Ultralow-loss single-mode optical fiber and preparation method thereof

Abstract

The invention relates to an ultralow-loss single-mode optical fiber and a preparation method thereof, the ultralow-loss single-mode optical fiber comprises a core layer and cladding layers, the radius R1 of the core layer is 6-7.5 microns, the relative refractive index difference delta n1 is 0.06-0.12%, the core layer is doped with alkali metal, the cladding layers comprise a transition layer, an inner cladding layer and an outer cladding layer, the transition layer is a pure silica glass layer, and the inner cladding layer is a pure silica glass layer. The radial single-side thickness of the transition layer is 0.5-6 [mu] m, the radius R2 of the inner cladding layer is 11-19 [mu] m, the relative refractive index difference delta n2 is-0.35 to-0.20%, the radius R3 of the outer cladding layer is 62.5 [mu] m, and the relative refractive index difference delta n3 is-0.20 to-0.14%. The pure silicon dioxide transition layer is arranged, so that alkali metal ions in the core layer can be effectively prevented from being migrated and diffused outwards in the subsequent high-temperature processing process, and the function target of reducing the virtual temperature can be achieved. The nitrogen element is doped into the outer cladding, so that the viscosity of the glass of the outer cladding can be improved, remarkable and controllable radial pressure stress can be generated on the core layer and the inner cladding in the optical fiber drawing and cooling process, and finally, the transmission loss of the optical fiber is remarkably reduced.

Description

Technical Field

[0001] This invention relates to an ultra-low loss single-mode optical fiber and its fabrication method, belonging to the field of optical communication technology. Background Technology

[0002] Transmission loss in optical fibers is a key performance indicator determining the signal transmission distance and repeaterless amplification range in optical communication systems. In silica-based single-mode fibers, especially within the 1550nm communication window, Rayleigh scattering, an intrinsic loss caused by fluctuations in the microstructure and composition of the glass network, is the primary factor limiting the theoretical loss limit of optical fibers. Therefore, effectively reducing Rayleigh scattering has become a crucial aspect of developing ultra-low-loss optical fibers.

[0003] To reduce Rayleigh scattering, those skilled in the art generally adopt the following two main technical approaches: First, reduce the "virtual temperature" of the fiber core. The virtual temperature of the fiber glass represents the temperature at which its glass network structure is "frozen". The higher the virtual temperature, the greater the structural fluctuations of the glass network, and the higher the Rayleigh scattering loss. By doping the core glass with alkali metals (such as Na, K, Rb, Cs, etc.), the viscosity of the glass network can be effectively reduced, making it easier for the structure to relax during the fiber drawing and cooling process, thereby forming the glass network at a lower temperature, i.e., obtaining a lower virtual temperature, and thus fundamentally reducing Rayleigh scattering. However, this method has a significant drawback: alkali metal ions have extremely high diffusivity during the sintering of the fiber preform and high-temperature drawing process. They diffuse from the high-concentration core to the inner cladding and even the outer cladding, leading to two adverse consequences: (1) the concentration of alkali metals in the core is reduced, which greatly reduces the effect of reducing the virtual temperature; (2) the alkali metal diffusion region forms a concentration gradient, causing refractive index distribution distortion and introducing additional scattering centers, which may increase the additional loss of the fiber. Second, apply compressive stress to the fiber. Both theory and experiments have demonstrated that applying appropriate compressive stress to the fiber core can compress the glass mesh, reduce its microstructure density fluctuations, and thus suppress Rayleigh scattering. Traditional stress application methods typically rely on the difference in the coefficients of thermal expansion (CTE) between the core and cladding. However, in pure silica fibers, it is very difficult to introduce significant compressive stress through the difference in CTE. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an ultra-low loss single-mode optical fiber and its preparation method in view of the shortcomings of the prior art. It optimizes the performance of the optical fiber by improving the cladding structure and material.

[0005] The optical fiber technology solution adopted by this invention to solve the above-mentioned problems is as follows: It includes a core layer and a cladding layer, characterized in that the core layer radius R1 is 6–7.5 μm, the relative refractive index difference Δn1 is 0.06–0.12%, and the core layer is doped with an alkali metal. The cladding layer includes a transition layer, an inner cladding layer, and an outer cladding layer. The transition layer is a pure silica glass layer with a radial thickness of 0.5–6 μm on one side. The inner cladding layer radius R2 is 11–19 μm, the relative refractive index difference Δn2 is -0.35–-0.20%, and the outer cladding layer radius R3 is 62.5 μm, the relative refractive index difference Δn3 is -0.20–-0.14%.

[0006] According to the above scheme, the pure silica glass layer of the transition layer is a high-purity pure silica layer (SiO2), whose OH... - The content is less than 1 ppm, and the content of metal impurities is less than one part per billion (ppb level).

[0007] According to the above scheme, the core layer is a silicon dioxide glass layer co-doped with one or more of germanium, chlorine, and bromine.

[0008] According to the above scheme, the alkali metal doping concentration in the core layer is 100 to 5000 ppm, preferably 300 to 4000 ppm, more preferably 400 to 3000 ppm, and even more preferably 500 to 1000 ppm.

