METHOD OF FABRICATING OPTICAL FIBER PREFORMS HAVING REDUCED SUSCEPTIBILITY TO RADIATION DAMAGE

Abstract: A method for producing an optical fiber preform (10) having a core portion of reduced HO<-> impurity content, wherein a doped silica core layer (12) is deposited onto the inner surface of a tube formed of fused silica in the presence of a reactive gas mixture of iodine and oxygen, the doped silica core layer (12) being formed by oxidizing chlorides of silicon and dopant elements to their respective oxides and then collapsing the tube (11) to form the preform (10).

Description: I METHOD OF FABRICATING OPTICAL FIBER PREFORMS HAVING REDUCED SUSCEPTIBILITY TO RADIATION DAMAGE 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 an optical fiber preform having reduced susceptibility to radiation damage. 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 glass fibers or optical fibers are produced by preparing a preform, and heating and drawing the preform into the fiber. The optical fibers are composed of silica and have an overall cross-sectional diameter of less than approximately 500 ym and are generally constructed of two sections: a transparent glass core and a trans- parent 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 0.01 to 0.05. The core layer generally has a diameter of greater than approximately 50 ym. A method widely used for fabricating an optical fiber is to first prepare a preform using a chemical vapor deposition (CVD) method. The CVD method comprises the steps of depositing a thin core layer composed 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 of cladding and then collapsing the resulting tube by heating to a solid preform free from any interior space. 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 oxides become the fiber core and the silica glass tube the cladding. The source of the vapor deposited core is generally a chloride, hydride of other compound of silicon as well as desired dopants tailoring the index of refraction between the core and the cladding. Dopant materials include compounds of, for example, germanium, titanium, aluminum and phosphorus which increase the index of refraction of the deposited silica core layer. In particular, liquid metal chloride materials such as GeCl4, SiCl4, POCI3 or the like are vaporized and entrained in a carrier gas such as oxygen and are flowed as a vapor stream at a temperature of about 40°C into the interior of the fuse tube which is rotated as a torch, which heats the tube to about 1000°C-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 creating oxides which deposit 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°C-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 that OH" containing impurities in optical fibers produce increased transmission losses at various wavelengths of interest in optical communications sytems. Transmission losses are particularly acute when the OH~ contaminated optical fiber is exposed to gamma radiation, thereby further limiting the use "of the fiber in radiation environments. The OH" impurities are contained in the core section of the fiber and are derived from water vapor which is an ubiquitous impurity that is often uncontrolled in all phases of materials in preparation and processing. Such water vapor impurities are frequently present in the core precursor chlorides used in the preparation of the optical fiber preform, and the removal of these impurities from the oxide core layer deposited by the CVD process is critical to the satisfactory performance and radiation resistance of the optical fiber prepared from the preform. SUMMARY OF THE INVENTION In accordance with the practice of the present invention, there is provided a method for reducing the amount of OH~ impurity in the core of an optical fiber preform, wherein the core is formed of metal oxide compounds deposited by the oxidation of a gaseous stream of corresponding metal chloride compounds passed into the interior of a silica tube from which the preform is formed, the method comprising incorporating a gaseous reactive mixture containing iodine and oxygen into the gaseous stream of metal chloride compounds whereby the I2/O2 reacts with the gaseous chloride compounds and deposited oxide compounds to remove OH~ impurities from 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 is a photomicrograph of optical fiber preform cross sections cut across the length of the preform wherein the metal oxide core of a first preform was deposited by the oxidation of volatilized metal chloride compounds in the presence of an I2/O2 reactive gas mixture in accordance with the present invention and is compared to a second preform having a similarly prepared core except that a CI2/O2 reactive gas mixture was substituted for the I2/O2 gaseous mixture of the present invention; and FIG. 4 presents infrared transmission curves for optical fibers prepared from preforms