Title of Invention	METHOD OF MAKING AN OPTICAL FIBER PREFORM
Abstract	A method for producing an optical fiber preform starting with a first-generation target comprising a substantially cylindrical pure silica rod. Plasma outside vapor deposition is used to deposic doped silica atop the rod to form a workpiece. The resulting workpiece is stretched and a doped silica rod having a center formed from the original target material, and an annular layer formed from doped silica. This doped silica rod is then used as a second-generation target and additional doped silica is deposited thereon, and the process repeated. A cladding layer of doped silica is deposited.atop the resulting structure to form a primary preform. Finally, the primary preforms are stretched and separated and a jacketing layer is deposited atop each to form the final fiber optic preform. The silica layers are deposited by a plasmatron using plasma outside vapor deposition. The dopants can be F, GeO<sub>2</sub>,P<sub>2</sub>O5. TiO<sub>2</sub>,A1<sub>2</sub>O<sub>3</sub> and the like. The products resulting from this process can be multimode and singlmode preform and fiber.

Title of Invention

METHOD OF MAKING AN OPTICAL FIBER PREFORM

Abstract

A method for producing an optical fiber preform starting with a first-generation target comprising a substantially cylindrical pure silica rod. Plasma outside vapor deposition is used to deposic doped silica atop the rod to form a workpiece. The resulting workpiece is stretched and a doped silica rod having a center formed from the original target material, and an annular layer formed from doped silica. This doped silica rod is then used as a second-generation target and additional doped silica is deposited thereon, and the process repeated. A cladding layer of doped silica is deposited.atop the resulting structure to form a primary preform. Finally, the primary preforms are stretched and separated and a jacketing layer is deposited atop each to form the final fiber optic preform. The silica layers are deposited by a plasmatron using plasma outside vapor deposition. The dopants can be F, GeO2,P2O5. TiO2,A12O3 and the like. The products resulting from this process can be multimode and singlmode preform and fiber.

