MXPA97005913A - Thermal treatment of glasses based on sil - Google Patents

Abstract

A dense glass concreting, the fiber optic preform added with aluminum oxide is then stretched and then heated to a temperature of 1490-1495øC to remove the bubbles without causing crystallization, subsequently, the stretched glass body is directly extracted into a fiber optic or overcoated , and then it is extracted in a fib

Description

THERMAL TREATMENT OF GLASSES BASED ON SILICA BACKGROUND OF THE INVENTION This invention relates to a method for producing elongated bubble-free glassware, and more particularly to a method for making optical fibers, especially amplification fibers that are used in fiber amplifiers. The so-called procedures e ternos; which include the external vapor deposition (OVD) method and the vapor axial deposition (VAD) method, produce homogenous and optically excellent glass preforms; In addition, these methods are cost efficient because they can produce preforms of large dimensions. Both methods of OVO and VOD u + ili.an burners to react talus precursor materials such as halides and organometallic compounds to produce a stream of glass particles or vitreous soot. In the OVD process, the soot stream is deposited on the outer peripheral surface of a mandrel and accumulates radially to form a porous body. After the mandrel has been removed from the porous body, it is inserted into a consolidation furnace where it dries and concretes. A mixture of chlorine-containing gas is flowed through the furnace to the heated preform to dry the porous body. Then, helium is flowed through the kiln and also into the opening of the preform to remove the residual chlorine and maintain an open longitudinal axis in the preform of the nuclous during concretion at temperatures that are often on the scale of 1440 ~ 1525 ° C for glass with high silica content »The resulting preformed dense glass can be extracted immediately into an optical fiber if it contains the proper ratio of core and coating glass. Frequently, the dense glass preform does not contain coating glass or only a portion of the required thickness of the coating glass. Said preforms are inserted in a reheating oven, wherein the longitudinal opening is evacuated, while the tip of the same is heated and stretched on an elongated rod. The opening closes and forms a "region of collapse of the long axis". The rod can be cut into core rods that are overcoated with additional coating glass to form extraction targets that are extracted in optical fiber. In the VAD process, the soot stream is deposited on the end of a target rod and accumulates axially to form a porous body that is similar to that produced by the OVD process, except that it lacks the axial opening. Therefore, the VAD preform of dense glass lacks the region of collapse of the longitudinal e. The rest of the process for producing the fiber is similar to that described above, in that the porous body dries, becomes dense glass, and is extracted into fiber .. Part of the helium to which the porous body is>. Orne e in the consolidation furnace remains in the glass in molecular form, ie, no bubbles can be observed. They have kept blanks from overcoating bubble books and they contain germ-oxide in 8-inch furnaces to degas the trapped helium molecules and improve the fiber's ability to stretch. However, some extraction targets contain visible gas bubbles that contain helium and / or other inert gases and, in some cases, additional oxygen. Fibers extracted from whites that contain bubbles break or experience variations in diameter during extraction. If only small fiber lengths are needed, the fiber can be drawn between the bubbles; However, starting the extraction procedure every time the fiber breaks is expensive and time consuming. An extraction target containing bubbles that are spaced along its length can be essentially unusable if they are to be extracted from the long fiber lengths of the transmission line. Optical fibers added with rare earths such as erbium