RU2764065C1 - Method for manufacturing single-mode optical fibers with a germanosilicate core - Google Patents

Abstract

FIELD: fiber optics.

SUBSTANCE: invention relates to fiber optics, in particular the technology of single-mode silica fibers with a core, doped with germanium dioxide. The method includes deposition of core glass layers, high-temperature compression of a quartz tube with deposited layers in several burner passes, etching of the inner channel with a fluorine-containing gas in the last high-temperature compression pass, fusion of a quartz pipe into a workpiece with a core diameter of 3-5 mm, application of a layer of quartz glass with a thickness providing single-mode radiation in a fiber. The workpiece is pulled to a rod with a diameter of 1.5-3 mm, which is installed coaxially inside the quartz tube and is pulled together with simultaneous fusion. More than 500 km of single-mode optical fibers can be obtained from one preform.

EFFECT: production process is simplified, its productivity is increased.

1 cl, 2 tbl

Description

The invention relates to fiber optics, in particular, to the technology of manufacturing single-mode fiber light guides by a modified chemical vapor deposition method (modified chemical vapor deposition - MCVD). Of all the known gas-phase methods for manufacturing optical fibers, the MCVD method is the most common in terms of manufacturing single-mode fibers with a wide range of applications. Such light guides are made on the basis of quartz glass, and their core is doped, as a rule, with germanium dioxide.

Traditional MCVD method for manufacturing germanosilicate single-mode fibers (Ainslie BJ Beales KJ Day CR and Rush JD Interplay of design parameters and fabrication condition on the performance of monomode fibers made by MCVD // IEEE J.Quant. Electron., 1981, v.QE- 17, No. 3, pp. 854-857) has a significant drawback: their optical loss increases with the fiber drawing temperature and the content of germanium in them. This phenomenon is due to the reduction of GeO ₂ in the process of high-temperature compression of the preform, at which the equilibrium pressure of oxygen for germanosilicate glass significantly exceeds 1 atm. Therefore, the oxygen content decreases on the inner surface of the core layer, which causes excessive optical loss.

It is possible to reduce the level of excessive losses in the manufacture of the workpiece by applying an internal barrier layer of a germanosilicate core with a reduced content of GeO ₂ (Patent EP 1612192). However, at a particularly high level of glass doping of the GeO ₂ core, its diameter for a single-mode optical fiber becomes so small that the use of this technical solution leads to a decrease in its effective refractive index. Therefore, the properties of the optical fiber deteriorate: its attenuation increases due to the scattering of radiation into the cladding and optical losses increase when the fiber is bent.

This disadvantage does not have the closest to the proposed technical solution MCVD method of manufacturing single-mode optical fibers with germanosilicate core (RF patent No. 2576686), taken as a prototype of the claimed invention. The essence of this technical solution lies in the fact that the MCVD method is used to manufacture a workpiece with an increased core diameter, the inner part of which, depleted in oxygen, is removed by gas-phase etching after preliminary high-temperature compression of the internal channel of a quartz tube with deposited layers of doped glass. Before the fiber is drawn, a layer of quartz glass is applied to the workpiece with a thickness that ensures single-mode radiation in the light guide.

The disadvantage of this method is the need to increase the outer diameter of the single-mode fiber preform to 45-75 mm with the recommended germanosilicate core diameter of 3-5 mm, which is difficult to implement on an MCVD installation for manufacturing fiber preforms (hereinafter - MCVD installation).

The technical problem to be solved is the complexity of the MCVD method for manufacturing single-mode optical fibers with a germanosilicate core.

Technical result achieved: simplification and increase in productivity of the MCVD process for manufacturing light guides.

The problem is solved by the proposed method for manufacturing single-mode optical fibers with a germanosilicate core, including the deposition of layers of core glass, high-temperature compression of a quartz tube with deposited layers in several burner passes, etching of the internal channel with fluorine-containing gas in the last pass of high-temperature compression, fusion of a quartz tube into a workpiece with a diameter core 3-5 mm, and applying a layer of quartz glass with a thickness that provides single-mode radiation in the light guide, characterized in that the workpiece is drawn into a rod with a diameter of 1.5-3 mm, it is installed coaxially inside the quartz tube and they are jointly stretched with simultaneous fusion .