Full Text	The present invention relates to methods for making optical fiber preform of both single node and multimode design using a plasma outside vapor deposition process. The prior art teaches various approaches for fabricating silica glass starter tubes, and for making optical fiber preforms. Starter tubes can be formed by heating silica and extruding it through an aperture. Both starter tubes and optical fiber preforms can be made by depositing doped or undoped silica onto a target using one of several techniques such as modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), outside vapor deposition (OVD). Each of these methods starts with prpviding a rotating target, typically shaped in the form of a tube or a solid rod, and formed from glass, ceramic or one of several other materials. In certain cases, the rod or tube becomes an integral peurt of the preform but, in other cases, the rod will be removed. A heat source, such as a gas burner or a plasma source is positioned beneath, above or laterally, across the rotating target. The heat source will provide the required energy for the glass-forming reactions to form glass particles. Depending upon the nature of the process, these deposited glass particles are ready for the next processing, drying and sintering steps such as VAD or OVD processes. If it is an MCVD process, these particles will be fused into vitreous quartz by the same heat source. When the target is mounted horizontally, the heat source travels along the length of the target to ensure uniform deposition. If the target is a tube, the glass forming particles and materials may be deposited either on the inside surface of the tube, in which case the outer diameter remains constant, or on the outside of the tube, in which case the outer dieuneter grows. When the target is mounted vertically, it rotates around its vertical axis, and with burners located either vertically above or laterally across, grows in both radial and axial directions. This results in a substantially cylindrical product whose diameter and length increase as deposition continues. UPS 3,737,292 to Keck et al. discloses a method of forming optical fibers. Multiple layers with predetermined index of refraction are formed by flame hydrolysis and deposited on the outside wall of a starting rod or member. After these layers of glass are coated on the rod the resulting hollow cylinder is heated and collapsed to form fibers. USP 4,224,046 to Izawa et al. teaches a method for manufacturing an optical fiber preform. Two gaseous glass" forming materials, oxygen, hydrogen and argon are jetted upwards in a burner towards a vertically mounted, rotating cylindrical start member. Soot-like glass particles are formed by flame hydrolysis and deposited on the lower end of the start member. The start member is gradually withdrawn upwards to maintain a constant spacing between the its growing end and the burner. Upon completion of the deposition, the resulting soot-like glass preform is then dried and sintered to form a transparent glass preform. UPS 4,217,027 to MacChesney et al. teaches the fabrication of prefoinns by what is usually referred to as /the Modified Chemical Vapor Deposition (MCVD) process. In this process, a vapor stream consisting of chlorides or hydrides of silicon and germanium with oxygen is directed to the inside of a glass tube. The chemical reactions among these chemicals, which are preferentially induced by a traversing hot zone, will under proper conditions result in the formation of glass on the inner wall of the tube. The particular matter deposited on the tube is fused with each passage of the hot zone. USP 4,412,853 to Partus discloses an MCVD process to form an optical fiber preform starter- tube. The process starts with a horizontally mounted; rotating tubular target formed from glass and having a preselected composition and optical characteristics. A vapor stream is fed through the tubular target as a heat source positioned beneath the tubular target, traverses along the latter"s length. This causes reaction products of the vapor stream to be deposited on, and fuse to, the interior surface of the tubular target. The deposited material has the same index of refraction as the tubular target, but a different composition. This reference also suggests that one may achieve the same effect by an" outside vapor-phase oxidation process or an outside vapor-phase axial deposition process, but does not explicitly teach how this can be done. USP 4,741,747 to Geittner et al. is directed to the Plasma Chenical Vapor Deposition (PCVD) method of fabricating optical fibers. In this PCVD method glass layers are deposited on the inner wall of a glass tube by heating the tube to a temperature between 1100° and 1300"» C, before passing the reactive gas mixture at a pressure between 1 and 30 hPa, and moving a plasma back and forth inside the glass tube. After the glass layers are deposited, this glass tvibe is collapsed to produce a solid preform, optical fibers can be drawn from this preform. USP 5,522,007 to Drouart et al. teaches the use of plasma deposition to build up an optical fiber prefona having high hydroxy1 ion concentration. In this reference, hydroxyl ions are deliberately entrained in a plasma generating gas by passing the gas through a water tank before it is introduced into one end of a plasma torch having an induction coll. The plasma torch projects molten silica particles mixed with hydroxyl ions onto a rotating substrate preform. This results in a preform having an average hydroxyl ion concentration lying in the range to 50-100 ppm deposited onto the target preform. According to Drouart et al., this technique results in optical fibers having an attenuation of 0.32 dB/km and 0.195 db/km at 1310 ran and 1550 ran, respectively. In addition to requiring multiple processing steps to fabricate preforms, some other disadvantages of the above processes are that: 1. the MCVD and PCVD processes are slower processes because of their low deposition rate; 2. the preform size is limited by the size of the deposition tube for MCVD and PCVD process; and 3. the OVD and VAD processes are based on flame tiydrolysis which generates excessive amounts of water and requires additional drying and sintering steps to produce liigh quality optical fiber preforms. SUMMARY OF THE INVENTION It is an object of the invention to provide a method for producing an optical fiber preform having low lydroxyl content at low cost by reducing the number of steps entailed in its manufacture, while increasing the size of a preform and increasing the rate of deposition. Phis and other objects are achieved by the present Lnventive method for forming an optical fiber preform. In one aspect of the present invention, a plasma source is placed in proximity to a starter rod formed from a primary material. The starter rod is held lorizontally at both ends and is arranged to rotate about Lts longitudinal axis. The plasma source is used to leposit silica doped with a known first doping :oncentration. The doped silica is deposited along the Length of the starter rod until the latter grows to a lesired diameter. The complex comprising the starrer rod ind the doped silica is then drawn down and a thinned section is extracted for use as a secondary rod. The secondary rod has a center formed from the primary laterial, and an outer layer formed from the doped silica. Additional silica, having the same doping concentration, is deposited atop this secondary rod until it, too, reaches a desired diameter, and then is.drawn down and a section extracted. The steps of depositing drawing down, extracting and depositing may be repeated a number of times. The result of this activity is