RU2790075C1 - Method for manufacturing optical fiber preforms from quartz support tubes with the presence of volumetric defects (variants) - Google Patents

Abstract

FIELD: optical fiber production.

SUBSTANCE: method for manufacturing an optical fiber preform from quartz support tubes with the presence of volumetric defects (variants) is claimed. The invention relates to fiber optics. The method according to the first variant includes the manufacture of the optical part of the preform by chemical internal vapor phase deposition (MCVD), or by chemical vapor phase deposition using a furnace (FCVD), or by plasma internal deposition from the gas phase (PCVD) with a modified proportion between the diameter of the shell and the core, in which for further complete etching of the quartz support tube in in a solution of hydrofluoric acid, after deposition of all layers of glass and the procedure of “collapsing” the workpiece, the diameter of the core is reduced by the value X, which is determined by the formula: X=D tube/K, where D tube is the thickness of the quartz support tube, K is the proportion between the shell and the core, calculated by the formula: K=D geometric cladding/D core, where D geometric cladding is the diameter of the geometric shell area of the preform, D core is the diameter of the core area of the preform. By the method for the second variant, after the manufacture of the optical part of the preform by the MCVD, or FCVD, or PCVD method, the complete etching of the quartz support tube in a solution of hydrofluoric acid is additionally carried out, after the deposition of all layers of glass and the procedure of “collapsing” the workpiece, as an intermediate step before the process of external deposition from the gas phase (OVD), or the process of jacketing, or by the process of fusing a rod blank in the center of a quartz pipe filled with quartz sand (”SAND”).

EFFECT: improvement in the quality of optical preforms made with the help of quartz support tubes in the presence of a significant number of volumetric defects located at an arbitrary depth.

2 cl, 3 dwg

Description

The invention relates to fiber optics and is an addition to methods for manufacturing the optical part of a preform (blank) by a modified chemical internal vapor deposition (MCVD) method or a furnace chemical vapor deposition (FCVD) method or a plasma internal vapor deposition (PCVD) method, and it can be effectively combined with the jacketing method (“flopping” an additional quartz tube over the optical cladding) or the external vapor deposition (OVD) method or the core blank fusion method in the center of a quartz tube filled with “SAND” quartz sand, and it can be used in the technological process of production of optical fibers at the stage of manufacturing an optical preform. Further, optical fibers are used in factories for the production of fiber optic cables.

The classic MCVD, FCVD, PCVD, OVD and jacketing techniques are well known in the art.

The optical part of the preform is understood as the region of the core and a small part of the shell, through which the optical power propagates. The optical part of the preform in the claimed invention is limited by the dimensions of the supporting quartz tube used in the MCVD/FCVD/PCVD methods.

When using inexpensive reference quartz tubes that do not have a high level of purity (they contain a significant amount of volumetric inclusions: microbubbles, impurities (hydroxyl group content > 1 ppm), etc.), in FCVD or MCVD or PCVD blank manufacturing methods it is possible the occurrence of a number of problems in already manufactured optical fibers. Problems can be associated not only with the apparent increase in optical loss, but also with the strength of the optical fibers. Optical fibers from blanks obtained on cheap reference quartz tubes with inclusions have high breakage during the proof testing procedure (rewinding the optical fiber under tension), which makes it impossible to manufacture strong optical fibers of the required length. Typically, for telecommunications optical fibers (e.g. G.652.D type), lengths of 50.4 km are common, and in some cases 25.2 km for multimode fibers (e.g. OM2) - lengths of the order of 17 are typical, 6 km. All other lengths are of little liquidity for fiber optic cable manufacturers and can lead to a significant number of restarts of the optical module and fiber optic cable production line, which leads to an increase in production costs. In this regard, it is necessary to strive to produce optical fibers with standardized lengths of 25.2/50.4 or 17.6 km with a strength of at least 8.8 N (0.69 GPa).

