RU2272003C1 - Method of high-temperature chemical treatment of glass surface - Google Patents

Abstract

FIELD: manufacture of fiber light conduits for communication lines and optical sensors.

SUBSTANCE: proposed method consists in high-temperature treatment of internal surface of quartz tube with envelopes applied in layers and core in the course of compression of blank and/or external surface of blank in the course of drawing the fiber by gas medium including fluorine-containing additives at content of 1-2 atomic % of fluorine. Hardening of light conduits is additionally achieved at drawing the fibers at tension mode of 0.1-1 Hpa, as well as at application of additional quartz layer alloyed with fluorine on blank before drawing the fiber.

EFFECT: improved optical and strength parameters of light conduits.

2 cl, 2 tbl

Description

The invention relates to fiber optics, in particular to a technology for manufacturing fiber optical fibers for communication lines and optical sensors.

The technology for manufacturing optical fibers consists in applying layers of a protective shell doped with P ₂ O ₅ and fluorine and layers of a core doped with an additive that increases the refractive index of silica glass on the inner surface of a quartz tube at a temperature of 1400-1500 ° C (GeO ₂ or P ₂ O ₅ ). A protective shell having a refractive index close to that of silica glass prevents the migration of light-absorbing impurities from the silica tube into the core layers. The tube with the deposited layers is heated by a moving gas burner to a temperature of 2000-2200 ° C. In two or three passes of the burner, the tube is compressed (collapsed) into a billet rod, which then, when heated in a graphite furnace to 1900-2150 ° C, is pulled into the fiber while a protective polymer coating is applied to it. To achieve small optical losses in the optical fibers, the initial quartz tubes are subjected to high-temperature chemical treatment in order to clean their inner surface from impurities that cause light absorption.

So, the known method of high-temperature chemical surface treatment of a glass with a mixture of oxygen with fluorine-containing additives (France, application No. 2473496 dated January 12, 1981, published July 17, 1981, IPC: C 03 V 15/00, 17/245; G 02 B 5/14) It is used for cleaning (etching) the inner surface of a quartz glass tube used for the manufacture of optical fiber blanks. The disadvantage of this method is that along with fluorine-containing additives, the introduction of hydrogen-containing additives is required in order to form an aggressive HF gas interacting with silica glass to form a gaseous SiF ₄ product. The presence of hydrogen in a gaseous medium leads to an increase in the optical loss of the optical fibers due to their contamination with OH-absorbing light groups.

The method closest to the proposed technical solution (France, application No. 25045514 dated 04.24.1981, published on 10.29.1982, IPC: C 03 B 37/075; G 02 B 5/14) was adopted by us as a prototype. In this method, after preliminary compression of the tube with deposited layers of the shell and core, a high-temperature chemical treatment of its inner surface with a mixture of oxygen and anhydrous fluorine-containing gases (CCl ₂ F ₂ , C ₂ F ₃ Cl ₃ , SF ₆ , etc.) with a concentration of more than 10 volume % at a temperature of more than 700 ° C after preliminary compression of the tube. Chemical treatment is used to remove the “dip” in the refractive index in the central part of the core of the preform: fluorine-containing reagents gasify the core layer depleted in germanium dioxide to form SiF ₄ and GeF ₄ . The presence of fluorine (and chlorine) in the gas atmosphere of high-temperature compression of the tube by several orders of magnitude reduces the content of OH groups in the core, thereby ensuring low optical loss of optical fibers.

The disadvantage of this method of chemical processing of the glass surface is the dependence of the thickness of the removed layer on the temperature of the inner surface of the pre-compressed tubular billet. This is due to fluctuations in the wall thickness and uneven emissivity of the quartz tube in the optical region of its transparency. For multimode optical fibers with a large core diameter, the variation in the thickness of the etched layer practically does not affect the optical properties of the optical fibers. However, for single-mode optical fibers having a small core diameter, such instability of the etching process leads to a significant change in the core diameter along the fiber length and, as a result, to a deterioration of some optical properties of the optical fibers (mode field diameter variation, higher mode cut-off length, and increased optical loss) . When the fiber is drawn in an inert medium, quartz glass evaporates, and finely dispersed SiO ₂ particles are deposited in the cold part of the preform together with dust particles from the furnace space. This leads to contamination of the surface layer of the preform and, as a result, to a decrease in fiber strength for both single-mode and multimode optical fibers.