a doped silica rod having a center formed from the primary material with a first diameter, and an annular layer formed from the doped silica with a second outer diameter. The doped silica rod is subject to further processing. Specifically, the plasma source is used to deposit an outer layer of doped silica atop the doped silica rod and the resulting structvire ma^ then be drawn down and a thinned section extracted, as before. The dopant used in forming the outer layer may be selected to ^^ either increase, or decrease, the index of refraction of the silica. If the dopant concentration is varied as the outer layer is being deposited, the outer layer is a graded layer. In such case, typically, the dopant concentration is varied from a maximvim, beginning concentration level when the outer layer is first being deposited, to a minimum, end concentration level when deposition of the outer layer is almost complete. If the dopant concentration is not varied as the outer layer is being deposited, the outer layer is a stepped layer. In such case, typically, a second dopant concentration, different from the first dopant concentration, is used throughout the deposition of the outer layer. In yet another aspect of the present invention, the complex comprising the doped silica rod and the outer layer is subjected to further processing. The plasma source is used to deposit a cladding layer atop the outer layer. If the outer layer was graded, the cladding layer may be formed from silica doped with the same dopant and same minimum, end concentration level. Alternatively, the cladding layer can be formed from pure silica, or even silica doped with some other dopant and at a third dopant concentration. If desired, the cladding layer may also have a graded doping. In yet another aspect of the present invention, the complex comprising the doped silica rod, the outer layer and the cladding layer, is provided with a jacket. The jacket can be added by either further plasma deposition, or alternatively, by providing a jacketing material over this complex and then applying heat to collapse the jacketing material into a finished preform. During plasma deposition, a dry plasma gas having a low hydroxyl concentration is used to form the plasma. A dry quartz source gas comprising SiCU, or other similar source gases having low hyroxyl concentration, and a dopant source gas such as GeCI4, which is sometimes co-doped with POCI3 or PCI5 are introduced in proximity to the plasma. This causes the material to be converted to silica (Si02), or silica doped with germanium oxide (Ge02) and or phosphorous pentoxide (P2O5) and deposited onto the target and fused into vitreous quartz in one simple step. Accordingly the present invention provides A method for making an optical fiber perform, said method comprising steps of: (a) providing a target rod formed from a first material; (b) concurrently depositing atop said target rod, a first silica layer doped with a first dopant provided at a first concentration, said first silica layer being deposited and sintered to a predetermined first thickness; (c) drawing down said target rod with said first silica layer deposited thereon to a predetermined first diameter, thereby forming a doped silica rod; (d) repeating steps (b) and (c) (d 1) for a predetermined number of times, or (d2) until said first material comprises a predetermined proportion of said doped silica rod; (e) depositing on said doped silica rod a second layer comprising silica doped with a second dopant provided at a second concentration, said second silica layer being deposited to a predetermined second thickness to thereby form an intermediate structure; (f) depositing a third layer on said intermediate structure, said third layer being deposited to a predetermined third thickness to thereby form a preform structure; and, at least one of said steps (b), (e) and (f) is performed by plasma outside vapor deposition, which comprises steps of: providing a high-frequency plasma torch comprising a coil, said plasmatron being selectively positionable along a length of said target with a 7 spacing of 30-55 mm separating the target from said coil; introducing a plasma gas having a hydroxyl content of less than 2 ppm into the plasmatron to form a plasma; injecting a source gas at least SiCI4 and a dopant into a region in communication with said plasma, said source gas having a hydroxyl content of less than 0.5 ppm; and depositing at least one reaction product of said plasma and said source gas onto the target while maintaining said spacing between the target and the coil. The invention will now be described more in detail with reference to embodiments given by way of example and shown in the accompanying drawings, in which; Fig. 1 shows an apparatus used to perform plasma deposition; Fig. 2 shows a partial side view of a plasmatron used in the apparatus of Fig. 1; Fig. 3 shows a top view of a plasmatron similar to that shown in Fig. 2; Fig. 4 shows a flow pattern of the plasma within the plasmatron of Fig. 3; Fig. 5 shows an optical fiber preform made in accordance with the method of the present invention. Fig. 1 shows an apparatus 20 used for plasma outside vapor deposition. The apparatus comprises a chamber 22-which is sealed so as to prevent impurities from being introduced into the final product. Within the chamber 22 is a lathe 24, such as that available from Heathway Ltd. or Litton Engineering Lab. The lathe 24 has a headstock 25 and a tailstock 26. The headstock 25 and the tailstock 26 are provided with a pair of opposing rotating spindle chucks 28 which hold the ends of an elongated target 30 having a substantially cylindrical outer wall. The spindle chucks 28 rotate target 30, as indicated by arrow Al. A movable carriage 32 movably mounted to the lathe 24 is arranged to travel in either direction along the target, as indicated by double headed arrow A2. A plasma source, shown generally as 40, is supported by carriage 32. Carriage 32 thus moves plasma source 40 along the length of the target 30. This results in the deposition of material on top of the target 30 to form an aptical fiber preform. The spindle chucks 28 rotate the barget 30 to ensure that material is uniformly deposited oy the plasma source 40 around the target so as to form a tubular member 34 having nearly perfectly cylindrical 3uter walls. In the preferred embodiment, the plasma source 40 jositioned on the carriage 32 moves in both directions ilong a substantial portion of the length of the target )0. This allows the plasma source 40 to travel along ;his portion of the target 30 and deposit materials :herealong. Instead of moving the plasma source 40 along the Length of the target, the target 30 may be moved while the plasma source 40 remains stationary. This can be realized by having the headstock 25 and the tailstock 26 the lathe move the target in a reciprocating fashion that all relevant portions of the target are brought lirectly above the plasma source 40. As another alternative, a plurality of plasma sources may be spaced apart along the length of the target. This allows for reduced movement of either the headstock 25 and tailstock 26 of the lathe 24 or the carriage 32 to which the plasma sources are attached, depending on which of the two is configured to move. In the extreme case where a great number of plasma sources are provided all along the length of the target, no movement of either the carriage 32 or the headstock 25 and tailstock 26 of the lathe 24 is needed. In the preferred embodiment, the plasma source 40 is a plasmatron torch having a dry plasma gas introduced into it through a first gas line 42 and a source gad introduced into it through a second gas line 44. The