are commonly used in fiber amplifiers. The nuclei of said fibers in amplification often contain Ge? 2 to increase the refractive index. Aluminum oxide is advantageously added to the core to ensure said improvements, since it reduces the amplification ion pool and, sometimes, improves the shape of the amplification spectrum. Therefore, it would be convenient to use aluminum oxide rather than germanium oxide in the amplifier fibers to achieve the desired optical characteristics, including increased retraction index of the core. However, aluminum oxide causes crystallization problems. The common temperature of the OVD and VAI methods of concretion produces crystallization in targets that contain more than some maximum permissible concentration of aluminum oxide, depending on the composition of the glass and the processing conditions ( see US Patent 5,262,365). A nucleation site for such crystallization appears to include the region of collapse of the longitudinal ee of concreted preforms of OVD. Crystallization can promote gas capture. Bubbles composed of helium and oxygen have usually been observed in concreted glass preforms added with aluminum oxide. According to the equilibrium phase diagram of S1O2-AI2O3, the porous body of soot must be concreted above the eutectic temperature of 1587 ° C to avoid nucleation and crystal growth. That exceeds the temperature of the operation of the silica-based muffles. Although the crystals can be melted in furnaces at temperatures above the eutectic point for mulite and cristobalite as demonstrated by the equilibrium phase diagram of AI2O3-S1O2, the resulting glass has unacceptably high bubble density and is poor in gravity, or well you inext rible. Absolute bubble-free extraction targets are required to extract large fiber lengths from the distributed fiber amplifiers. Only relatively short lengths of fibers added with rare earths are needed for discrete fiber amplifiers. However, the fibers must be free of defects such as bubbles composed of trapped gases (also known as seeds). Even when discrete amplification fiber lengths between the bubbles can be exceeded, the process becomes expensive when the extraction target contains too many bubbles. Therefore, attempts have been made to reduce the occurrence of bubbles and crystallization in extraction targets. Irruppliers such as fluorine and P2O5 have been added to the core to reduce the risk of crystallization by aluminum oxide. However, the addition of additional impurities may increase the cost, and the presence of such indicators in the core is often undesirable. Therefore, there is a need for a process by which bubble-free or low-bubble-dense glass-containing targets containing aluminum oxide can be produced, without introducing other impurities that are undesirable in the core region. Prior art processes typically have aluminum oxide at a maximum in the longitudinal axis. See the patents of E.U.A. Nos. 4/323, / 7 <;3; 5,058,976; and 5,155,621, wherein the concentration of aluminum oxide is highest in the center of the core. In the patent "27 < 3, aluminum oxide is used to adjust the refractive index of the fiber and also to inhibit the loss of the fluorescent nitride during processing." the "976 patent, the core has regions that begin with the center having aluminum oxide and germam oxide, followed by a subsequent region comprising erbium and aluminum oxide, and then a third region comprising only oxide oxide impurity. The erbium is not in the center, but is in an annular region about half the radius of that of the nucleus, erbium diffuses into the central layer of aluminum oxide, but does not reach the center of the fiber. In the 621 patent, aluminum oxide is limited to the center of the nucleus, and the erbium is added uniformly throughout the body of the core, which is assumed to reduce spontaneous emissions. OVD which have this concentration profile of aluminum oxide are especially at risk of undergoing crystallization in the region of collapse of the longitudinal e.

