METHOD FOR INTRODUCING DOPANTS IN OPTICAL FIBER PREFORMS

Abstract: A method for incorporating a metal dopant in the doped silica core of an optical fiber preform, the dopant precursor compound being a metal chelate having a sufficiently high vapor pressure whereby the chelate can be volatilized at relatively low temperatures (e.g. 100<o> to 250<o>C) and be readily incorporated in the gaseous core precursor compound streams normally used for the deposition of the doped core, the metal chelate being derived from a fluorinated beta -diketone ligand having the general formula (I), wherein R, R1 and R2 are perfluorinated alkyl groups consisting of 1-7 carbons or perfluorinated aryl groups.

Description: METHOD FOR INTRODUCING DOPANTS IN OPTICAL FIBER PREFORMS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an optical fiber preform and a method for producing the same, and more particularly to a method of incorporating dopants in the silica core of the preform. 2. Description of Related Art Optical communications systems, that is systems operating in the visible or near visible spectra, utilize clad glass fibers as the transmission medium. These clad fibers or optical fibers are produced by preparing a preform and then heating and drawing the preform into the fiber. The optical fibers are composed of silica and have an overall cross-sectional diameter of about 125 ym and are generally constructed of two sections, a transparent glass core and a transparent glass cladding, the cladding surrounding the core having a lower refractive index relative to the core, with a typical variation in refractive index from core to cladding being in the range of about 1.01 to 0.05. The core generally has a diameter of 50 ym. A method widely used for fabricating an optical fiber is to first prepare a preform, from which the fiber is drawn, using a chemical vapor deposition (CVD) method. The CVD method comprises the steps of depositing a thin layer of silica doped with various metals on the inner surface of a fused silica tube, the doped silica layer having a higher index of refraction than that of the outer surface of the tube or cladding, and then collapsing the resulting tube by heating to form a solid preform free from any interior space, with the thin doped layer then becoming the core. In the CVD process, layers of fused doped silica are built up on the inside of a long silica glass tube by the oxidation of vaporized core precursor compounds dispersed in an oxygen carrier resulting in the formation of metal oxide particles which deposit and are fused on the inner wall of the tube to form the doped core of the preform. When a fiber is drawn from the preform, the deposited fused metal oxide layer becomes the fiber core and the silica glass tube the cladding. The source of the vapor deposited core is generally a chloride of silica as well as desired dopants tailoring the index of refraction between the core and the cladding. The most widely used dopant materials include chloride compounds of, for example, germanium, titanium, aluminum and phosphorus which increase the index of refraction of the deposited silica core. The metal chloride dopant compounds (except aluminum) are normally liquids which are vaporizable at relatively low temperatures, e.g. 100° to 250°C and the vapors of such materials such as GeCl4, S1CI4, POCI3 or the like are entrained in a carrier gas such as oxygen and flowed as a vapor stream, at a temperature of about 100°C, into the interior of a glass cladding tube which is rotated while a torch which heats the tube to about 1000° to 1600°C repeatedly traverses its length. As the vapor stream passes through the tube and encounters a heat zone adjacent the torch, it reacts, pyrolyzing the metal compounds and forming oxides which deposit and fuse on the interior surface of the tube. After numerous traversals of the torch along the length of the tube to deposit the oxide core layer, the tube is heated to even higher temperatures (e.g., 1900° to 2000°C) by the torch in several traversals to shrink the tube and in a final traversal, the tube is collapsed, resulting in a solid rod-shaped preform. Thereafter, the solid collapsed preform is drawn into an elongated filament which comprises the optical fiber. It is known to the art that certain metals, e.g., cerium, arsenic, europium and samarium, when used to dope optical fiber preforms, aid in. reducing radiation damage due to ionizing radiation. Metal species such as Ce3+, Ce^ + , Sm3+ and Eu^ + , when incorporated in the core glass, act as ^electron sinks, and/or "hole" sinks which act to reduce