wherein the core layer is deposited by the oxidation of gaseous metal chloride compounds in the presence of an I2/O2 reactive gas mixture in accordance with the present invention, as compared to the transmission curve for a similarly prepared core except that a CI2/O2 reactive gas mixture was substituted for the I2/C2 gaseous mixture in the preparation of the core layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description relates to a method of reducing the level of OH~ impurities in the metal oxide core layers of optical fiber preforms wherein the core is deposited by a CVD process using metal chloride compounds entrained in a gaseous oxygen carrier. By incorporating an I2/O2 reactive gas' mixture in the gaseous metal chloride compound stream fed to the interior of the preform tube, removal of OH~ impurities is effected from the deposited metal oxide core layer. The following reaction sequences involving the SiCl4 component are believed to be typical of the process: In the CVD process, SiCl4 is oxidized as follows: SiCl + 0 2 > Siθ2 + 2CI2 (1) The CI2 produced by the oxidation of the SiCl4 interacts with I2 in the I2/O2 reactive gas mixture in accordance with the equation: 12 + CI2 > 2IC1 (2 ) The compound IC1 dissociates more readily than CI2 to form atomic chlorine and iodine in accordance with the equation: IC1 > I + Cl (3) At the temperatures employed for the CVD process, I2 dissociates more readily than CI2. For instance at 1000°K, the equilibrium dissociation constant of I2 is four orders of magnitude larger than Cl 2 » I2 dissociates in accordance with the equation: l 2 21 ( 4 ) The availability of atomic iodine serves as a catalyst for the dissociation of CI2 in accordance with the equation: I + Cl 2 > IC1 + Cl (5) The overall effect of equations (3) and' (5) is: C i 2 > 2 Cl Thus, the presence of the I 2 in the process of the present invention serves to enhance the formation of atomic chlorine, i.e., by reaction of CI2 with I 2 or I. The enhancement effect on the dissociation kinetics of CI2 ranges from one to two orders of magnitude as the temperature increases from 1000°K to 2000°K. The presence of the chlorine and iodine atoms the gaseous core precursor compound streams passed into the preform tube remove hydrogen impurities from the deposited core oxides by displacing hydroxyl ion therefrom in accordance with the equation: X + OH- > X- + OH (6) where X is Cl or I. The tradeoff of X~ for OH~ as illustrated by Equation (6) does not stop there. The condensed phase that has been cleansed of its OH" content thereafter loses the X" by another displacement reaction, as follows: 1/2 0 2 + 2X~ > 0 = + X 2 +, (7) which illustrates the active role of the carrier gas, O2. The free hydroxyl species so formed volatilizes as an unreacted species or as the reacted species XOH and is thereby removed from the deposited core layer in the stream of core precursor gas which passes through the tube interior. Since the CVD process is effected by the passage of metal chloride compounds in the form of a vapor coiointly with oxygen gas, it is preferable that the metal chloride compounds have a vapor pressure so that they are vaporizable and flowable in the form of a gas, at relatively low temperatures, e.g., 50°C-100°C. It is critical to the practice of the present invention that metal chlorides be used as the core precursor compounds, as iodide compounds are impractical for CVD practice; the iodides are less volatile and more thermally unstable than the chlorides. Thus, whereas SiCl4 has a boiling point of 58°C, SU4 does not achieve ebullition until 288°C. The dopant chloride compound GeCl4 has a boiling point of 85°C whereas Gel4 vaporizes with decomposition at 440°C. Dopant forming compounds which may be used in the practice of the present invention are, for example, metal chloride compounds which can be volatilized at relatively low temperatures, e.g., 50°C-100°C, and which increase the refractive index of the deposited silica core layer. Examples of such metal chloride compounds include GeCl4, TiCl4, AICI3, POCI3 which oxidize to form the dopants Geθ2, Tiθ2 AI2O3 and P2O5. respectively. In preparing the core precursor materials for passage to the interior of the tube from which the optical fiber preform is fabricated, the metal chloride compounds, which are generally liquids at room temper¬ ature, are heated to above their vaporization temperature and the so-produced metal chloride vapors are mixed with oxygen gas which acts as a carrier for the vaporized metal chlorides, whereby the metal chlorides are entrained in the gaseous carrier. Generally, the gaseous mixture passed into the tube is comprised of SiCl4, a dopant metal chloride, and oxygen in sufficient proportions so as to result in a core of 80% Siθ2 and 20% metal oxide. When the vaporized metal chloride compounds entrained in the oxygen gas carrier are flowed through the interior