[0009] According to the above scheme, the alkali metal source compound is an alkali metal halide, and the halogens include F, Cl, Br, I, and At. The alkali metal halide is any combination of the two, such as NaF, NaCl, NaBr, KCl, KBr, etc.

[0010] According to the above scheme, the radial thickness of the transition layer is 2 to 4 μm on one side.

[0011] According to the above scheme, the relative refractive index difference of the inner cladding layer increases, that is, the relative refractive index difference increases with the increase of the diameter.

[0012] According to the above scheme, the outer cladding layer is a fluorine-nitrogen co-doped silica glass layer, wherein the nitrogen doping concentration is 100 to 2000 ppm, preferably 300 to 1500 ppm, and more preferably 500 to 1000 ppm.

[0013] The technical solution of the optical fiber fabrication method of the present invention is as follows: An alkali metal-doped core layer and a pure silicon dioxide transition layer are sequentially deposited using chemical vapor deposition (CVD), or an alkali metal-doped core layer is deposited within a pure silicon dioxide liner. In an atmosphere containing Cl2 or He, the porous core layer and transition layer are sintered and condensed into a transparent glass core rod at a temperature of 1700℃-1900℃. Deposit the inner cladding layer on the outside of the glass core rod. An outer cladding layer is deposited outside the inner cladding layer. During the deposition of the outer cladding layer, a nitrogen-containing reactive gas (such as NO, N2O, or NH3) is introduced as a reaction source while doping with fluorine. By controlling the process parameters, nitrogen is incorporated into the SiO2 matrix of the outer cladding layer, thus producing an optical fiber preform with an outer cladding layer containing both fluorine and nitrogen. Finally, the optical fiber preform is drawn into optical fiber at a drawing furnace temperature of 2000℃-2200℃. This temperature range ensures that the alkali metal does not diffuse excessively, the transition layer function is effective, and the outer cladding stress is formed smoothly.

[0014] According to the above scheme, the outer cladding layer is a fluorine-nitrogen co-doped silica glass layer, wherein the nitrogen doping concentration is 100 to 2000 ppm, preferably 300 to 1500 ppm, and more preferably 500 to 1000 ppm.

[0015] According to the above scheme, the pure silica glass layer of the transition layer is a high-purity pure silica layer (SiO2), whose OH... - The content is less than 1 ppm, and the content of metal impurities is less than one part per billion.

[0016] According to the above scheme, the core layer and transition layer are deposited using MCVD or PCVD processes, and the inner cladding layer and outer cladding layer are deposited using OVD processes.

[0017] The beneficial effects of this invention are as follows: 1. By setting a pure silica transition layer as a diffusion barrier layer, its high purity and chemical stability can effectively block the migration and diffusion of alkali metal ions in the core layer during subsequent high-temperature processing (such as sintering and fiber drawing), achieving chemical confinement of the core layer. The alkali metal content in the core layer can achieve the "reduction of virtual temperature" function, ensuring that its virtual temperature reduction effect is fully utilized. 2. The addition of nitrogen to the cladding layer significantly increases the viscosity of the cladding glass. During fiber drawing, the high-viscosity cladding layer mainly bears the drawing tension, enabling it to generate significant and controllable radial compressive stress on the core layer and inner cladding layer during fiber drawing and cooling. This prevents the inner cladding layer and core layer glass network from breaking under tensile stress, reduces density fluctuations, and thus further reduces Rayleigh scattering loss. At the same time, it can suppress alkali metal diffusion during the drawing process, ensuring the alkali metal concentration in the core layer, and ultimately significantly reducing fiber transmission loss. 3. Through the reasonable setting of the fiber waveguide structure and the effects brought about by the above-mentioned material changes, the fiber of this invention can achieve ultra-low attenuation. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the radial structure of an embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram of the refractive index profile according to an embodiment of the present invention.

[0020] Figure 3 This is a schematic diagram of the refractive index profile of another embodiment of the present invention.

[0021] Figure 4 This is a schematic diagram of the fiber stress distribution according to an embodiment of the present invention.

[0022] Figure 5 This is a schematic diagram of fiber stress distribution according to another embodiment of the present invention.

[0023] Figure 6 This is a schematic diagram of the fiber stress distribution according to the third embodiment of the present invention.

[0024] Figure 7 This is a flowchart of one embodiment of the preparation method of the present invention. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0026] An ultra-low loss optical fiber based on transition layer and nitrogen doping stress modulation includes a core layer and a cladding layer. The core layer has a radius of R1 and a relative refractive index difference of Δn1. The core layer is doped with an alkali metal. The cladding layer includes a transition layer, an inner cladding layer, and an outer cladding layer. The transition layer is a pure silica glass layer with a radial thickness of 0.5–4 μm on one side. The inner cladding layer has a radius of R2 and a relative refractive index difference of Δn2. The outer cladding layer has a radius of R3 of 62.5 μm and a relative refractive index difference of Δn3.