the generation of optical absorption sites in a preferred spectral use region so tha't the optical transmission properties of the fiber drawn from the preform are maintained at an optimized level. Although the metals immediately enumerated above exhibit beneficial doping activity in optical fiber cores, these metals normally form compounds that have very low vapor pressures at temperatures of 100° to 250°C and it is, therefore, difficult, if not impossible, to vaporize these compounds to the level necessary for their entrainment in the gaseous metal chloride/02 streams normally used to prepare doped core layers for optical fiber preforms. Although it is known to the art that alco ' holates of these desirable doping metals can be vaporized at relatively low temperatures, e.g., 100° to 300°C, the oxidation of the alcoholates produces an undesirable water vapor by-product. Under the conditions prevailing in CVD processes, the water vapor by-product, in turn, generates OH~ which is an impurity that when it is incorporated in the doped core, produces undesirable transmission losses in the optical fiber at the various wavelengths of interest in optical communication systems. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a method for incorporating a metal dopant in the silica core of an optical fiber preform, the dopant precursor compound being a metal chelate having a sufficiently high vapor pressure whereby the chelate can be volatilized at relatively low temperatures (e.g., about 100° to 250°C) and thereby readily incorporated i- n the gaseous core precursor compound streams normally used for deposition of the doped core layer in 'the preform, the metal chelate being derived from a fluori¬ nated betadiketone ligand having the following general structures: 3 2 R2 Keto Enol where R, R^ and R2 are either fluorine or perfluorinated alkyl. The negative ion formed by the removal of the proton from the above keto-enol structures serves as a coordinating ligand to almost any positive ion of a metal element and forms with such ion what is known as a complex. The organo-metallic compounds thus derived from the coordination of the ligand ions and metal ions are known as metal β-diketonates or metal β-ketoenolates; for the purpose of this application, these compounds are referred to as metal β-diketonates. The metal β-diketonates have relatively high vapor pressures at temperatures of 100° to 250°C and as such may be readily vaporized in a helium stream for intro¬ duction into the gaseous 02/SiCl4 streams used to prepare the core layers of optical fiber preforms. As the metal β-diketonates used in the practice of the present invention contain limited hydrogen substituency and preferably do not contain hydrogen atoms, the opportunity for the creation and incorporation of water vapor derived impurities into the core layer of the preform is minimized or eliminated. As the metal β-diketonates can be sublimed or otherwise puri'fied before introduction into the gaseous core precursor streams, the presence of tramp contaminants in the metal chelates used in the practice of the present invention is also eliminated and therefore high purity dopant metal oxides are produced by the oxidative decomposition reaction used in the formation of the core layer. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an optical fiber preform prepared in accordance with the method of the present invention; FIG. 2 illustrates in " schematic form, an apparatus suitable for carrying out the process of the present invention; FIG. 3 illustrates in schematic form, an apparatus suitable for vaporizing a metal β-diketonate; FIG. 4 is a cross-sectional view of the apparatus of FIG. 3 illustrating electrical leads for a heat source; and FIG. 5 is a second cross-sectional view of the apparatus of FIG. 3 illustrating the relationship between a heat source and a dopant source. DETAILED DESCRIPTION OF THE INVENTION The metal β-diketonates used in the practice of the present invention are prepared by reacting a fluorinated β-diketone ligand having the formula: wherein R, Rx, and R2 are fluorine or perfluorinated alkyl or aryl, or combinations thereof, with a metal salt of the formula: M(X) n (2) wherein M is a metal cation, X is halogen, nitrate, sulfate or acetate and n is an integer which corresponds to the electroequivalence of M. Illustrative examples of such metals include copper, zinc, mercury, indium, lanthanum, cerium, praseodymium, neodymium, samarium, zirconium, chromium, uranium, manganese, iron, 'cobalt, nickel, platinum, palladium, cadmium, scandium, thorium, vanadium, gallium, thallium, yttrium, europium, gado¬ linium, hafnium, lead and plutonium, while the halogen ion, of course can be chlorine, bromine, iodine and the like. Illustrative examples of the fluorinated β-diketone ligand used to prepare the metal β-diketones include 1,1,1,3,5,5,5-heptafluoro-2,4-pentanadione; 3-trifluoro- methyl 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; and 1,1,1,2,2,3,3,7,7,8,8,9,9,10,10,10-hexadecylfluoro-5- trifluoromethyl-4,6-decanedione. The single H on the number 3 or 5 carbon is needed for the enol form to be made. Otherwise, R]_, R2 and R3 must be perfluorinated alkyls or perfluorinated aryls. R and 2 may range from 1 to 5 carbon atoms. In the preferred practice of the present invention, the β-diketonate ligand is completely devoid of hydrogen substituency to prevent the incorporation of hydrogen as OH" impurity in the doped core layer of the preform. The reaction to prepare the metal β-diket'onate from e.g., a metal chloride and fluorinated β-diketone is preferably conducted in the presence of an inert organic polar solvent such as CCI4 at a temperature of about 25° to 50°C. The fluorinated metal β-diketonates used in the practice of the present invention are typically solid compounds having a relatively high vapor pressure, e.g., about 10 to 50 mm Hg. at temperatures in the range of 200° to 250°C. As such, they can be readily vaporized and introduced into the gas.eous core precursor streams used to prepare optical fiber preforms. Dopant forming compounds normally used to prepare the doped core layer may also be used in the practice of the present invention and include, for example, metal chloride compounds which can be volatilized at relatively low temperatures, e.g., about 50° to 100°C, and function to increase the refractive index of the deposited silica layer. Examples of such metal chloride compounds include GeCl4, TiCl4, AICI3, POCI3 which oxidize to form the dopants Geθ2, Tiθ2, I2O3 P2O5, respectively. The detailed description which follows is presented with respect to metal chlorides; however, it will be readily appreciated that other metal salts may be employed in place of metal chlorides, as discussed above. In preparing the core precursor materials for admission to the interior of the tube from which the optical fiber preform is fabricated, the metal β-diketonates and the metal chloride core precursor compounds (which are generally liquids at room temper- ature) are individually heated to above their vaporization temperature and carried along by a carrier gas such as He or O2, and the individual metal β-diketonate and metal chloride vapors are then mixed with oxygen gas which acts as a carrier for the vaporized mixture of metal β-diketone and metal chlorides whereby the vaporized mixture is entrained in the gaseous oxygen carrier. Generally, the gaseous mixture flowed into the tube from which the preform is fabricated is comprised of about 90 to 94 volume percent oxygen, about 2 to 4 volume percent SiCl4, about 0.1 to about 0.4 volume percent of the metal β-diketonate, and about 2 to about 3 volume percent of the various other dopant metal chloride compounds. When the vaporized metal β-diketonate/metal chloride compound mixture entrained in the oxygen gas carrier is flowed through the interior, of a silica tube which is heated at a temperature of about 1000°C to 1900°C and preferably about 1700° to 1800°C, the metal β-diketonate and metal chlorides are decomposed and oxidized to their respective oxides and uniformly deposited onto the interior surface walls of the tube by thermophoresis whereby the surface layer of the tube is modified with a doped silica layer of a higher refractive index than the tube walls. The time required for the deposition of the doped core layer depends upon the ' flow rate of the gaseous mixtures and the concentration of the various core forming compounds, with the tendency that the greater flow rate (within the limits of achieving deposition of the decomposition products) and the higher the concen- trations of the various core forming compounds, the shorter the processing time to deposit the doped core layer. Using the volume percent concentrations of metal β-diketonate and core precursor compounds enumerated above, a core layer of 250 μm thickness can be deposited in about 2 to 3 hours in combination with a traveling heat source, the heat source being repeatedly or recipro¬ cally moved a required number of times, e.g., 50 at a speed of 10 cm/min to maintain the walls, on which the core layer is deposited, at a temperature of about 1000°C to 1900°C. Generally, the -gaseous