An object of the present invention is to improve the optical and strength properties of optical fibers. The technical result is achieved by selecting the optimal conditions for high-temperature chemical treatment in the process of collapse of the workpiece and its stretching.

The problem is solved in a new way, which consists in high-temperature chemical treatment with a gas medium containing fluorine-containing additives, the inner surface of the quartz tube with deposited layers of shells and core in the process of compression and (or) the outer surface of the workpiece in the process of drawing fibers from it, in which from the prototype, the content of fluorine-containing gases in the carrier gas corresponds to 1-2 atomic% of fluorine.

To increase the strength of the optical fibers, the fiber is drawn under a tension of 0.1-1 GPa.

Also, with the aim of hardening, an additional layer of quartz glass is applied to the outer surface of the preform before drawing the fiber by the method of external gas-phase deposition from a vapor-gas mixture containing 1-2 atomic% fluorine.

During the chemical treatment with fluorine-containing reagents during high-temperature compression of the tube with the deposited layers and (or) when the optical fiber is drawn, the content of these reagents in the gas phase is maintained at the level of 1-2 atomic% fluorine. At such a low concentration of fluorine-containing substances and a temperature of 2000-2100 ° C, etching does not occur, and fluorine from the gas phase is almost completely introduced into the glass. The presence of fluorine in the germanium silicate core reduces the amount of additional optical losses that increase with the drawing temperature. The instability of the optical properties of single-mode optical fibers, caused by the variation in the core diameter in the prototype due to the uneven etching process, is eliminated. The presence of 1-2 atomic% fluorine and chlorine in the gas phase provides a low content of OH groups in the core of the workpiece.

Such a concentration of fluorine-containing additives in an atmosphere of drawing out optical fibers eliminates the deposition of dusty particles in the furnace space and SiO ₂ particles onto the cold parts of the preform, turning them into SiF ₄ gas. The introduction of fluorine into the surface layer of the preform reduces the viscosity of silica glass, so that with an increase in the pulling force of the fiber, compressive stresses arise in its surface layer. Preliminary deposition on the outer surface of the workpiece a layer of quartz glass doped with fluorine with a thickness of at least 0.5 mm, leads to an increase in the thickness of this compressive layer. These factors provide hardening of the optical fibers. The proposed tensile limit of 0.1-1 GPa is selected empirically and creates optimal conditions for ensuring the strength of the fiber.

The combination of the above features and analysis of differences from the prototype according to the existing level of technology allows us to conclude about the "novelty" and "inventive step" of the new method.

The method is implemented as follows. On the inner surface of a quartz tube (with a wall thickness of 2 mm and an outer diameter of 20 mm), 20 layers of a quartz glass doped with 2 mol.% P ₂ O ₅ and 0.5 at.% Fluorine were applied by the known gas-phase deposition method when the reaction zone was heated moving torch up to 1450 ° С. Then, in two passes of the burner at 1600 ° С, a shell of quartz glass doped with 1 mol.% GeO ₂ and 0.5 at.% Fluorine was formed. Such a shell is necessary to isolate the core from the phosphorus-containing shell, which has increased optical losses in the spectral range of 1.2-1.7 μm due to light absorption by P-OH vibrations. The deposition process of the layers was completed by applying a layer of a silica glass core doped with 10 mol% GeO ₂ . When it was heated by a moving torch to 2000-2100 ° С, the tube was compressed to an internal channel diameter of 3-4 mm. During this operation, the inner channel was purged at a flow rate of 1000 ml / min with a mixture of oxygen with Freon-12 (CF ₂ Cl ₂ ), setting its concentration in three equal sections of the preform equal to 0; 0.5; 1 volume%, which corresponds to a gas content of 0; one; 2 at.% Fluorine. The content of impurity moisture in the initial oxygen was at the level of 0.0001 volume%. Then, the final fusion of the inner channel was performed. On a R-101 refractometer, we measured the radial profile of the change in the refractive index in three parts of the workpiece processed at different concentrations of freon. A single-mode fiber with a fiber diameter of 125 μm was drawn from a round billet thus obtained, using a resistance furnace with a graphite heater. The furnace was purged with a mixture of argon with 0.15-0.3 vol.% SF ₆ . During the drawing process, the fiber was coated with an epoxy acrylate protective coating with a thickness of 40 μm, UV curable. The fiber was drawn at a speed of 60 m / min and a fiber tension of 0.01-1 GPa, which was controlled by changing the temperature of the furnace heater. Before the fiber was pulled out, a 0.5 mm layer of quartz glass doped with fluorine at a concentration of 1.5 ± 0.5 atomic% in the reaction products was deposited on a 50 mm-long blank using an external gas-phase deposition.