plasma gas is substantially comprised of nitrogen and oxygen in an appropriate, predetermined proportion. Air may serve as the plasma gas. In such case, filtered air first passes through a first dryer 46 to remove moisture before entering the first gas line 42. This ensures that the hydroxy1 concentration of the plasma gas is low, on the order of 2.0 ppm, or less. The total volume of gas being delivered will be regulated by a mass flow controller (HFC) 80 or by a flowmeter, as an alternative. The source gas comprises a source chemical such as SiCl4, and at least one carrier gas, such as oxygen 0, or nitrogen N2. The carrier gases enter the Asecond dryer 48 to remove moisture. This ensures that the hydroxyl concentration of the source gas is also very low, on the order of 0.5 ppm. After the carrier gases are dried, they proceed to a MFC 81 before entering a bubbler 50 to pick up the source chemical. Depending upon the characteristics of the MFC, it is also possible to use it downstream of the bubbler. The gas stream comprising carrier gases laden with the source chemical then proceeds to the second gas line 44. Optionally, by opening valve 52, a dopant gas may be introduced into the gas stream before it reaches the plasmatron torch. In the preferred embodiment, the source chemical Is SiCl4. This chemical is chosen for its retctive properties in a plasma. Specifically, the SiCl2 serves as a source of Si to form SiO2 which is deposited on the target 30. The dopant can be a fluorine dopant gas in the form of SiF4 or SiF2. Fluorine dopants will lower the index of refraction and also change the viscosity of the quartz. In addition, fluorine dopants result in increased design flexibility for optical fiber preforms. As is well known, however, if one wishes to increase the index of refraction, Geo, or other equivalent substance may be used as the dopant. In the preferred embodiment, the source chemical for GeO2 is GeCl4. This chemical is chosen for its purity because of its having similar physical ami chemical properties SiCl4. The delivery of the GeCl4 will be similar to SiCl4. The carrier gas from the dryer 48, can be split to another branch where it will be regulated by a MFC 82, before proceeding to a bubbler 83 to pick up the source chemical GeCl4. Similar to the control of chemical SiClf4 the HFC can also be located downstream of the bubbler. This gas stream can feed into the gas line 44 and form a mixture before entering the plasmatron torch. It is also possible to directly introduce the GeCl4 gas stream by a separate line 84 to the plasmatron torch. One advantage of using the separated delivery lines is to minimize the competing chemical reactions between GeCl^ and SiCl4. Other source chemicals that can be used for doping instead of germanium oxide (Geo,) or co-doping with germanium oxide are materials such as POCI3, pels, and other similar index increasing dopants such as Aluminum and Titanium containing chemicals. Fig. 2 shows a cutaway side view of the plasmatron torch 40 positioned below the target 30. The plasmatron torch 40 comprises a substantially tubular torch housing 50 formed from quartz. The housing has a diameter of 60 mm and a height of 220 mm. However, diameters ranging from 40-80 mm and heights between 180-400 mm may also be used. A copper induction coil 52 is provided around the upper portion of the housing 50. The coil 52 comprises a plurality of windings 54 having a diameter of approximately 72 mm and spaced apart from each other by 6 mm. A gap between the housing and the coil can be between 2-10 mm. The uppermost portion of the coil 52, as indicated by uppermost winding 54", is separated from the outer surface of the tubular member 34 by a spacing designated by L, which is on the order of 30-55 mm. As the quartz glass is deposited, its outer diameter increases. However, the spacing L is maintained by" adjusting the height of a support stand 56 on which the plasma torch 40 is placed. Support stand 56, in turn, is mounted to carriage 32, and moves laterally therewith. Initially, the support stand 56 is set at a predetermined height, and this height is reduced as the diameter of the deposited material increases during deposition. This maintains a predetermined distance between the plasma torch 40 and the deposited material. An optical or other sensor mounted on the carriage 32 and connected to a controller may be used to gauge the distance of the radially growing tubular member 34 from the carriage, and adjust the height of the support stand 56, accordingly. On either side of the uppermost portion of the housing 50 is a plasma stabilizer bar 58. Each stabilizer bar is formed from quartz and comprises a U-shaped gutter extending laterally from the rim of the housing 50. The stabilizer bars 58 have a diameter of 60 mm and extend 20 mm on diametrically opposite sides of the housing rim, although diameters in the range of 40-80 mm and lengths of 15-40 mm may also be used. When the plasmatron torch 40 is in use, the stabilizer bars 58 are aligned parallely to the target. This arrangement helps spread the reactive source chemicals being deposited onto the growing tubular member 34. A pair of injection ports 60 connect the second gas line 44 carrying the source chemicals to the plasmatron torch 40. The injection ports 60 enter the housing at substantially the same height along the housing 50, at a point between the uppermost windings 54" of the coil 52 and the stabilizer bars 58. The injection ports comprise quartz tubing having a diameter of 5 mm, although tubing diameters on the order of 3-10 mm may be used with the plasmatron torch 40 of the present invention. In the preferred embodiment, a pair of injection ports 60 enter the housing 50 at the same height and are positioned diametrically across from each other, instead of just two such ports, however, three or even more ports, symmetrically arranged, may be provided. In Pig. 2, the two injection ports 60 are shown to be directly beneath the stabilizer bars. This, however, is not an absolute necessity, and the injection ports 60 may be angularly offset from the stabilizer bars 58, in a top view of the plasmatron torch, as shown in Fig. 3. A pair of plasma gas inlets 62 connect the first gas line 42 carrying the plasma gases to the plasmatron torch 40. The plasma gas inlets 62 enter the housing at substantially the same height, proximate to the base of the housing. These inlets 62 comprise stainless steel tubing having a diameter of 5 mm, although a range of diameters may suffice for this purpose. The plasmatron torch 40 is also provided with a coolant inlet 64 and outlet 66. During use, a coolant, such as water, passes through the inlet 64, circulates within the outer wall of the housing 50, and exits through the outlet 66. The coolant inlet and outlet are formed from stainless steel and have a diameter of 5 mm. As with the plasma gas inlet and the injection port, this diameter may also vary. The plasma gas inlets 62, the coolant inlet 64 and the coolant outlet 66 are all formed in a stainless steel chamber 68. The chamber 68 is a stainless steel square block 80 mm on a side, and having a heigh of approximately 40 mm. The chamber 68 is mounted onto the support stand 56 which, in turn, is mounted on the carriage 32 for movement along the target 30. A high frequency generator (not shown) is electrically connected to the coil 52, powering the latter with a variable power output up to 80 kW at a frequency of approximately 5.0 MHz. In the preferred embodiment, the generator is Model No. T-80-3MC from Lepel Corporation. This generator is driven with a 60 Hz, 3-phase 460 V power supply to energize the plasmatron torch 40. As an alternative, a Model No. IG 