From the prior art, classical methods for manufacturing an optical fiber preform are known by MCVD/FCVD/PCVD methods, however, these methods do not detail the principles/methods/techniques for working with low-quality (low-purity) raw materials for the production of optical preforms. For the claimed invention, from the list of raw materials for the production of optical preforms, it is important to focus on supporting quartz tubes, due to the import component of this type of raw material.

The closest technical solution to the first variant of the claimed solution is a method of chemical etching of quartz glass pipes, including their treatment in a solution of hydrofluoric acid, followed by washing in distilled water. The pipe is placed horizontally with a thick wall down the reactor, the acid is poured to a level preferably equal to 1/2 of the pipe diameter, after which the acid is evenly drained, while the time of contact of the pipe with the acid in the process of draining it is equal to the ratio of the value of the circumferential thickness difference to the double rate of quartz glass etching ( RU 2477713, IPC C03C15/00, published 03/20/2013).

The disadvantage of the prototype is that the etching method considers only one type of defect in the supporting quartz tubes - circumferential thickness variation. In this regard, this method bleeds off a layer of glass with a value only necessary to eliminate the circumferential thickness variation of the quartz tube. However, reference quartz pipes may have defects/inclusions located throughout their entire volume.

The closest technical solution to the second variant of the claimed solution is the method of building up the technological shell - this is a combination of the MCVD and OVD methods, where the core and the reflective shell are formed by the MCVD method, and the bulk of the technological shell is formed by the OVD method (access mode: https://siblec.ru /telekommunikatsii/fiziko-tekhnologicheskie-osnovy-volokonno-opticheskoj-tekhniki/3-metody-polucheniya-zagotovok-kvartsevykh-ov Date of access: 05/06/2022). Also, the combination of MCVD + OVD methods is described in the method of increasing the technological shell, in which the combination of the MCVD method is carried out, by which the core and the reflective shell are formed, and the bulk of the technological shell is formed by the OVD, APVD and "SAND" methods (access mode: https:// op.vlsu.ru/fileadmin/Programmy/Magistratura/12.04.05/Metod_doc/Metod_samostASTL_120305LT_120405LTm.pdf Publication date: 2014).

However, the known combined methods do not completely bleed the reference quartz tube after the optical preform is made by MCVD or FCVD or PCVD as an intermediate step before the OVD or PCVD or "SAND" process in order to improve the quality of the preform.

The technical result of the claimed invention is to improve the quality of optical preforms made using supporting quartz tubes with a significant amount of volumetric inclusions (with a low level of purity, the content of microbubbles and impurities (hydroxyl group content> 1 ppm), etc.), located on an arbitrary depth.

The quality of a preform is understood as its optical purity, which makes it possible to manufacture optical fibers of the best quality that meets a number of criteria for telecommunication optical fibers of the corresponding category (IEC 60793-2-50 or GOST R IEC 60793-2-50). Among the group of requirements for the claimed invention, the attenuation coefficient and strength of optical fibers are especially important.

The essence of the invention lies in the fact that the method for manufacturing an optical fiber preform from reference quartz tubes with volumetric defects according to the first variant includes manufacturing the optical part of the preform by the MCVD or FCVD or PCVD method with a changed proportion between the diameter of the cladding and the core (Fig. 3). In turn, the proportion between the diameter of the shell and the core (hereinafter referred to as the proportion) is changed in such a way that after the stage of deposition of all the inner layers of the glass and the “collapse” procedure of the workpiece, it is possible to etch off all reference quartz tube and obtain the proportion required according to the refractive index profile of the type of optical fiber being manufactured.

The modified proportion discussed above is achieved by reducing the diameter of the core by the value X, which is determined by the formula:

,

,

where D tube is the thickness of the reference quartz tube,

K is the ratio between the cladding and the core required by the type of optical fiber being manufactured, calculated by the formula:

,

,

where D geometric cladding is the diameter of the preform geometric shell area; D core is the diameter of the preform core area.