metal β-dike ' tonate and core precursor compounds are flowed through the tube at a pressure of about ambient. In the preferable embodiments of the invention, the gaseous mixture of metal β-diketonate and core precursor compounds is used at a total pressure of about 750 to 760 mm Hg. and a flow rate of about 1000 to 3000 cc/min. FIG. 1 depicts a portion of an optical fiber preform 10 constructed of a glass core 12 formed from a plurality of doped silica layers and an outer wall or cladding 11. FIG. 2 is a schematic drawing of a glass working lathe 20 which can be used to fabricate optical preform 10. Glass lathe 20 comprises apparatus known in the art and includes rotary chucks 22 and 22' which grasp tube 14 and provide rotary motion as required. The internal mechanical and drive structure of chucks 22, 22' are not shown for clarity in presenting the elements of the present invention. The starting glass tube 14 , which will form outer wall 11, is mounted between synchronous rotatable chucks 22-22" . As tube 14 is rotated, as for example, in the direction shown by the arrow 15, it is heated by a gas hydrogen/oxygen burner 26 which repeatedly traverses its length slowly from left to right and makes a fast return to the left after each traversal. Gaseous materials such as SiCl4, metal -diketonates and other gaseous dopants such as, but not limited to GeCl4 and POCI3 are introduced into tube 14 during heating through a manifold 24. These gaseous materials will be deposited on inner wall 16 where they react to form layers 18 which become part of core 12 as discussed further below. Manifold 24 receives the input gases through a tube or tubes which are in turn connected to source material reservoirs, not shown, which are normally flasks or other containers containing liquid chloride compounds heated to the vaporization temperature of the individual compounds. Manifold 30 is a gas tight chamber providing a substantially sealed transfer volume between the source materials and tube 14. It is necessary that manifold 24 provides a substantially gas tight rotary seal 25 where it contacts tube 14. For this reason the preferred embodiment of manifold 24 is a chamber constructed from polytetrafluoroethylene material which provides a gas tight seal while permitting unrestrained rotary motion. The material properties of metal -diketonate sources generally prevent their introduction into manifold 24 in the same manner as other gaseous materials. Instead, a special heat probe 30 is employed which vaporizes the -diketonate material within the interior of tube 14. Heat probe 30 is shown in position in FIG. 2 inserted into manifold 24 through a gas tight seal 27 and into the interior of tube 14. Seal 27 is such that probe 30 can be moved axially within the interior of tube 14 in order to provide optimum positioning of the β-diketonate source. Heat probe 30 is canitilevered into tube 14 and manifold 24 by use of the walls of the probe and seal 27.. However, it will be apparent to those skilled in the art that additional clamping means or guiding and support structure, not shown, can be employed in order to position probe 30 within tube 14 and maintain a gas tight seal for manifold 24. A more detailed sectional view of heat probe 30 is provided in FIG. 3 which shows the interior details for vaporizing the β-diketonate material. The main structure of heat probe 30 comprises a fused silica tube 34. Within heating probe 30, there is placed a fused silica boat 40 which contains pressed pellets 42 or loose powder of the metal β-diketone to be vaporized. The heating probe is provided with a fused silica porous filter plug 38, through which the vaporized metal β-diketonate is exhausted and passed to the inner surface 16 of silica tube 14. Heating probe 30 is further provided with heating elements 50 which comprise resistive chromium/nickel wire connected by copper electrical leads 52 to an electrical control device, not shown for clarity, whereby heating elements 50 are activated, by applying a controlled voltage thereto, to raise the temperature of the elements to a predetermined elevated temperature at which pellets 40 are vaporized. Heating elements 50 are enclosed in a fused silica liner 54 to prevent any contact between the metal leads 52 and the gas stream. Leads 52 are enclosed in fused silica tubing 56 which is joined to the liner 54 on one end and exits through the walls 34 of probe 30 at the other end. This is illustrated in FIG. 3 and FIG. 4. The radial position of lead ' s 52 is illustrated in FIG. 4. FIG. 5 further illustrates the radial disposition of elements 