60/5000 generator is available from Fritz Huttinger Electronic GmbH of Germany. Fig. 4 depicts the plasma jet 70 forred within the plasmatron torch 40 when the dry plasma gas is fed through the inlets 62 and converted into a plasma. The plasma jet 70 is substantially symmetric about the torch"s longitudinal axis A". The position of the injection ports 60 is such that the source chemicals are introduced into the plasma just above a point V where the vertical velocity of said plasma is zero. This provides the needed structure of hydrodynamic and thermal flow of the source chemical jet into the border layers to realize efficient deposition onto the growing tubular member 34. And while the preferred embodiment has the injection ports entering laterally into the housing, this is not an absolute requirement. Instead, the source gases may introduced into the center of the plasma jet 70 by a water cooled probe extending along the longitudinal axis A" of the plasmatron torch 40. Fig. 5 illustrates a well-known procedure which can performed with a lathe 124, such as Model No. PFH842XXLS Precision Quartz and Glass Working Lathe, manufactured by Heathway. The headstock 125 and tailstock 126 of the lathe 124 can move longitudinally relative to one another. This allows for easy loading and unloading of a finished workpiece 130 of length L3 which has been leposited atop an initial target. More significantly, it also allows one to draw down a portion of a workpiece into a secondary rod of a reduced diameter comparable to that of the original target. This is accomplished by-keeping the headstock 125 stationary and moving the tailstock 126 away from the headstock 125 while the plasma source 140 is moved in a direction opposite to that of tailstock 126. Alternatively, this can also be accomplished by placing a plasma source 140, or other heat source, at one end of the workpiece 130 to soften it. Then the headstock 125 and tailstock 126 are moved in the same direction, but with different speeds by distances L5, L4, respectively, to the positions shown in phantom 125", 126". The result is a thin, secondairy rod 132, which can (but need not) have the same diameter as the original target. As is known to those skilled in the art, the secondary rod has the same cross-sectional composition as the workpiece from which it is derived, as so has a center whose consistency along is substantially similar to that of the original target, and outer layer substantially similar to the materials deposited atop the target during the formation of the workpiece. The lathe 124 allows the headstock 125 and tailstock 126 to be moved far enough longitudinally to stretch the secondary rod to a distance L4, which is substantially the same as the length L3 of the workpiece from which it is derived. The secondary rod 132 may be cut from the workpiece, mounted on the lathe 124 in place of the workpiece 130, and used as a target for subsequent deposition with the plasma source 140. Thus, the original, or first-generation, target is used to create a first-generation workpiece, from which a secondary rod can be drawn to be used as a second-generation target. Deposition atop this second-generation target can will thereby form a second-generation workpiece, and so on. This iterative process of plasma deposition on a target to form a workpiece, stretching one end of the workpiece to form a reduced-diameter rod, and using this reduced- diameter rod as a subsequent target for further deposition can be repeated an arbitrary number of times. If the material being deposited atop the target is unchanged through the iterations, the result of N iterative steps is an N-th generation rod having a very small center which is substantially identical in composition to the original target, and an annular layer reflective of the materials deposited atop the target. For instance, if the original target has a diameter Dl and the finished workpiece has a diameter D2 = M x Dl, then the proportion of the original target material in the first-generation workpiece is approximately 1/M2. If a second-generation target of diameter Dl is drawn from this workpiece and material sufficient to form a second-generation workpiece of diameter D2 is deposited thereon, the proportion of the original target material in the second generation workpiece is approximately 1/M2. Thus, it can be seen that one may readily form a workpiece having a predetermined proportion of the original target material therein by controlling M during deposition, along with the total number of iterations. A method for forming a multimode optical fiber preform using the aforementioned iterative technique will now be described. In order to provide a more detailed explanation, some dimensions are given. However, it must be noted that in the actual process, many different value are possible. The method begins by providing a first generation target, horizontally mounted on a lathe, such as that shown in Fig. 5. The target is preferably formed from pure silica, in which case it may be purchased from a commercial vendor, such as Product no. F300, available from Heraeus Amersil of Georgia. Alternatively, the first-generation target may be an Nth-generation doped silica rod formed using the current process. In the preferred embodiment, the first-generation target has a length of one meter and a diameter Dl = 6 mm. silica doped with GeO2 is deposited atop the first-generation target using the plasma source described above. The dopant concentration for the Geo2, depends on the desired numerical aperture (NA) of the multimode optical fiber being produced. For instance, to form a fiber with a NA of 0.2, the maximum GeO2 dopant concentration is approximately 10%. And to form a fiber with a NA of 0.275, the maximum GeO2 dopant concentration will be approximately 18%. The dopant concentration may be held at the same level during deposition, in which case a stepped layer, is formed. Alternatively, the dopant concentration may be gradually varied to form a graded layer. This is done by automatically controlling, by means of. -i microprocessor or like, an adjustable flow meter through which the dopant is introduced. It should be noted that stepped and graded layers may succeed one another in subsequent generations of workpieces, and that layers having different, constant doping concentrations may succeed one another, as well. Thus, a graded layer may be deposited on the first-generation target, and a stepped layer may be deposited atop the second-generation target formed after drawing down the first-generation workpiece. Similarly, one may deposit a stepped layer atop a graded layer, which has been deposited atop an original first-generation target. Also, a first stepped layer, having a first dopant concentration, may be deposited atop a target, and a second stepped layer, having a second dopant concentration, deposited atop the next generation target. Additional layers, either graded or stepped, may be deposited atop any of the above structure. In the preferred embodiment, silica doped with 18 % GeO2 is deposited as a stepped layer atop the 6 mm diameter first generation target until a workpiece having a length of one meter and a diameter of D2 = 48 mm is formed (i.e., M = 8). This resulting first-generation workpiece has approximately 64 times the cross-sectional the original target, two layers having substantially the same optical properties and fairly indistinguishable from one another, and a third, graded layer. In the preferred embodiment, the 80 mm diameter third-generation workpiece is subject to additional processing to form a primary optical fiber preform. Specifically, a cladding, or barrier, layer is deposited atop the third-generation