For multimode optical fibers of categories OM2-OM5, the parameter K=125 µm/50 µm=2.5.

The validity of the formula for the definition of X can be verified by expressing X from the following equation:

In FIG. Figure 3 shows the structure of the preform (conventional designations: D core – diameter of the preform core area; D optical cladding – diameter of the preform optical shell area; D tube – thickness of the supporting quartz tube in MCVD/FCVD/PCVD methods; D geometric cladding – diameter of the preform geometric shell area ; X is the amount by which it is required to reduce the diameter of the core so that after etching the supporting quartz tube with a thickness (D tube) the proportion (K) between the shell and the core does not change).

It should be taken into account that after reducing the diameter of the core of the manufactured preform by the value X (to maintain the proportion between the core and the shell), there remains a smaller number of layers to “record” the required refractive index profile. In this regard, it is necessary to use supporting quartz tubes of the maximum allowable diameter, so as not to face the need to “record” the refractive index profile without alternative using the PCVD method. In order for the bleed of the support tube to make no contribution to the preform manufacturing process, it is possible to use the internal diameter of the support tube with the full bleed process equal to the outer diameter of the support tube without the bleed process of the support tube after the preform synthesis process. However, it should be noted that the use of large diameter quartz support tubes is limited by the size of the furnace in the FCVD method. The typical size of the support tubes for creating the optical part of the preform is (44 +/- 1.5) mm. The use of larger diameter quartz tubes may entail technical modernization (increase in size) of the furnace.

The method for manufacturing an optical fiber preform from reference quartz tubes with the presence of volumetric defects according to the second variant includes manufacturing the optical part of the preform by the MCVD or FCVD or PCVD method without changing the proportion due to the fact that the bleed glass volume of the reference tube is compensated by the OVD method or the jacketing method or the SAND method. That is, after the stage of deposition of all the inner layers of the glass, the procedure of "collapsing" the workpiece and after etching the support tube, it is possible to increase (compensate) the thickness of the geometric shell, for example, due to the OVD method or the jacketing method or the "SAND" method. In this case, there is no need to change the proportion between the diameter of the shell and the core.

Inclusions in the supporting quartz pipes in both versions can be located at an arbitrary depth (Figs. 1 and 2). In FIG. 1 and 2 show typical microscopic images of the breakage site (the end of the optical fiber) after the proof testing procedure, and Fig. 2 additionally shows a highlighted area showing the region of the core of the optical fiber. In FIG. Figure 1 shows microbubbles localized in the quartz optical cladding (the so-called "internal" breaks) of an optical fiber drawn from a preform made using quartz tubes with inclusions. In FIG. 2, the microbubble is localized inside the reference quartz tube (this can be seen from the “mirror” area localized in the shell).

Both options use the practical concept that the purity of the deposition quartz reference tube is usually not as high as the purity of glass deposited by MCVD/FCVD/PCVD methods, and the reference quartz tube with inclusions will not be part of the future preform, but is only temporary. base/base for deposition of glass layers.

In both cases, preliminary treatment of supporting quartz tubes in an aqueous solution of highly pure hydrofluoric acid, although it provides more effective cleaning of their surface, containing an increased amount of impurities in a layer 10-15 μm thick (Leko V.K., Komarova L.A. Study of the distribution of impurities in the surface layers of quartz glass pipes (Optico-mechanical industry, 1974, No. 6, pp. 33-35), however, such treatment cannot guarantee that microbubbles and other inclusions will not occur at great depths. For this reason, the claimed invention recommends complete etching of the reference quartz tube after deposition of all layers of glass and the billet "collapse" procedure.

In the claimed invention, in both cases, it is possible to use a solution of hydrofluoric acid without special requirements for its purity, followed by washing with distilled water (Sommer R.G., Deluca R.D., Burke G.E. New glass system for low-loss optical waveguides. Elecron. lett., 1976, v .12, No. 16, p.408-409).