50 about the circumference of probe 30. High purity helium is ' introduced into heating probe 30 from a source (not shown) through a valve 32 at a flow rate of 1200 to 1500 cc/min. The helium gas is passed through probe 30 and over the heated metal β-diketonate pellets 40 whereby the vaporized metal β-diketonate is entrained in the flowing helium gas and the gaseous mixture is immediately exhausted from the tube via outlet 38 and into tube 14 of the preform fabricating apparatus 20. Actually, the heated section is inside tube 14, about 15 to 20 cm away from the maximum return position of the flame. In this way, the dopant is introduced very close to the heated zone. The gaseous metal β-diketonate/He mixture passed into tube 14 is comprised of about 0.1 to 2 volume percent metal β-diketone and about 98 to about 99.9 volume percent helium. The metal β-diketonate/He gaseous mixture flowed into tube 14 is caused to be admixed with the other core forming gaseous materials introduced through manifold 24 and this mixture of gaseous materials is passed into interior of tube 14. As the mixture of metal β-diketonate and other core forming gaseous components encounters the heat zone (approximately 1000° to 1900°C) caused by the burner 26, the gaseous compounds are decomposed and reacted with oxygen to produce deposits which are generally oxides of the gaseous metal compounds which serve as core precursor materials. Thus, as the oxidized gaseous metal β-diketonates and metal chlorides are decomposed and converted to oxides, they are deposited by thermophoresis and fuse on the interior surface of tube 14, as a core layer, and unconsolidated soόt and extra carrier gases pass out of tube 14 via exhaust line 28. A plurality of traversals of burner 26 (e.g., about 50) are accomplished until a predetermined thickness of core layer 12 is attained. After the requisite deposit of doped core layer 12 has been attained, tube 14 is heated to a higher temperature, e.g. , 1900° to 2300°C, to cause the tube to soften, shrink and finally collapse to form the solid optical fiber preform 10 shown in FIG. 1. This is accomplished by elevating the temperature of burner 26 to provide a localized heat zone which is slowly traversed (e.g. , about 0.5 to 0.2 mm/sec) along the tube 14 to effect localized softening of the tube wall. A number of traversals (e.g. , 6) of the elevated heat zone along the length of tube 14 progressively shrinks the diameter of the tube until, on a final collapse traversal which moves the heat zone from left to right, the tube with core layer 12 deposited therein, is completely collapsed to form preform 10 in the absence of any gas pressure. The present invention is understood more readily with reference to the following example. The example, however, is intended to illustrate the present invention and is not to be construed to limit the scope of the present invention. EXAMPLE A typical metal chelate (cerium diketonate) was formed as follows: 18.25 g (25 mol) Ce(S04)2 were dissolved in 200 ml water and neutralized with NH4OH to pH of about 10. To this solution was added 31.6 gm (.14 mol) 1,1 ,1 ,5,5,5-hexafluoro-2,4-pentanedione dissolved in 200 ml benzene. The aqueous phase was maintained alkaline with periodic additions of NH4OH. The mixture was allowed to react for 1 1/2 hrs at 70°C under reflux. The mixture was cooled and the immiscible layers separated in a separatory funnel. The aqueous phase was discarded. The benzene was removed by distil¬ lation. Benzene was then added to the solid residue and water removed by azeotrbpe distillation. A yield of ~50 gm yellowish solid was obtained for a yield of ~80%. No melting point data was collected. In a typical preparation of a doped preform, tris(6,6,7,7,8,8,8-heptafluoro-2,2-diemthyl-3,5-octane dionato)europium, available commercially from Aldrich Chemical Co. (Milwaukee, WI) was used as the dopant. The crystalline solid was pressed into 2 mm diameter pellets and 1.5 to 2 grams of these pellets were charged to a fused silica boat which was placed in the heater section 23 of probe 30 shown in FIG. 3. A flow of dry helium gas was initiated through the probe and the heater elements were activated to raise the temperature of the boat contents to 200° to 250°C to effect the vaporization of the metal β-diketonate. A tube 14 of high purity fused silica of 50 cm length x 18 mm ID x 24 mm OD sealed to larger "handles" of commercial silica on each end was first cleaned by immersion in hydrofluoric acid solution for 3 minutes and was rinsed with deionized water, followed by methanol and dried by passage of 2 