workpiece. The Thickness of the cladding layer depends on the type of finished optical fiber preform to be made. For a 62.5/125 fiber preform, the finished primary preform will have a final diameter of about 93 mm. For a 50/125 fiber preform, the finished primary preform will have a final diameter of about 96 mm. The cladding layer is formed by depositing silica doped at the seune concentration of GeO2 as the minimum doping concentration level used to form the third layer, i.e., 10% GeO2. This results in a structure having the original target material at the center, a constantly doped pair of second layers having the same optical properties, a graded layer having a dopant concentration varying from a maximum value to a minimum value, and a cladding layer comprising silica doped at the minimum value. Once the cladding layer is applied, the finished primary preform must be stretched to form the final preforms. From a single, 1 meter long 62.5/125 preform having a diameter of 93 mm diameter, one can obtain eight, one-meter long preform pieces, each having an outer diameter of 32 mm. And from a single, 1 meter long 50/125 preform having a diameter of 96 mm diameter, one can obtain twelve, one-meter long pieces, each having an outer diameter of 27 mm. A jacketing layer may be applied atop the cladding layer of these preform pieces. The jacketing layer preferably has the same index of refraction as pure silica. The jacket may be applied by plasma outside vapor deposition using pure silica. Alternatively, a tube or sheet of pure silica, having an appropriate diameter or width, may be provided around a preform piece, and heat applied to fuse the jacket onto the preform piece to form the final optical fiber preform. In the preferred embodiment, the final optical preform has an outer diameter of about 56 mm. This final preform may then be drawn into approximately 200 Km of fiber having a diameter of 125 µM. Although, for best performance, a cladding and then a jacketing layer is applied, it should be noted that one may dispose of the cladding step and directly apply a jacketing tube to the third-generation workpiece, once it has been stretched. A similar method for making single moie optical fiber preform can be achieved by using the following procedure. The starting target can be a pure silica rod that can be either a F300 rod purchased from Heraeus or a pure s;.lica Nth-generation rod fabricated in house. Multiple fluorine doped silica layers with constant concentration are deposited on the target until it reaches a desired diameter. Single mode optical fibers can be drawn from this preform. There are many different glass index modifiers such as F, GeOj, P2O5, TiOj, AI2O3, etc., and in the proper combination, they can be used to make the doped core and/or doped cladding. In the preferred embodiment, the target is a Nth-generation GeOj doped rod with pure silica or doped silica cladding layers deposited on it. The preform is completed when the desired diameter is reached. While the present invention has been disclosed with reference to certain preferred embodiments, these should not be considered to limit the present invention. One skilled in the art will readily recognize that variations of these embodiments are possible, each falling within the scope of the invention, as set forth in the claims below. WE CLAIM: 1. A method for making an optical fiber perform, said method comprising steps of; (a) providing a target rod formed from a first material; (b) concurrently depositing atop said target rod, a first silica layer doped with a first dopant provided at a first concentration, said first silica layer being deposited and sintered to a predetermined first thickness; (c) drawing down said target rod with said first silica layer deposited thereon to a predetermined first diameter, thereby forming a doped silica rod; (d) repeating steps (b) and (c) (d 1) for a predetermined number of times, or (d2) until said first material comprises a predetermined proportion of said doped silica rod; (e) depositing on said doped silica rod a second layer comprising silica doped with a second dopant provided at a second concentration, said second silica layer being deposited to a predetermined second thickness to thereby form an intermediate structure; (f) depositing a third layer on said intermediate structure, said third layer being deposited to a predetermined third thickness to thereby form a preform structure; and, at least one of said steps (b), (e) and (f) is performed by plasma outside vapor deposition, which comprises steps of: providing a high-frequency plasma torch comprising a coil, said plasmatron being selectively positionable along a length of said target with a spacing of 30-55 mm separating the target from said coil; introducing a plasma gas having a hydroxyl content of less than 2 ppm into the plasmatron to form a plasma; injecting a source gas at least SiCU and a dopant into a region in communication with said plasma, said source gas having a hydroxyl content of less than 0.5 ppm; and depositing at least one reaction product of said plasma and said source gas onto the target while maintaining said spacing between the target and the coil. 2. The method as claimed in claim 1, comprising the step of; (g) applying a jacketing layer stop said perform structure, said jacketing layer consisting essentially of pure silica and being applied to a predetermined fourth thickness. 3. The method as claimed in claim 2, comprising the step of: drawing down said preform structure to a predetermined third diameter after step (f) and prior to step (g). 4. The method as claimed in claim 1, wherein the first material is selected from the group consisting of silica and silica doped with a dopant. 5. The method as claimed in claim 4, wherein the dopant is an index modifying material which is selected from the group consisting of F, Ge02, P2O5 Ti02 and AI2 O3. 6. The method as claimed in claim 1, comprises the step of varying the second concentration as the second silica layer is deposited. 7. The method as claimed in claim 6, wherein the second depoant is fluorine , and said second concentration is varied from a minimum value when said second silica layer is first being deposited, to a maximum value when deposited of said second silica a layer is nearing completion. 8. The method as claimed in claim 6, wherein said second concentration is varied from a maximum value when said second silica layer is first being deposited, to a minimum value when deposition of said second silica is nearing completion. 9. The method as claimed in claim 8, wherein said maximum value of said second concentration is substantially the same as said first concentration. 10. The method as claimed in claim 8, wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with said second dopant at said minimum value. 11. The method as claimed in claim 8 wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with fluorine. 12. The method as claimed in claim 1, wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with fluorine. 13. The method as claimed in claim 1, wherein the source gas is introduced just above a point in the plasmatron at which the vertical velocity of the plasma is zero. coil comprises a plurality of windings, and the target is separated from a winding closest to the target by said spacing. 15. The method as claimed in claim 1, wherein the plasma gas is dried before it is introduced into the plasmatron. 16. The method as claimed in claim 1, wherein the first and second dopants and the first and second concentrations, are the same. 17. A method for making an optical fiber preform substantially as herein above described with reference to the accompanying drawings.