In both versions, the process of etching the supporting quartz tube from the finished preform occurs by placing the preform in a vertical position inside a hermetically sealed reactor tube made of polymeric materials. A concentrated 40% solution of hydrofluoric acid and distilled water is poured into the reactor to a level that completely covers the preform, and so that the solution does not spill out of the reactor. The rate of etching of quartz glass in acid at room temperature is approximately 50-100 µm/h. At the end of the etching process, the hydrofluoric acid solution is drained off and the preform is washed in distilled water. Finished preform thickness can be controlled using the 2600 Series Preform Analyzer or caliper.

The present invention in the first embodiment has the disadvantage of increasing the synthesis time of the optical part of the preform and increasing labor intensity due to the need for the etching process. Despite the fact that the claimed methods suggest manufacturing only the optical part of the preform, the increase in preform synthesis time can be significant, which is a feature of internal deposition methods. Below is an estimate of the synthesis time of the optical part of the preform for a multimode optical fiber of category OM2 by the FCVD method. At a typical carriage speed of 125 mm/min, for a preform 500-600 mm long, one layer is typically deposited within 6-7 minutes. For the optical cladding diameter of the finished preform of 21 mm and the core diameter of about 8.4 mm (proportion to be set 2.5), for the case when the supporting quartz tube forms the optical cladding, with an oxygen flow through silicon of about 802 slm, we have an experimental Cross Section Area for one layer CSA _layer = 1.65 mm ² . CSA _core area = 55.40 ^mm2 , optical cladding area CSA _{optical_cladding} = 346.2 ^mm2 , number of layers for core deposition:

number of layers for shell deposition:

In this regard, the deposition time of the optical core is 7 min per layer * 34 layers = 238 min ≈ 4 h; the synthesis time for a preform with a diameter of 21 mm during the formation of the core and shell by the FCVD method is 7 min per layer * 210 layers = 1470 min = 24.5 h ≈ 1 day. Hence, we see that there is a significant (6-fold) increase in the preform synthesis time by the FCVD method in the case when one tries to form an optical shell of such a diameter as to compensate for the etched reference quartz tube. Therefore, this method of etching the reference quartz tube must be combined with the jacketing method (“flopping” an additional quartz tube over the optical cladding) or the OVD method or the “SAND” method to form a geometric shell to compensate for the etched diameter. Due to this, the increase in synthesis time will not be so insignificant and is compensated by a larger diameter jacket tube or by a slight increase in the deposition time by a more productive OVD method or by a larger volume occupied by quartz grains in the SAND method.

In both versions, the etching rate can be increased by increasing the concentration of hydrofluoric acid in the solution (above 40%) or by heating the solution (the etching rate of quartz glass can be increased by a factor of 3 when the solution temperature is raised to 40–50 °C).

In addition, the present invention in the first embodiment has the disadvantage of a small mass of the produced preform, due to which the potential of the stretched mileage of the optical fiber is reduced. For an example of a manufactured multimode blank: let the preform length be 600 mm, and the outer diameter of the preform be 21 mm, with a quartz glass density of 2201 kg/m ³ , we have a mass of one preform of about 0.457 kg, which is about 16.93 km of optical fiber with preforms (on average, the weight of 1 km of fiber is 27 grams). After etching the support tube from the preform up to 16 mm, the mass of the preform will be reduced to 0.265 kg, which is about 9.83 km of optical fiber from one preform. At the same time, for a preform with a length of 900 mm, the weight of the preform will be 0.686 kg, which will be about 25.40 km of optical fiber from one preform. After etching the support pipe from the preform up to 16 mm, the mass of the preform will be reduced to 0.398 kg, which is about 14.74 km of optical fiber from one preform. Therefore, for a preform 600 mm long, the loss of the potential exit of the fiber from the preform will be (16.93-9.83) km=7.1 km, i.e. (7.1 km/16.93 km)*100%=41.9%, and for preforms with a length of 900 mm, the loss of the potential output of the fiber from the preform will be (25.40-14.74) km \u003d 10.66 km, that is, the same (10.66 km / 25.40 km) * 100% \u003d 41.9%. that the loss of mass of the preform, and, consequently, the loss of the maximum mileage of the optical fiber, cannot be compensated by increasing the length of the manufactured preform. However, the present invention in the second embodiment does not experience the described disadvantage.

