was mainly caused by the synergistic effect of the chemical action of trace sodium at the interface and the hypothetical decrease in core temperature, rather than a single factor. While achieving the aforementioned loss reduction effect, it fully retains the original mode bandwidth and bending performance of multimode fiber, and its hydrogen sensitivity and mechanical strength are comparable to those of conventional fiber, without any side effects.

[0094] It should be noted that, under the premise that the sodium concentration in the core layer of Example 1 is less than 0.1 ppm, the hypothetical temperature of the core layer is even lower than that of Comparative Example 2, which directly does not contain 40 ppm sodium in the core layer, and the NBOHC concentration is comparable to that of Comparative Example 2.

[0095] This counterintuitive effect stems from the fact that while direct sodium doping in Comparative Example 2 reduced the glass viscosity, it also altered the coordination environment of GeO2 and Na2O in the SiO2 network, introducing additional network defect sites and partially offsetting the structural ordering benefits from the hypothetical temperature reduction. In contrast, Example 1, through the reconstructed compressive stress field of sodium doping in the cladding layer, promotes the densification of the core glass network purely through a thermodynamic-mechanical coupling path without altering the core chemical composition and network coordination structure, avoiding the chemical side effects of alkali metals directly entering the core network. Therefore, the indirect control path achieves structural ordering effects comparable to or even slightly superior to direct sodium doping while maintaining the chemical purity of the core layer. Example 5:

[0096] The only difference between this embodiment and Example 1 is that the aluminum-doped silicon dioxide diffusion barrier layer is replaced with a fluorine-doped silicon dioxide barrier layer, with a fluorine doping concentration of 1000 ppm and a thickness of 2.5 μm. All other preparation process parameters are exactly the same as in Example 1.

[0097] Test results: SIMS testing showed that the sodium diffusion depth was 1.9 μm, and the sodium concentration in the core layer was <0.1 ppm, indicating a blocking effect comparable to that of the aluminum-doped layer.

[0098] The residual stress at the core center is -37 MPa, the hypothetical temperature is 1218°C, the attenuation at 850 nm is 2.14 dB / km, and the absorption at 650 nm is 0.02 dB / km. The performance is consistent with that of Example 1.

[0099] Conclusion: Fluorine-doped silica can also serve as an effective sodium diffusion barrier layer, demonstrating that barrier layer design is versatile and not limited to aluminum-doped materials. Example 6:

[0100] High-power transmission tests were conducted on the optical fibers of Example 1 and Comparative Example 1 using an 850nm, 10W high-power VCSEL laser, with a test length of 100m.

[0101] Test results: Comparative Example 1 (undoped): When the input power is 10W, obvious thermal damage occurs at the output end of the optical fiber, and the attenuation increases by 0.5dB / km.

[0102] Example 1 (40ppm sodium): At an input power of 10W, there was no thermal damage and the increase in attenuation was <0.01dB / km.

[0103] Mechanism analysis: In this embodiment, the fiber core layer is under compressive stress, which improves the glass's resistance to thermal shock and laser damage threshold. Example 7:

[0104] This embodiment is used to solve the technical problem of controlling the axial uniformity of sodium doping in external plasma spraying process. Based on the 40ppm sodium doping scheme of Example 1, the powder pretreatment and spraying process are optimized.

[0105] The specific preparation method is as follows: In step S2, the synthesized quartz powder is first sieved by air classifier to control the particle size D50 to 20±2μm. Then, the classified powder is mixed with an aqueous sodium acetate solution and placed in an ultrasonic dispersion tank. It is ultrasonically dispersed at 40kHz for 30 minutes with continuous stirring to ensure uniform adsorption of sodium ions on the powder surface. After soaking for 18 hours, the powder is removed and dried in a 120°C oven for 12 hours, followed by calcination at 950°C for 10 hours.

[0106] When depositing the sodium-doped outer coating using an external plasma spraying process, an online X-ray fluorescence monitoring device is added to scan the sodium content every 1 mm of deposition along the mandrel axis. When the sodium concentration in a certain section of the axis deviates from the target value by ±10%, the powder feeding rate of that section is dynamically adjusted. After spraying, the preform is rotated and kept at 1400°C for 2 hours to homogenize the sodium distribution using surface tension, and then naturally cooled to room temperature.

[0107] The remaining steps S1, S3 and process parameters are exactly the same as in Example 1.

[0108] Verification results: Five equidistant points along the axis of the preform were sampled, and the sodium content was tested by ICP-OES. The results were 39.2 ppm, 40.5 ppm, 38.8 ppm, 41.2 ppm, and 39.6 ppm, respectively, with a standard deviation of 0.9 ppm and an axial non-uniformity of << ±3%.

[0109] The uniformity of fiber diameter after drawing is ±0.3μm (better than ±0.5μm in Example 1). Axial testing of 850nm EMB was performed at five equidistant points: 5820, 5910, 5780, 5860, and 5890MHz·km, with a fluctuation range of ±65MHz·km and a standard deviation of 48MHz·km.