BRIEF DESCRIPTION OF THE INVENTION Therefore, it is an object of the present invention to provide a method for reducing the occurrence of bubbles in optical fiber extraction targets. Another objective is to provide an optical fiber extraction target that has a concentration gradient of aluminum oxide that reduces the risk of crystallization, especially in extraction targets produced by OVD. Briefly, the present invention relates to a method for reducing bubbles in a glass article. Vitreous soot is deposited on a substrate to form an elongated cylindrical body, at least a portion of which is porous. The porous portion of the body is dried and concreted to convert the porous portion to a dense glass having a given cross-sectional area. The resulting glass preform is extracted to form a glass shank in which the cross-sectional area of the concreted dense glass is smaller than the given cross-sectional area. The cane is thermally treated at a fairly high temperature to remove the bubbles and, for a fairly short time, to avoid the crystallization of the cane. This method can be used to improve glass rods produced by processes such as OVD, VAD and in the MCVD process added with solution. In the OVD process, the deposition step comprises depositing vitreous soot on the outer peripheral surface of a mandrel to accumulate a porous coating, and remove the mandrel. In the VAD process, the deposition step comprises vitreous soot deposition on the end of a target rod and accumulating the coating in an axial direction. In the MCVD-based process, the deposition step comprises depositing vitreous soot on the inner surface of a substrate tube. In a useful embodiment in the OVD and VAD processes, the soot vi t reo is deposited on a substrate to form an elongated cylindrical porous body that has a first external diameter. The porous body is dried and concreted to consolidate the porous body in a form having a second diameter that is smaller than the first outer diameter. The preform is removed to form a core rod that has a third diameter that is smaller than the second diameter. The resulting cane is then thermally treated. The method of the invention is especially useful for manufacturing amplification fibers for fiber amplifiers by the OVD method. Said fibers employ a rare earth such as erbium; aluminum oxide is also present to prevent the clumping of rare earth ions. The glass rods formed by this process may exhibit crystallization in the region of collapse of the longitudinal axis when the aluminum oxide content is too high. Therefore, the method includes the step of depositing a plurality of soot layers on a mandrel to accumulate a coating, one or more of the first < soot deposits deposited containing a first concentration of aluminum oxide. The concentration of aluminum oxide of the rest of the plurality of layers is greater than the first concentration.