Full Text

The present invention relates to methods for making optical fiber preform of both single node and multimode design using a plasma outside vapor deposition process.
The prior art teaches various approaches for fabricating silica glass starter tubes, and for making optical fiber preforms. Starter tubes can be formed by heating silica and extruding it through an aperture. Both starter tubes and optical fiber preforms can be made by depositing doped or undoped silica onto a target using one of several techniques such as modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), outside vapor deposition (OVD). Each of these methods starts with prpviding a rotating target, typically shaped in the form of a tube or a solid rod, and formed from glass, ceramic or one of several other materials. In certain cases, the rod or tube becomes an integral peurt of the preform but, in other cases, the rod will be removed. A heat source, such as a gas burner or a plasma source is positioned beneath, above or laterally, across the rotating target. The heat source will provide the required energy for the glass-forming reactions to form glass particles. Depending upon the nature of the process, these deposited glass particles are ready for the next processing, drying and sintering steps such as VAD or OVD processes. If it is an MCVD process, these particles will be fused into vitreous quartz by the same heat source.
When the target is mounted horizontally, the heat source travels along the length of the target to ensure uniform deposition. If the target is a tube, the glass forming particles and materials may be deposited either on the inside surface of the tube, in which case the

outer diameter remains constant, or on the outside of the tube, in which case the outer dieuneter grows.
When the target is mounted vertically, it rotates around its vertical axis, and with burners located either vertically above or laterally across, grows in both radial and axial directions. This results in a substantially cylindrical product whose diameter and length increase as deposition continues.
UPS 3,737,292 to Keck et al. discloses a method of forming optical fibers. Multiple layers with predetermined index of refraction are formed by flame hydrolysis and deposited on the outside wall of a starting rod or member. After these layers of glass are coated on the rod the resulting hollow cylinder is heated and collapsed to form fibers.
USP 4,224,046 to Izawa et al. teaches a method for manufacturing an optical fiber preform. Two gaseous glass" forming materials, oxygen, hydrogen and argon are jetted upwards in a burner towards a vertically mounted, rotating cylindrical start member. Soot-like glass particles are formed by flame hydrolysis and deposited on the lower end of the start member. The start member is gradually withdrawn upwards to maintain a constant spacing between the its growing end and the burner. Upon completion of the deposition, the resulting soot-like glass preform is then dried and sintered to form a transparent glass preform.
UPS 4,217,027 to MacChesney et al. teaches the fabrication of prefoinns by what is usually referred to as /the Modified Chemical Vapor Deposition (MCVD) process. In this process, a vapor stream consisting of chlorides or hydrides of silicon and germanium with oxygen is directed to the inside of a glass tube. The chemical reactions among these chemicals, which are preferentially induced by a traversing hot zone, will under proper conditions result in the formation of glass on the inner wall of the tube. The particular matter deposited on the tube is fused with each passage of the hot zone.

USP 4,412,853 to Partus discloses an MCVD process to form an optical fiber preform starter- tube. The process starts with a horizontally mounted; rotating tubular target formed from glass and having a preselected composition and optical characteristics. A vapor stream is fed through the tubular target as a heat source positioned beneath the tubular target, traverses along the latter"s length. This causes reaction products of the vapor stream to be deposited on, and fuse to, the interior surface of the tubular target. The deposited material has the same index of refraction as the tubular target, but a different composition. This reference also suggests that one may achieve the same effect by an" outside vapor-phase oxidation process or an outside vapor-phase axial deposition process, but does not explicitly teach how this can be done.
USP 4,741,747 to Geittner et al. is directed to the Plasma Chenical Vapor Deposition (PCVD) method of fabricating optical fibers. In this PCVD method glass layers are deposited on the inner wall of a glass tube by heating the tube to a temperature between 1100° and 1300"» C, before passing the reactive gas mixture at a pressure between 1 and 30 hPa, and moving a plasma back and forth inside the glass tube. After the glass layers are deposited, this glass tvibe is collapsed to produce a solid preform, optical fibers can be drawn from this preform.
USP 5,522,007 to Drouart et al. teaches the use of plasma deposition to build up an optical fiber prefona having high hydroxy1 ion concentration. In this reference, hydroxyl ions are deliberately entrained in a plasma generating gas by passing the gas through a water tank before it is introduced into one end of a plasma torch having an induction coll. The plasma torch projects molten silica particles mixed with hydroxyl ions onto a rotating substrate preform. This results in a preform having an average hydroxyl ion concentration lying in the range to 50-100 ppm deposited onto the

target preform. According to Drouart et al., this technique results in optical fibers having an attenuation of 0.32 dB/km and 0.195 db/km at 1310 ran and 1550 ran, respectively.
In addition to requiring multiple processing steps to fabricate preforms, some other disadvantages of the above processes are that:
1. the MCVD and PCVD processes are slower processes because of their low deposition rate;
2. the preform size is limited by the size of the deposition tube for MCVD and PCVD process; and
3. the OVD and VAD processes are based on flame tiydrolysis which generates excessive amounts of water and requires additional drying and sintering steps to produce liigh quality optical fiber preforms.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for producing an optical fiber preform having low lydroxyl content at low cost by reducing the number of steps entailed in its manufacture, while increasing the size of a preform and increasing the rate of deposition. Phis and other objects are achieved by the present Lnventive method for forming an optical fiber preform.
In one aspect of the present invention, a plasma source is placed in proximity to a starter rod formed from a primary material. The starter rod is held lorizontally at both ends and is arranged to rotate about Lts longitudinal axis. The plasma source is used to leposit silica doped with a known first doping :oncentration. The doped silica is deposited along the Length of the starter rod until the latter grows to a lesired diameter. The complex comprising the starrer rod ind the doped silica is then drawn down and a thinned section is extracted for use as a secondary rod. The secondary rod has a center formed from the primary laterial, and an outer layer formed from the doped silica. Additional silica, having the same doping