of a fused silica tube which is heated at a temperature of about 1,700°C to about 2,000°C and preferably about 1,800°C to about 2,000°C, the metal chlorides are oxidized to their respective oxides and uniformly deposited onto the interior surface walls of the tube followed by diffusion, dissolution or chemical reaction with the fused silica, whereby the surface layer of the tube is modified with a doped silica layer of a higher refractive index than the tube walls. The cojoint passage through the interior of the tube of the reactive I2/CI2/O2 gaseous mixture removes any OH" existing on the surface of the deposited metal oxide core layer so that the collapsing of the tube following the desired deposition of the doped core ■layer produces a preform core which is substantially free of OH" impurity.. To effect the removal of OH~, the gaseous I2/O2 mixture generally comprises approxi- mately 13 volume percent I2. The time required for the deposition of the doped core layer and the scrubbing of the deposited layer with the I2/CI2/O2 reactive gas mixture to remove OH~ impurities depends upon the flow rate of the gaseous mixtures and the concentration of the various core forming compounds and reactive gas components, with the tendency that the greater the flow rate and the higher the concentrations of the various core forming compounds and reactive gas components, the shorter the processing time to obtain a deposited doped core layer substantially free of OH" impurity. Particularly advantageous results are obtained when the gaseous mixture of core precursor compounds preferably comprises about 73 volume percent SiCl4, 27 volume percent of the dopant compounds and the I2/O2 reactive gas mixture preferably comprises about 13 volume percent I 2 in the oxygen carrier gag. Using such concentrations of compounds in combination with a travelling heat source, a core layer of about 3 mm in diameter which is substantially free of OH", can be deposited in less than about 3 to 6 hours. The heat source must be repeatedly or reciprocally moved a required number of times (e.g., 50) at a speed of 20 per hour to maintain the walls, on which the core layer is deposited, at a temperature of 1000°C-1600°C. FIG. 1 depicts a portion of an optical fiber preform 10 constructed of a glass tube having an outer wall 11 and a core 12 formed from a plurality of doped silica layers. FIG. 2 is a schematic drawing of a glass working lathe 13 which can be used to fabricate the optical preform 10. The starting glass tube 11 is mounted between synchronous rotatable chucks 14-14' . As the tube 11 is rotated, as for example, in the direction shown by the arrow 15, it is heated by a gas oxygen burner 16 which repeatedly traverses its length slowly - from left to right and makes a fast return to the left after each traversal. Metal chloride compounds which are to " be oxidized to metal oxides and deposited on the inner wall of the glass tube 11 to form the core 12 of the preform 10, such as SiCl4 and dopants GeCl4 and POCI3, are heated to their vaporization temperatures and introduced into manifold 17 via inlet tubes 17a-d, which are in turn connected to heated source material reservoirs, not shown. The carrier gas, oxygen, is admitted to the manifold 17 via inlet 17d from the reservoir not shown. The vaporized metal chlorides and oxygen are mixed in the manifold 17 so that there is emitted from the manifold a constant stream of the mixture of vaporized silicon tetrachloride and dopant metal chlorides dispersed in the gaseous oxygen carrier which flow into the central opening 18 of the tube 11 through inlet pipe 17' . Also connected to manifold 17 is inlet 17e through which a gaseous reactive mixture of dry oxygen and iodine comprised approximately of about 13% iodine in the dry oxygen is passed into the manifold 17 from the iodine reservoir 19. In preparing the gaseous mixture of oxygen and iodine, high purity (99.99%) oxygen from a source, not shown, is passed at a flow rate of 25 cmVmin into a cold trap, also not shown, where residual water is removed from the oxygen by freezing in a dry ice/acetone mixture contained in the trap. The dried oxygen is then passed into the iodine reservoir 19 containing iodine vapor. The iodine vapor.is produced by activating heat elements 20. to raise the temperature in the reservoir 19 to approximately 110°C. The dry oxygen passed into the reservoir 19 from inlet I7f mixes with the iodine vapor, and gaseous reactive I2/O2 mixture is caused to flow out of the reservoir 19 via outlet 17e into the pipe 17'. Not shown is the arrangement of mixing valves and shutoff valves which may be utilized to meter flows- and make other necessary adjustments to the various gaseous compositions passed into the pipe 17' and thereafter in the central opening 18 of the tube 11. The I2/O2 reactive gaseous mixture is caused to be admixed with the core forming gaseous