DESCRIPTION OF THE DRAWINGS Figure 1 shows a concreted glass body produced by an external procedure. Figure 2 shows schematically the heat treatment of the body of Figure 1 after it has been drawn. Figure 3 shows the deposition of vitreous soot on a rnandp 1. Figure 4 is a cross-sectional view of the porous body after the mandrel has been removed. Figure 5 shows schematically the drawing of a rod or cane from a concrete preform made of dense glass. Figure 6 shows the rod during the heat treatment in the hot area of a furnace. Figures 7 and 8 show two concentration profiles of aluminum oxide. Lae figures 9 and 10 show two concentration probes of erbium oxide.

DETAILED DESCRIPTION OF THE INVENTION Figures 1 and 2 schematically show the principle of the present invention. A body of elongated porous soot is formed by an external procedure such as the OVD or AVD procedures. The soot body is dried and concreted to form the dense glass preform 10 having an outer diameter D2. The preform 10 may contain bubbles that would adversely affect the extraction procedure of the fiber and / or the resulting fiber. Therefore, it is desirable to eliminate or reduce the number of bubbles in the glass. The thermal binding of the preform 10 has not always resulted in a reduction in the bubble count. In accordance with this invention, the reformer 10 is extruded and stretched by known techniques to form a glass rod 14 of reduced diameter D3. Then, the rod 14 is subjected to heat treatment in the furnace 16. The furnace 16 can heat the rod 14 along its entire length for a controlled time. Alternately, the length of the rod may be treated increasingly with heat by a sweeping type furnace, or by gradually inserting the rod in an oven having a narrow hot area. Regardless of the type of furnace, each length segment of the cane is heated to a high enough temperature for sufficient time to remove or decrease the number of bubbles therein. The reed 30 of the small diameter core, compared to that of the concreted preform, makes it easier for the gases in the bubbles to escape. The diameter D3 is preferably in the range of 5 to 10 mm to achieve this result. The structural integrity of the reeds that have diameters less than about 5 mrn, would make them difficult to use if they had a great length. The diameter D3 could be less than m if only short lengths of cane had to be overlapped 1 and extracted in pound. It is thought that the key factors that influence the successful removal of bubbles include temperature, duration of treatment, size of bubbles and composition of gas. A given temperature and a given treatment duration may be suitable for removing bubbles from a rod having a given composition. If the duration of the treatment is decreased, the temperature must be increased correspondingly to achieve a similar result. Also, a successful procedure may include inserting a cane through the heated area of an oven at a given speed. If the speed is increased, the temperature must also be increased. Several factors affect the temperature of the treatment. Glasses containing aluminum oxide tend to crystallize. Therefore, the temperature needs to be high enough and the duration at high temperature needs to be short enough to avoid crystallization and crystal growth and, however, to remove the bubbles from said glasses. For glasses added with aluminum oxide, the prevention of crystallization is improved by rapidly cooling the heat-treated cane. This can be achieved by quickly removing the shank from the oven at the end of the heat treatment. The bubbles were successfully removed on silica glass rods added with germnanium oxide and up to about 2) 5% by weight of aluminum oxide, by passing the rods through the heated area of an oven at approximately 1440-150p ° OR. When the concentration of the aluminum oxide increased to an amount greater than about 2.35% by weight, an undesirable level of crystallization was observed. Even when less crystallization is observed in said canes with higher concentrations of aluminum oxide, after they were maintained at lower temperatures (1100-1 00 ° C) for approximately 2 hours, said lower temperatures did not reduce the number of bubbles. The benefits of the thermal treatment described above are not limited to glasses added with aluminum oxide, but also are applicable to other compositions that do or do not form crystals during processing. Said compositions include such indicators as ferric oxide, which is the doped! - with increasing refractive index that is used more extensively. When rods containing germanium oxide are heat treated, the temperature should not exceed 1725 ° C, the decomposition temperature of gernanium oxide. If the temperature of the treatment is high enough to decompose the germnanium oxide, the pre-existing bubbles may increase evenly in size. Another limitation on the procedure is the deformation of the cane by stretching, in a modality in which the rods were heated at 1495-1550 ° C and the insertion speed was about 6 mm / minute, the rañas were stretched high. around 0.25 m, an acceptable amount. However, the process run at a temperature of 1845 ° 0 and an insertion speed of 15 rnm / minute resulted in the formation of an unacceptable stretched rod having a diameter that decreased from 7 mm to 3"9 nm. . This invention is especially suitable for manufacturing optical fibers. However, it can also be used for bulk glass specimens. For example, glass rods obtained by an external deposition process can be separated into small pieces that are formed into optical devices such as lenses, windows, prisms and the like.The concreted dense glass preform can be stretched and treated thermally before forming the optical dies The heat treatment process of this invention is especially suitable for use in conjunction with the core rods of fiber amplifiers obtained by the OVD process. Nucleus rods were obtained from silica added with gernanium oxide, aluminum oxide and erbium oxide The precursor materials used to form the oxides are included in table 1.