The essence of the invention lies in the fact that:

- The operation of applying a layer of quartz glass of the required thickness of ≈ 20-30 mm onto the workpiece, which is technically difficult on the MCVD installation, is excluded.

- The use of the proposed technical solution makes it possible to produce at least 70 meter-long rods from one blank with an outer diameter of 15 mm and a core of 3-5 mm. When pulling out such a rod, together with a quartz tube with an outer diameter of 25 and a wall thickness of 3 mm, at least 10 km of light guides with a diameter of 125 μm can be pulled out. And in total, from one blank with a diameter of 15 mm, made on an MCVD installation, at least 700 km of optical fiber can be obtained.

- The recommended range of rod diameters of 1.5-3 mm is due to the different content of GeO ₂ in the core (from 5 to 20 mol.%).

- The operation of forming a layer of quartz glass with a thickness that provides single-mode radiation in the light guide is carried out not on the MCVD installation, but in the process of pulling the rod together with the quartz tube. Due to this, the time of using the MCVD installation per unit fiber length is reduced by 7 and 70 times in comparison with the prototype and analogue of the proposed technical solution.

The claimed technical solution is confirmed by three examples.

Example No. 1. Fabrication of an isotropic single-mode gemosilicate fiber (hereinafter - OGF) with a core doped with 5 mol % GeO ₂ .

The manufacturing process of the light guide consisted of the following steps:

- obtaining pre-preparation by MCVD method;

- hauling the preform into a rod with a diameter of 1.5 mm;

- drawing the fiber from a rod coaxially located in a quartz tube with an outer diameter of 25 mm and a wall thickness of 3 mm, with simultaneous application of an epoxyacrylate sheath.

The OGS prefabrication was made on the OFC-12-729 automated complex manufactured by Nextrom. On the inner surface of a one-meter tube made of F-300 quartz glass with an outer diameter of 20 mm and a wall thickness of 2 mm, layers of a shell of pure quartz glass and layers of a core doped with 5 mol % GeO ₂ were deposited. By high-temperature compression of the pipe at 2200–2250°C, the diameter of the inner channel was reduced to 2–3 mm. During this operation, the pipe was purged with oxygen containing no more than 5⋅10 ^-4 vol % moisture. The subsequent operation of internal etching with a mixture of SF ₆ and oxygen at a temperature of 1700-1800°C removed up to 30% of the inner layer of the core, depleted in oxygen. The final fusion of the inner channel was carried out at a temperature of 2250°C.

The R-101 refractometer measured the difference between the refractive indices (RI) of the core and cladding (∆n), as well as the geometrical parameters of the preform cross section (Table 1).

25 meter rods with a diameter of 2.4 mm were drawn from the part of the preform normalized by the stability of the core diameter.

Table 1. Pre-harvesting parameters

∆n Workpiece diameter, mm Core diameter, mm Shell diameter, mm 0.007 13.3 5 6

A light guide with a fiber diameter of 125 μm and a length of 15 km was pulled out from one rod, installed coaxially in a tube of quartz glass F 300 with an outer diameter of 25 and a wall thickness of 3 mm. During the drawing process, a two-layer epoxy-acrylate coating with a thickness of 60 μm was applied to the fiber.

The optical loss spectrum of the OGS in the wavelength range 1100–1600 nm was measured by the cutoff method using a Yokogawa AQ6370C optical spectrum analyzer. The cutoff wavelength of the highest mode LP _{11 was} determined on a two-meter fiber segment by bending on the same instrument.

The cutoff wavelength of the LP ₁₁ mode is ≈ 1.25 µm, the optical loss at a wavelength of 1.31 and 1.55 µm is 0.5 and 0.3 dB/km, respectively. From the remaining 24 rods, another 360 km of OGS can be pulled out.