metal chloride compounds in the pipe 17' and this mixture of gaseous materials is passed from the pipe 17' through input gas line 21 into the interior 18 of the tube 11. As the mixture of core forming, oxygen dispersed metal chloride compounds and I2/O2 reactive gas mixture encounters the heat zone (approximately 1400°C-1600°C) caused by the burner 16, the metal chloride compounds are decomposed and reacted with oxygen to produce deposits which are composed of oxides of the metal chlorides. Thus, as the oxidized metal chlorides are converted to their respective oxides, the oxides are deposited on the interior surface of the tube 11 ahead of the heat zone on the cooler surfaces of the tube 11. Gaseous and solid particles not deposited on the interior surface of the tube 11 as a doped core layer pass out of the tube 11 via exhaust tube 23. A plurality of traversals of the burner 16 (e.g., about 50) are accomplished until a predetermined thickness of the doped core layer 12 is attained. Concommitant with the deposit of the d.oped core layer, the cojoint presence of the I2/CI2/O2 reactive mixture effects a reaction with OH" impurities existing at the surface of the deposited core layer which are then removed as volatile unreacted hydroxyl species. After the requisite deposit of doped core layer 12 has been attained, the tube 11 is heated to a higher temperature (e.g., 1900°C-2000°C) to cause the tube 11 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 the burner 16 to provide a localized heat zone which is slowly traversed (e.g., less than 0.5 mm/sec.) along the tube 11 to effect localized softening of the tube wall. A number of traversals (e.g., 6) of the elevated heat zone along the length of the tube 11 progressively shrinks the diameter of the tube until, on a final collapse traversal which moves the heat zone from right to left, the tube with the doped core layer 12 deposited therein, is completely collapsed to form the preform 10. The present invention provides, as described above, a method of fabricating an optical fiber preform which may be used to form optical fibers which exhibit reduced susceptibility to transmission loss, and particularly to gamma radiation exposure, by reducing the OH" impurity in the core layer. The present invention will be 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 tube of commercial grade fused silica of 0.5 meter length 13 mm ID x 20 mm OD was first cleaned by immersion in hydrofluoric acid-nitric acid solution for 3 minutes and was rinsed with deionized water. The tube was mounted in an apparatus of the type shown in FIG. 2. Doped core material was deposited on the interior wall surfaces as the tube was rotated at 100 rpm. Before the doped core precursor compounds were introduced into the interior of the tube, the tube was flushed with a continuous stream of an I2/O2 mixture comprising 13% 12 in oxygen flowed into the tube at a flow rate of 25 cm^/min while traversing with an oxyhydrogen burner sufficient to bring the wall temper¬ ature of the tube to 700°C to 800°C in order to remove or scrub any OH" impurities present on the interior wall surface. Following a period of about 10 minutes, a mixture of gaseous SiCl4, GeCl4 and POCI3 was introduced entrained in an oxygen carrier, the flow rate being 80 cm 3 /min for SiCl4, 186 cm 3 /min for GeCl 4 , 37 cmVmin for POCI3, 150 cm 3 /min for O2, and 750 cm 3 /min for He. Parallel with the introduction of the chloride compounds, a mixture of about 13% iodine in oxygen was flowed into the interior , of the tube at a flow rate of approximately 25 cm 3 /π.in. Forty additional passes of the burner were made over a 2 hour period with the parallel flow of SiCl4/GeCl4/POCl3/θ2 core precursor compounds and I2/O2 reactive gas mixture through the tube. After this period of time, the flow of doped core precursor compounds was discontinued and the flow of the I2/O2 reactive gas mixture was continued for several additional passes of the burner. Collapse of the tube was initiated with the l2/°2 9 as mixture still flowing by reducing the rate of traverse of the burner so that the temperature of the tube walls was raised to 2,200°C. After almost complete collapse of the tube, the I2/O2 flow was stopped with a final collapse producing a finished preform having a diameter of 10 mm composed of a silica cladding, and a doped silica core of 3 mm thickness. To determine the effect of gamma (γ) radiation on the transmission properties of the finished optical fiber preform, a section A was cut across the length of the preform and exposed to 10 6 rads of γ-radiation from a cobalt-60 source. In FIG. 3, there is shown a photomicrograph of the irradiated finished preform Section A. For purposes of comparison, the procedure of the Example was repeated to prepare an optical fiber preform, with the exception that a CI2/O2 reactive gas mixture with CI2. resulting from the dissociation