TABLE 1 Reactive Oxide (s) Erbium oxide Er (F0DÍ3 Oxide of u inio AICI3 and Al (HFA) 3 Germanium oxide GeC Silica SiC Referring to Figures 3 to 5, a series of schematic diagrams illustrating an OVD method for producing a core rod is shown. Those skilled in the art are well aware of the individual steps of this process, and only those portions of the procedure that are necessary to explain the features of the invention are repeated here. For other detailed descriptions of the external vapor deposition process, those skilled in the art are referred to the pu.ent of F.U.A. Nos. 4,453,961, ,043,002, 5,211,732, 4,906,267 and 4,251,251, whose full descriptions are incorporated in the present invention as re fe re n c a 1 a. Figure 3 shows an OVD deposition system, L wherein the vitreous soot streams from the burners 24 and 26 are directed towards the mandrel 20 to deposit on them a < uei po poroso de soot vi tren) 22. Two burners are shown; the deposition runs were made using one or two burners. When two burners were used, a first burner was supplied with Er (F0D) 3 and Al (HFA) 3 or AICI3, and the second burner was supplied with S1CI and GeCl-i. The aluminum oxide precursor is sometimes fed to the second burner more than the first. TI mandrel is removed to form a porous body of soot 22 having an axial opening 25. The internal and external diameters are D0 and Di, respectively. In the next step (not shown), the porous body of soot 22 is dried and concreted to provide a dense glass preform 24. The concreted preform 24 has an outer diameter D2 that is substantially smaller than the diameter i of the porous body soot 22. The diameter D0 of axial opening 25 'is smaller than J to opening 25, but it is still recognizable. The dense glass preform 24 is then mounted in a conventional extraction oven, where the tip of the same is heated by heating means 26 (see Figure 5). A gas with a high helium content typically flows through the furnace muffle. A glass rod 28 can be fixed to the bottom of the preform 24. Impellers 32 push the rod 28 downwards, thereby driving a rod rod 30 in the form of a rod. A vacuum accessory (not Ib shown) is attached to the upper end of the preform 24. As the shank 30 is removed, the opening 25 closes quickly, since the pressure in it is low relative to the pressure of the environment. The shaft 30 of the core has a diameter D3 that is smaller than D2 - In one embodiment, D2 was approximately 38 inm, and D3 was approximately 7 nm, depending on which D3 is less than 20% of 2. The rod 30 The core is sufficiently long, so that it is typically separated into a plurality of sections. In the most successful mode of operation, the rod 30 was heated in an oven 42 (figure 5). The hot area of the oven 44, which was generated by heating means 45, was maintained at a temperature between 1490 and 1495 ° C. A clamping piece 46 or other suitable means supported one end of the shank 30. An axle 48 engaged the clamping piece 48 to a motor (not shown) that rotated the shank 30. The shank 20 was brought back and toward forward through the hot area 44 at a constant speed, preferably around 6 mrn per minute. During its passage in and out of the muffle, the rod was spun at a speed of approximately 3.5 revolutions per minute. After the cane is heat treated, it can be overcoated with silica lining glass and extracted into amplification fibers. The OVD process described above can be used to manufacture amplification fibers, the nuclei of which have a uniform aluminum oxide radial concentration of up to about 1.3% by weight. Some crystals begin to form along the region of collapse of the longitudinal axis at that concentration; however, many good fibers can be extracted from whites. The aluminum oxide content may be higher in fibers produced by external processes such as VAD which do not result in a collapse region of the longitudinal axis. It has been discovered that amplification fibers having excellent amplification spectrum configuration can be produced by maintaining the concentration of the aluminum oxide at levels up to about 2.35% by weight in all regions of the core, except the region of collapse of the longitudinal ee, wherein up to about 1.3% by weight is maintained. Two types of aluminum oxide profiles that were used are shown in Figures 7 and 8. The profiles of the erbium oxide were typically as shown in Figures 9 and 10. The average concentration of heavy erbium oxide in the area in the core region of the core cane was typically on the scale of about 0.3 wt% to about 0.5 wt% for discrete amplifier fibers. However, useful amplifier fibers with erbium oxide concentrations outside this scale have been obtained. Distributed fiber amplifiers contain very little erbium.