The second variant of the method makes it possible to partially compensate for the increase in preform synthesis time due to a jacket tube of larger diameter or due to a slight increase in the time of the deposition process by the more productive (compared to the MCVD/FCVD/PCVD methods) OVD method or due to the larger volume occupied by quartz grains in the method SAND. Additionally, the second version of the method can produce a larger volume of optical fiber from one preform than in the first version.

The first option is applicable in cases where the preform is made entirely by using the MCVD/FCVD/PCVD methods (not combined with other methods of building a geometric shell, such as OVD, SAND or jacketing). In this case, the preform cladding consists only of an optical part and a core with a proportionally reduced diameter by X to maintain scale ratios with the required refractive index profile of the optical fiber after the pipe is etched. The first method is inferior to the second in many respects, however, the first method has no alternative in cases where there are no OVD and SAND installations and in cases where jacketing is impossible due to the lack of high-purity quartz jacket pipes.

The above information demonstrates the feasibility of using the claimed methods for manufacturing optical preforms (optical fiber preforms) using MCVD/FCVD/PCVD methods obtained using reference quartz tubes with volumetric defects as part of a critical import substitution process. The present invention seems to be especially effective in the second variant due to the absence or partial elimination of the described disadvantages of increasing the synthesis time and reducing the mass of the preform.

The claimed variants of the method are connected by a single inventive concept, since they make it possible to manufacture high-quality optical fiber preforms from supporting quartz tubes with a significant amount of volumetric inclusions (with a low level of purity, the content of microbubbles and impurities (hydroxyl group content> 1 ppm), etc.), located at an arbitrary depth.

Claims (8)

1. A method for manufacturing an optical fiber preform from reference quartz tubes with volumetric defects, including manufacturing the optical part of the preform by chemical internal vapor deposition, or chemical vapor deposition using an oven, or plasma internal vapor deposition from a gas phase with a changed proportion between the cladding diameter and the core, in which for further complete etching of the reference quartz tube in a solution of hydrofluoric acid, after deposition of all layers of glass and the “collapse” procedure of the workpiece, the diameter of the core is reduced by the value X, which is determined by the formula

where D tube is the thickness of the reference quartz tube, K is the ratio between the shell and the core, calculated by the formula

where D geometric cladding is the diameter of the preform geometric shell area, D core is the diameter of the preform core area. 2. A method for manufacturing an optical fiber preform from reference quartz tubes with volumetric defects, including manufacturing the optical part of the preform by chemical internal vapor deposition, or by chemical vapor deposition using an oven, or by plasma internal vapor deposition from a gas phase, building up a geometric shell by jacketing , or by the method of external deposition from the gas phase, or by the method of fusion of a rod blank in the center of a quartz tube filled with quartz sand, to achieve the required proportion between the core and the shell, characterized in that after the manufacture of the optical part of the preform by the method of chemical internal vapor deposition, or by the method of chemical vapor-phase deposition using an oven, or by the method of plasma internal vapor deposition, the supporting quartz tube is additionally completely etched in a solution of hydrofluoric acid, after deposition of all layers of the stack preforms, as an intermediate step before the external vapor deposition process, or the jacketing process, or the process of fusion of the core blank in the center of a quartz tube filled with quartz sand in order to improve the quality of optical preforms made using reference quartz tubes with the presence of a significant number of volumetric defects located at an arbitrary depth.

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