for several hours. The tube was mounted in an apparatus of the type shown in FIG. 2. Core precursor compounds were deposited on the interior wall surfaces as the tube was rotated at 100 rpm. Before the core precursor compounds were introduced into the interior of the tube, the tube was flushed with a continuous stream of- oxygen flowed into the tube at a flow rate of about 1500 to 2000 cc/min while traversing with the burner 26 a sufficient number of times to bring the wall temperature of the tube to 1900°C in order to volatilize any impurities present on the interior wall surface. Following a period of about 15 seconds, a mixture of gaseous SiCl4, GeCl4 and POCI3 was introduced into the interior of the tube entrained on an oxygen carrier, the flow rate being 58 cc/min, SiCl4, 49 cc/min, GeCl4, 0.24 cc/min, POCI3 and 1500 cc/m'in O2. the gaseous mixture being comprised of approximately 3.5 percent SiCl4, 2.9 percent GeCl4 and 90 percent O2. Cojointly with the introduction of the core precursor chloride compounds, a mixture of about 9.0% of vaporized europium heptafluoropentanedionate in helium carrier (500 cc/min) in probe 30 was mixed with the gaseous chloride compounds the interior of the tube to yield a total flow rate of approximately 2000 cc/min. Additional passes of the burner were made over a 3 hour period with the parallel flow of SiCl4/GeCl4/POCl3/θ2 core pre¬ cursor compounds and europium heptafluoropentanedionate/ He gas mixture through the tube interior. Collapse of the tube was initiated by reducing the rate of traverse of the burner so that the temperature of the tube walls was raised to 2300°C. After final collapse and cooling, there was obtained a finished preform having a diameter of 7 mm composed of a silica cladding, and a doped silica core of 2 to 3 mm diameter composed of approximately 83% Siθ2, 15% Geθ2, 2% P2O5 and <0.03% EU2O3. When tested for U.V. fluorescence by illumination with light of predominantly 254 nm (Hg discharge), the core emitted a red fluorescence indicating the presence of Eu + 3 ion. While specific components of the present invention are defined in the working .example above, many other variables may be introduced which may in any way affect, enhance or otherwise improve the present invention. These are intended to be included herein. Although variations are shown in the present application, many modifications and ramifications may occur to those skilled in the art upon reading the present disclosure. These, too, are intended to be included herein.

Claims: CLAIMS What is Claimed is: 1. A method for producing an optical fiber preform comprising the steps: (a) flowing a gas mixture of a volatile silica compound and a volatile metal chelate entrained in a gaseous oxygen carrier through the interior of a silica glass tube, the metal chelate being derived from a fluorinated β-diketone ligand having the general formula: 0 Ri 0 II I B R- C C — - c -R 2 H where R, R_ and R2 are radicals selected from the group consisting of fluoroalkyl; (b) oxidizing the gas mixture to deposit a silica layer doped with an oxidized metal of the chelate on the interior walls of the tube and then; and (c) collapsing the tube. 2. The method of Claim 1 wherein the metal chelate is the reaction product of the fluorinated β-diketone ligand and a metal salt having the formula: M(X) n wherein M is a metal cation, X is a halogen, acetate, sulfate or nitrate and n is an integer which corresponds to the electroequivalence of M. 3. The method of Claim 1 wherein the metal is a rare earth metal. 4. The method of Claim 1 wherein the metal is europium. 5. The method of Claim 2 wherein the metal salt is europium trichloride acetate or nitrate. 6. The method of Claim 1 wherein the β-diketone ligand is 1,1,1,3,5,5,5-heptafluoro-2,4,pentanedione. 7. The method of Claim 1 wherein the β-diketone ligand is 3-trifluoromethyl-1,1,1,5,5,5-hexafluoro-2,4- pentanedione. 8. The method of Claim 1 wherein the β-diketone ligand is 1,1,1,2,2,3,3,7,7,8,8,9,9,10,10-hexadecyl- fluoro-5-trifluoromethyl-4,6-decane^ione. 9. The method of Claim 1 wherein the metal chelate is europium heptafluoropentanedionate. 10. The method of Claim 1 wherein the gas mixture is flowed through the interior of the tube while the tube is heated at a temperature of about 1000° to 1600°C. 11. The method of Claim 1 wherein the metal chelate is vaporized by heating the chelate to a temperature of about 100° to 250°C.

Inventors: BLAIR RICHARD G, BLAIR, RICHARD, G.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1986
Main classification: C03C13
IPC codes: C03B37, C03C1, C03C13, G02B6