[0110] Conclusion: By using powder classification, ultrasonic dispersion, online monitoring, and rotary homogenization annealing, the axial sodium concentration fluctuation can be controlled within ±3%, which significantly improves the uniformity of fiber diameter and the stability of mode bandwidth, and solves the problem of axial performance fluctuation caused by uneven powder adsorption in mass production of APS spraying process.

[0111] This invention is not limited to the optional embodiments described above, and anyone can derive other various forms of products based on the inspiration of this invention. The specific embodiments described above should not be construed as limiting the scope of protection of this invention; the scope of protection of this invention should be determined by the claims, and the specification can be used to interpret the claims.

Claims

1. A low-loss multimode optical fiber, comprising, from the inside to the outside in the radial direction, a graded-index core, an inner cladding, and an outer cladding, characterized in that: The gradient refractive index core layer is a gradient refractive index distribution core layer made of silica-based glass. The inner cladding layer includes an aluminum-doped silicon dioxide diffusion barrier layer, and the outer cladding layer is a sodium-doped silicon dioxide glass layer, wherein the sodium doping concentration in the outer cladding layer is 10 ppm to 80 ppm by weight. The inner cladding consists of an inner layer of pure silicon dioxide and an outer layer of aluminum-doped silicon dioxide diffusion barrier layer.

2. The low-loss multimode optical fiber according to claim 1, characterized in that: The gradient refractive index core layer is GeO2-doped silica glass with a power-law refractive index profile, the power-law exponent α being 1.9-2.2, and the relative refractive index difference Δ1 between the core layer center and pure quartz being 0.9%-1.2%. The low-loss multimode fiber has a 50 / 125μm structure and operates at wavelengths of 850nm, 953nm, or 1300nm.

3. The low-loss multimode optical fiber according to claim 1, characterized in that: The total thickness of the inner cladding layer is 12.5 μm, the thickness of the aluminum-doped silicon dioxide diffusion barrier layer is 2.5 μm, and the aluminum doping concentration is 500 ppm-1000 ppm.

4. The low-loss multimode optical fiber according to claim 1, characterized in that: The sodium doping concentration in the outer cladding layer is 10ppm-40ppm by weight; the sodium concentration in the graded refractive index core layer is less than 0.1ppm; and the peak sodium concentration at the interface between the graded refractive index core layer and the inner cladding layer is 1ppm-5ppm.

5. A low-loss multimode optical fiber according to claim 1, characterized in that: The residual stress in the central region of the gradient refractive index core layer is a compressive stress of -30MPa to 45MPa, and the hypothetical temperature at the center of the core layer is 1200°C to 1260°C.

6. A preparation method, characterized in that, The method for preparing a low-loss multimode optical fiber according to any one of claims 1-5 comprises the following steps: S1. A plasma chemical vapor deposition process is used to sequentially deposit an aluminum-doped silicon dioxide diffusion barrier layer, a pure silicon dioxide inner cladding layer, and a GeO2-doped gradient refractive index core layer on the inner wall of the substrate tube. After melting and stretching, a core rod is obtained. S2. High-purity silica powder is mixed and soaked with sodium acetate aqueous solution, dried, and then calcined at 900°C-1000°C to obtain sodium-doped silica powder. The sodium-doped silica powder is deposited on the outer surface of the core rod using an external plasma spraying process to form a sodium-doped outer cladding layer, thereby obtaining an optical fiber preform. S3. The optical fiber preform is drawn into fibers at 1950°C-2050°C, and then forced to cool with helium gas at a cooling rate ≥10. 4 °C / s.

7. The preparation method according to claim 6, characterized in that: In step S1, the parameters of the plasma chemical vapor deposition process are: microwave power 3kW-4kW, substrate tube internal pressure 1mbar-1.5mbar, deposition temperature 1000°C-1200°C, and number of deposition layers 2000-2500.

8. The preparation method according to claim 6, characterized in that: In step S2, the soaking time is 12-24 hours, and the calcination time is 8-12 hours; the parameters of the external plasma spraying process are: plasma power 35kW-45kW, powder feeding rate 8g / min-12g / min, mandrel rotation speed 40rpm-50rpm, and reciprocating movement speed 3mm / s-4mm / s; the radial deposition thickness of the sodium-doped outer coating is 3mm-5mm.

9. The preparation method according to claim 6, characterized in that: In step S3, the wire drawing speed is 400m / min-600m / min and the wire drawing tension is 40g-60g.

10. The preparation method according to claim 6, characterized in that: In step S2, the high-purity silica powder is synthetic quartz powder, with an initial aluminum residue of no more than 0.1 ppm and an initial sodium residue of no more than 0.1 ppm; the sodium doping concentration in the outer cladding is controlled to be 10 ppm-80 ppm by adjusting the concentration of the sodium acetate aqueous solution.

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