[0027] The preparation method includes sequentially depositing an alkali metal-doped core layer and a pure silica transition layer through chemical vapor deposition, or depositing an alkali metal-doped core layer in a pure silica liner. In an atmosphere containing Cl2 or He, the porous core layer and transition layer are sintered and condensed into a transparent glass core rod at a temperature of 1700℃-1900℃, forming the first master rod. An inner cladding layer is deposited on the outside of the glass core rod, and an outer cladding layer is deposited on the outside of the inner cladding layer. During the deposition of the outer cladding layer, a nitrogen-containing reactive gas (such as NO, N2O or NH3) is introduced as a reaction source while doping with fluorine. By controlling the process parameters, nitrogen is incorporated into the SiO2 matrix of the outer cladding layer to form an optical fiber preform containing fluorine and nitrogen elements, forming the second master rod. Finally, the optical fiber preform is drawn into an optical fiber at a drawing furnace temperature of 2000℃-2200℃ and coated with a resin coating layer.

[0028] Table 1 lists the refractive index profile parameters in the preferred embodiments of the present invention, and Table 2 corresponds to the doping element content of the optical fiber.

[0029] Table 1. Refractive index profile parameters in the embodiments of the present invention

[0030]

[0031] Table 2 Elemental doping parameters in this invention

[0032]

[0033] Table 3. Fiber Optic Parameters in the Embodiments of the Invention

[0034]

Claims

1. An ultra-low loss single-mode optical fiber, comprising a core and a cladding, characterized in that... The core layer has a radius R1 of 6–7.5 μm and a relative refractive index difference Δn1 of 0.06–0.12%. The core layer is doped with an alkali metal. The cladding includes a transition layer, an inner cladding layer, and an outer cladding layer. The transition layer is a pure silica glass layer with a radial thickness of 0.5–6 μm per side. The inner cladding layer has a radius R2 of 11–19 μm and a relative refractive index difference Δn2 of -0.35–-0.20%. The outer cladding layer has a radius R3 of 62.5 μm and a relative refractive index difference Δn3 of -0.20–-0.14%.

2. The ultra-low loss single-mode optical fiber according to claim 1, characterized in that... The transition layer is a high-purity silica glass layer with OH groups. - The content is less than 1 ppm, and the content of metal impurities is less than one part per billion.

3. The ultra-low loss single-mode optical fiber according to claim 1 or 2, characterized in that... The core layer is a silicon dioxide glass layer co-doped with one or more of germanium, chlorine, and bromine.

4. The ultra-low loss single-mode optical fiber according to claim 3, characterized in that... The alkali metal doping concentration in the core layer is 100 to 5000 ppm.

5. The ultra-low loss single-mode optical fiber according to claim 4, characterized in that... The alkali metal source compound is an alkali metal halide, and the halogens include F, Cl, Br, I, and At.

6. The ultra-low loss single-mode optical fiber according to claim 1 or 2, characterized in that... The radial thickness of the transition layer is 1–4 μm on one side.

7. The ultra-low loss single-mode optical fiber according to claim 1 or 2, characterized in that... The relative refractive index difference of the inner cladding is increasing, that is, the relative refractive index difference increases with the increase of the diameter.

8. The ultra-low loss single-mode optical fiber according to claim 1 or 2, characterized in that... The outer cladding layer is a fluorine-nitrogen co-doped silica glass layer, wherein the nitrogen doping concentration is 100 to 2000 ppm.

9. A method for fabricating any of the ultra-low loss single-mode optical fibers according to claims 1-8, characterized in that... An alkali metal-doped core layer and a pure silicon dioxide transition layer are sequentially deposited using chemical vapor deposition (CVD) technology. Depositing an alkali metal-doped core layer in a pure silica liner. In an atmosphere containing Cl2 or He, the porous core and transition layers are sintered and condensed at a temperature of 1700℃-1900℃. Transparent glass core rod An inner cladding layer is deposited on the outside of a glass core rod. An outer cladding layer is deposited outside the inner cladding layer. During the deposition of the outer cladding layer, a nitrogen-containing reactive gas is introduced simultaneously with fluorine doping. The reaction source incorporates nitrogen into the SiO2 matrix of the outer cladding layer by controlling process parameters, thus producing an optical fiber preform with an outer cladding layer containing fluorine and nitrogen. Finally, the optical fiber preform is drawn into optical fiber at a drawing furnace temperature of 2000℃-2200℃, and this temperature range is maintained. This ensures that alkali metals do not diffuse excessively, the transition layer functions effectively, and the stress in the outer cladding layer is successfully formed.

10. The method for fabricating ultra-low loss single-mode optical fiber according to claim 9, characterized in that... The outer cladding layer is a fluorine-nitrogen co-doped silica glass layer, wherein the nitrogen doping concentration is 100 to 2000 ppm.

11. The method for fabricating ultra-low loss single-mode optical fiber according to claim 9, characterized in that... The transition layer is a high-purity silica glass layer with OH groups. - The content is less than 1 ppm, and the content of metal impurities is less than one part per billion.

12. The method for fabricating ultra-low loss single-mode optical fiber according to claim 9, characterized in that... The core and transition layers are deposited using MCVD or PCVD processes, and the inner and outer cladding layers are deposited using OVD processes.

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