of the metal chlorides, was substituted for the I2/CI2/O2 reactive gas mixture used in the Example. A section B was cut out of this comparative preform which was also exposed to 10 6 rads of γ-radiation. FIG. 3 also contains a photomicrograph of this irradiated comparative preform. An examination and comparison of photomicrographs of preform sections A and B immediately indicate that the core 12 of preform A had undergone substantially less γ-radiation damage than the core 12a of comparative preform B. The cross-sections of the preforms were obtained by cutting the preform and polishing the ends. Exposure to γ-radiation darkens the fiber preforms after 1 mega-rad dose. The remarkable feature here is the difference in transparency between the I2/CI2/O2 reactive atmosphere (RAP) preform (12), and the CI2/O2 RAP preform. A portion of the preform prepared in the Example was drawn to result in a fiber having an overall diameter of approximately 175 ym, with a doped core area defined as a region within the fused silica outer layer having a diameter of approximately 100 ym. The length of fiber drawn was approximately 500 m. Portions of the drawn fiber were then exposed to varying doses of γ-radiation from a cobalt-60 source. The induced transmission loss at 0.82 μm wavelength at the different radiation doses was measured by monitoring the transmission during irradiation. Co^O radiation was used in these tests. Curve A in FIG. 4 presents the induced transmission loss data for the first optical fiber prepared in accordance with the Example which had been subjected to varying doses of γ-radiation. Curve B presents the induced transmission loss data for the second optical fiber obtained from the same preform as in A, this second optical fiber having also been irradiated with varying doses of γ-radiation. For purposes of comparison, the procedure of the Example was repeated to prepare an optical fiber, with the exception that a reactive mixture of CI2 in oxygen was substituted for the I2/CI2/O2 reactive gas mixture, the CI2 obtained from the dissociation of the metal chlorides. A portion of the comparative optical fiber was exposed to varying doses of γ-radiation. Curve C in FIG. 4 presents the induced transmission loss data for the comparative optical fiber. A comparison of the data recorded in Curves A and B of the graph in FIG. 4 with that of Curve C immediately indicates that the comparative optical fibers prepared from a glass-preform in which the core was deposited under an CI2/O2 atmosphere undergoes a substantially greater transmission loss than optical fibers prepared from a glass-preform in which the core was deposited under an I2/CI2/O2 atmosphere in accordance with the present invention. Thus, Curve A indicates that the optical fiber prepared from a first preform processed in accordance with the practice of the present invention at dosages up to 2 x 10 1 * rads undergoes an initial induction, or radiation-dose threshold behavior during which there is little transmission loss. Curve B indicates that the second optical fiber prepared from the first preform processed in accordance with the present invention does not undergo a transmission loss until the fiber is subjected to a radiation dose of 15 x 10 1 * rads. Curve C, on the other hand, indicates that the comparative optical fiber immediately encounters a significant transmission loss when exposed to γ- radiation. At a dose of about 2 x 10 1 * rad, the transmission loss in the comparative fiber rises to about 95 db/km above the baseline value, while the fibers prepared from preforms processed in accordance with the present invention exhibits an incremental transmission loss of only 10 db/kπt. 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 a highly radiation- damage resistant optical fiber preform having a core portion of reduced OH" impurity content, comprising depositing a doped silica core layer onto the inner surface of a tube formed of fused silica in the presence of a reactive gas mixture of iodine and oxygen, the doped silica core layer being formed by oxidizing chlorides of silica and dopant elements to their respective oxides and then collapsing the tube to form the preform. 2. The method of Claim 1 wherein the dopant elements are selected from the group consisting of Ge, Ti, Al and P. 3. The method of Claim 2 wherein the dopant elements are Ge and P. 4. The method of Claim 1 wherein the doped silica core layer is deposited at a temperature of about 1000°C to about 1600°C. 5. The method of Claim 1 wherein the reactive gas mixture is comprised of about 13 volume percent gaseous iodine and about 87 volume percent oxygen.

Inventors: GORRE LUISA E, PASTOR RICARDO C, TANGONAN GREGORY L, GORRE, LUISA, E., PASTOR, RICARDO, C., TANGONAN, GREGORY, L.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1986
Main classification: C03B37
IPC codes: C03B8, C03B20, C03B37, C03C13, G02B6