IH The profiles of the germ-oxide of the fibers were confi gured gradually with a small decrease in the concentration in the longitudinal axis. A sufficient amount of oxy or germenum was added to the core to provide a determined change in the refractive index. The concentrations of aluminum oxide of some of the fibers of this modality are given in Table 2. The concentrations of aluminum oxide listed are the maximum concentrations that occurred in the longitudinal eye region and in the rest of the nucleus. The concentrations were obtained by means of analysis of the cane with microprobe.

TABLE 2 P re f o rr o n idrio No, Percentage by weight of AI2O3 E e Longitudinal Core l 0.98 2.08 n 1.11 2.32 3 1.24 2.35 4 1.28 2.27 5 0.21 0.06 6 0.21 0.62 7 0.43 0.80 8 0.54 1.22 9 0.68 0.83 Each concreted glass preform was stretched to a glass rod that was separated into a plurality of core reeds of 7 mm diameter. Two of the rods of Form No. 3 were heat treated. The first thermally treated cane lacked crystals. The second rod of pro forma No. 3 had a few agglomerated crystals. Four of the reeds of No. 4 were processed thermally. One of these reeds had an individual crystal. The three remaining rods of reform No. 4 lacked crystals. The principles of the present invention can be applied to the method of impregnation of solution to liase of MCVD, in which a porous glass layer is concreted in helium (see U.S. Patent No. 5,262,365, which is incorporated in the present invention). as reference). According to that technique, glass having a relatively low refractive index is deposited on the inner peripheral surface of a silica glass substrate tube by means of an ordinary rtCVD method to form a coating glass layer. . Each step of the burner along the subsurface tube during the deposition of the coating layer produces a concrete layer, by means of which the entire coating layer is formed of a dense glass. A porous glass layer is deposited on the inner surface of the coating layer by means of a MOVD method carried out at a relatively low temperature. Subsequently, a rare earth element and aluminum in solution are introduced into the pores of the glass porous layer that forms the core until they are saturated with the solution. Then, the porous layer impregnated with the solution and forming the core is dried, dehydrated and concreted in a flow of helium gas to convert it into a dense non-porous glass layer. The coated substrate is heated until it collapses and eliminates the central opening, and is then extracted into fiber. The concreted dense glass layer can be subjected to thermal treatment of the type described in the present invention to eliminate bubbles that formed or trapped helium and the like. Prior to the passage of the heat treatment, the preform is heated and stretched to decrease the cross-sectional area of the concrete layer containing the aluminum oxide. Thus, the gas in the bubbles escaped more easily. The stretched preform t thermally tied can be extracted into fiber.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. A method for reducing bubbles in a glass article, comprising the steps of: depositing-glassy soot on a substrate to form an elongated cylindrical body, at least a portion of which is porous; drying and concreting the porous portion of said body to convert said porous portion to a dense glass having a given cross-sectional area; extracting the resulting glass preform to form a glass shank in which the cross-sectional area of said dense glass is smaller than said given cross-sectional area; and heat treating the cane at a high enough temperature to remove the bubbles and, for a short time, to avoid the crystallization of the cane.

2. The method of claim 1, wherein the deposition step comprises a step of deposition of a soot stream, selected from the group comprising the deposition of vitreous soot on the outer peripheral surface of a mandrel to accumulate a porous coating, and removing the mandrel, or depositing the vitreous soot on the end of a target rod and accumulating said coating in an axial direction, or depositing the vitreous soot on the inner surface of a substrate tube. 7

3. - A method for reducing the bubbles in a glass article, comprising the steps of: depositing the glass soot on a substrate to form a porous elongated cylindrical body having an outer diameter edge; drying and concreting the porous body to consolidate said porous body in a preform having a second diameter that is smaller than said first external diameter; extracting the preform to form a core rod having a third diameter that is smaller than said second diameter; and treat the cane at a fairly high temperature to remove the bubbles and, for a short time, to avoid crystallization of the cane.

4. The method of claim 1 to 3, wherein the vitreous soot contains an irnpupficant selected from the group consisting of erbium oxide, alumino oxide, germ-oxide, and combinations thereof.

5. The method of claim 1 or 1, wherein the vitreous soot contains aluminum oxide, the maximum concentration of which is up to about 2.35% by weight.

6. The method of claim 1 or 3, wherein the vitreous soot contains aluminum oxide, having an initial concentration in a central region of the cane and increasing in concentration at radial distances greater than the outer radius of said central region. .

7. The method of claim 1 or 3, wherein the step of deposition of vitreous soot comprises depositing an? '! a plurality of layers of soot on a mandrel to accumulate a coating, one or more of the first layers of soot deposited containing a first concentration of aluminum oxide, the concentration of the aluminum oxide being the rest of said plurality of layers greater than said first concentration.

8. The method of claim 7, wherein the concentration of the aluminum oxide in said central region is less than about 1.3% by weight, the concentration of the aluminum oxide increasing radially up to a maximum of up to about 2.35% by weight .

9. The method of claim 1, having one or more of the characteristics selected from a) the step of extracting the preform near the central opening, b) the temperature is between 1300 ° C and 1500 ° C, c) the temperature is greater than about 1450 ° C, d) the diameter of the cane is less than about 10 nm, and e) the diameter of the cane is between 5 nm and 7 mm.

10. An optical fiber added with aluminum oxide, wherein the aluminum oxide has a first maximum concentration in a central region, and a second maximum concentration greater than the first maximum concentration in a greater radius than that of said central region , optionally the aluminum oxide in the longitudinal axis of the rod being approximately 1.3% by weight and radially increasing to a maximum of approximately 2.35% by weight, optionally also containing an impinger selected from the group consisting of erbium, oxide of gernanium and combinations thereof.