Example #2. Fabrication of an anisotropic single-mode gemosilicate fiber (AOGS) with an elliptical core doped with 20 mol % GeO ₂ .

The AOGS manufacturing process consisted of the following steps:

- obtaining pre-preparation by MCVD method;

- cutting lateral grooves for pre-cutting;

- high-temperature rounding of the preform;

- reupholstery of a round billet up to a diameter of 1.5 mm;

- fusion of the waist with a quartz pipe with an outer diameter of 25 mm and a wall thickness of 3 mm.

On the inner surface of a one-meter tube made of F-300 quartz glass with an outer diameter of 25 mm and a wall thickness of 3 mm, layers of a shell of pure quartz glass and layers of a core doped with 20 mol.% GeO ₂ were deposited. Subsequent compression of the pipe at a temperature of 2200 - 2250 °C reduced the diameter of the inner channel to 2-3 mm. During this operation, the pipe was purged with oxygen containing no more than 5⋅10 ^-4 vol % moisture. The subsequent operation of internal etching with a mixture of SF ₆ and oxygen at a temperature of 1700-1800 °C removed up to 30% of the inner layer of the core, depleted in oxygen. The final compression of the pipe was carried out at a temperature of 2250°C.

The R-101 refractometer measured ∆n and the geometrical parameters of the preform cross section (Table 2).

Table 2. Pre-harvesting parameters

∆n Workpiece diameter, mm core diameter,
mm Shell diameter, mm 0.03 15.5 2.6 5.6

The elliptical shape of the core was created by the known method of cutting lateral grooves on the preform and its high-temperature rounding. 70 meter rods with a diameter of 1.5 mm were pulled out of the preform. The rod in the process of fiber drawing was fused with a quartz tube with an outer diameter of 25 mm and a wall thickness of 3 mm. Received in this way 15 km AOGS with a diameter of 125 mm with an outer diameter of the polymer epoxyacrylate shell of 250 μm and a core with axes 4.7 and 1.9 μm long.

The cutoff wavelength of the AOGS LP ₁₁ mode is ≈ 1.31 µm, the optical loss at a wavelength of 1.31 and 1.55 µm is 4 and 2 dB/km, respectively. The degree of conservation of radiation polarization (h -parameter) ≈ 10 ^-5 m ^-1 . From the remaining 69 rods, at least 1000 more km of AOGS can be pulled out.

Example #3. Fabrication of a thin anisotropic single-mode gemosilicate fiber with an elliptical core doped with 20 mol % GeO ₂ .

The manufacturing process of AOGS is similar to example No. 2, with the exception of the final operation, in which the diameter of the drawn fiber was reduced from 125 to 45 microns. The diameter of the outer single-layer polymer shell is ≈ 100 µm. AOGS 60 km long was received. The optical characteristics were measured on a 300 m long fiber segment.

The cutoff wavelength of the AOGS LP ₁₁ mode is ≈ 0.76 µm, the optical loss at a wavelength of 0.85 µm is 3.5 dB/km. The degree of conservation of radiation polarization (h -parameter) ≈ 10 ^-5 m ^-1 . From the remaining 68 rods, at least 5000 more km of such AOGS can be pulled out.

Thus, the technical result is achieved.

The above information confirms the obvious industrial applicability of the proposed method for manufacturing preforms for high-aperture single-mode fibers with a germanosilicate core.

Claims (1)

A method for manufacturing single-mode optical fibers with a germanosilicate core, including deposition of core glass layers, high-temperature compression of a quartz tube with deposited layers in several burner passes, etching of the inner channel with fluorine-containing gas in the last high-temperature compression pass, fusing the quartz tube into a workpiece with a core diameter of 3-5 mm, and deposition of a layer of quartz glass with a thickness that provides single-mode radiation in the light guide, characterized in that the workpiece is drawn to a rod with a diameter of 1.5-3 mm, which is installed coaxially inside the quartz tube, and they are jointly drawn with simultaneous fusion.