Tables 1 and 2 show the influence of the conditions of high-temperature compression of the tube and the pulling of optical fibers on their optical and strength properties. Optical losses were measured by the termination method on optical fibers with a length of 500 m at a wavelength of 1.3 μm (α _1.3 ) and at an absorption wavelength by OH groups (α _1.39 ). The cut-off wavelength of the highest mode (λ _s ) was estimated by comparing the optical signal in the straight and curved sections of the fiber.

Static strength (durability) was determined by the duration of the process of destruction of the fiber, curved by a loop in a calibrated hole with a diameter of 2.6 mm at room temperature and relative humidity of 40-50%. Each durability value presented in the table is an arithmetic average of 10 measurements.

Table 1
Effect of tube compression conditions on the optical properties of optical fibers Fiber number The content of freon in the gas, vol. % Compression Temperature, ° С Fiber pull tension, GPa (α _1.39 ), dB / km (α _1,3 ), dB / km λ _s , μm one 0 2000 0.01 29th 0.8 1,2 2 0.5 2000 0.01 4.2 0.63 1.15 3 one 2000 0.01 3 0.6 1.13 four one 2100 0.01 3 0.55 1.10

table 2
The influence of fiber drawing conditions on its strength properties Fiber number The content of SF ₆ in gas, vol. % The tension when pulling the fiber, GPa The durability of the fiber when bending with a diameter of 2.6 mm, sec Note 5 0 0.01 10 - 6 0.15 0.1 fifty - 7 0.3 0.5 one hundred - 8 0.3 1.0 80 Broken fiber 9* 0.3 1.0 130 - * - the fiber had a cladding of quartz glass doped with fluorine.

The results of the presented examples indicate a decrease in optical loss with an increase in the compression temperature of the tubular billet and an increase in the content of freon in oxygen, which can be explained by the positive effect of fluorine introduced into the core. At lower temperatures of chemical treatment (less than 2000 ° C) and a higher concentration of fluorine-containing reagents (more than 10 vol.%), Characteristic of known technologies, glass etching predominantly occurs without the introduction of fluorine. An increase in the temperature of chemical treatment of more than 2100 ° C during compression of the tube leads to a decrease in the viscosity of the glass and, as a consequence, to deformation of the layers of the workpiece. An increase in the freon content of more than 2 vol.% Leads to etching of the glass of the core and, as a result, to a decrease in its diameter and wavelength of cutoff of the highest mode.

The results presented in table 2 indicate an increase in the durability of optical fibers elongated in an inert medium containing SF ₆ . The upper limit of fiber tension (1 GPa) at low temperature is due to its possible breakage. A decrease in the drawing temperature leads to a decrease in the thickness of the outer fluorine-containing diffusion layer having compressive stresses. The increase in the thickness of the outer layer due to the deposition of a fluorine-containing layer on the workpiece by the method of external deposition (fiber guide No. 9) eliminates the breaking of the fiber. A decrease in tension of less than 0.1 GPa is impractical, since it leads to a decrease in the durability of the optical fibers.

The above information confirms the obvious industrial applicability of the method of high-temperature chemical treatment of the glass surface with a gaseous medium with fluorine-containing additives in the manufacture of optical fibers based on quartz glass.

Claims (2)

1. A method of high-temperature chemical surface treatment of a glass in the manufacture of optical fibers, comprising treating with a gaseous medium containing fluorine-containing additives the inner surface of a quartz tube with layers of shells and a core applied to it during compression of the tube and / or the outer surface of the workpiece while drawing the fiber, characterized in that use a gas environment in which the content of fluorine-containing reagents corresponds to 1-2 at.% fluorine, and high-temperature chemical surface treatment preforms in the process of stretching it are produced at a temperature providing a fiber tension of 0.1-1 GPa. 2. The method according to claim 1, characterized in that the high-temperature chemical treatment of the surface of the workpiece by drawing the fibers is carried out after applying a layer of quartz glass doped with fluorine in an atmosphere containing 1-2 at.% Fluorine on the workpiece.