concentration, is deposited atop this secondary rod until it, too, reaches a desired diameter, and then is.drawn down and a section extracted. The steps of depositing drawing down, extracting and depositing may be repeated a number of times. The result of this activity is a doped silica rod having a center formed from the primary material with a first diameter, and an annular layer formed from the doped silica with a second outer diameter.
The doped silica rod is subject to further processing. Specifically, the plasma source is used to deposit an outer layer of doped silica atop the doped silica rod and the resulting structvire ma^ then be drawn down and a thinned section extracted, as before. The dopant used in forming the outer layer may be selected to ^^ either increase, or decrease, the index of refraction of the silica.
If the dopant concentration is varied as the outer layer is being deposited, the outer layer is a graded layer. In such case, typically, the dopant concentration is varied from a maximvim, beginning concentration level when the outer layer is first being deposited, to a minimum, end concentration level when deposition of the outer layer is almost complete.
If the dopant concentration is not varied as the outer layer is being deposited, the outer layer is a stepped layer. In such case, typically, a second dopant concentration, different from the first dopant concentration, is used throughout the deposition of the outer layer.
In yet another aspect of the present invention, the complex comprising the doped silica rod and the outer layer is subjected to further processing. The plasma source is used to deposit a cladding layer atop the outer layer. If the outer layer was graded, the cladding layer may be formed from silica doped with the same dopant and same minimum, end concentration level. Alternatively, the cladding layer can be formed from pure silica, or

even silica doped with some other dopant and at a third dopant concentration. If desired, the cladding layer may also have a graded doping.
In yet another aspect of the present invention, the complex comprising the doped silica rod, the outer layer and the cladding layer, is provided with a jacket. The jacket can be added by either further plasma deposition, or alternatively, by providing a jacketing material over this complex and then applying heat to collapse the jacketing material into a finished preform.
During plasma deposition, a dry plasma gas having a low hydroxyl concentration is used to form the plasma. A dry quartz source gas comprising SiCU, or other similar source gases having low hyroxyl concentration, and a dopant source gas such as GeCI4, which is sometimes co-doped with POCI3 or PCI5 are introduced in proximity to the plasma. This causes the material to be converted to silica (Si02), or silica doped with germanium oxide (Ge02) and or phosphorous pentoxide (P2O5) and deposited onto the target and fused into vitreous quartz in one simple step.
Accordingly the present invention provides A method for making an optical fiber perform, said method comprising steps of: (a) providing a target rod formed from a first material; (b) concurrently depositing atop said target rod, a first silica layer doped with a first dopant provided at a first concentration, said first silica layer being deposited and sintered to a predetermined first thickness; (c) drawing down said target rod with said first silica layer deposited thereon to a predetermined first diameter, thereby forming a doped silica rod; (d) repeating steps (b) and (c) (d 1) for a predetermined number of times, or (d2) until said first material comprises a predetermined proportion of said doped silica rod; (e) depositing on said doped silica rod a second layer comprising silica doped with a second dopant provided at a second concentration, said second silica layer being deposited to a predetermined second thickness to thereby form an intermediate structure; (f) depositing a third layer on said intermediate structure, said third layer being deposited to a predetermined third thickness to thereby form a preform structure; and, at least one of said steps (b), (e) and (f) is performed by plasma outside vapor deposition, which comprises steps of: providing a high-frequency plasma torch comprising a coil, said plasmatron being selectively positionable along a length of said target with a
7

spacing of 30-55 mm separating the target from said coil; introducing a plasma gas having a hydroxyl content of less than 2 ppm into the plasmatron to form a plasma; injecting a source gas at least SiCI4 and a dopant into a region in communication with said plasma, said source gas having a hydroxyl content of less than 0.5 ppm; and depositing at least one reaction product of said plasma and said source gas onto the target while maintaining said spacing between the target and the coil.
The invention will now be described more in detail with reference to embodiments given by way of example and shown in the accompanying drawings, in which;
Fig. 1 shows an apparatus used to perform plasma deposition;
Fig. 2 shows a partial side view of a plasmatron used in the apparatus of Fig. 1;
Fig. 3 shows a top view of a plasmatron similar to that shown in Fig. 2;
Fig. 4 shows a flow pattern of the plasma within the plasmatron of Fig. 3;
Fig. 5 shows an optical fiber preform made in accordance with the method of the present invention.

Fig. 1 shows an apparatus 20 used for plasma outside vapor deposition. The apparatus comprises a chamber 22-which is sealed so as to prevent impurities from being introduced into the final product.
Within the chamber 22 is a lathe 24, such as that available from Heathway Ltd. or Litton Engineering Lab. The lathe 24 has a headstock 25 and a tailstock 26. The headstock 25 and the tailstock 26 are provided with a pair of opposing rotating spindle chucks 28 which hold the ends of an elongated target 30 having a substantially cylindrical outer wall. The spindle chucks 28 rotate target 30, as indicated by arrow Al. A movable carriage 32 movably mounted to the lathe 24 is arranged to travel in either direction along the target, as indicated by double headed arrow A2.
A plasma source, shown generally as 40, is supported by carriage 32. Carriage 32 thus moves plasma source 40 along the length of the target 30. This results in the deposition of material on top of the target 30 to form an aptical fiber preform. The spindle chucks 28 rotate the barget 30 to ensure that material is uniformly deposited oy the plasma source 40 around the target so as to form a tubular member 34 having nearly perfectly cylindrical 3uter walls.
In the preferred embodiment, the plasma source 40 jositioned on the carriage 32 moves in both directions ilong a substantial portion of the length of the target )0. This allows the plasma source 40 to travel along ;his portion of the target 30 and deposit materials :herealong.
Instead of moving the plasma source 40 along the Length of the target, the target 30 may be moved while the plasma source 40 remains stationary. This can be realized by having the headstock 25 and the tailstock 26 the lathe move the target in a reciprocating fashion that all relevant portions of the target are brought lirectly above the plasma source 40.

As another alternative, a plurality of plasma sources may be spaced apart along the length of the target. This allows for reduced movement of either the headstock 25 and tailstock 26 of the lathe 24 or the carriage 32 to which the plasma sources are attached, depending on which of the two is configured to move. In the extreme case where a great number of plasma sources are provided all along the length of the target, no movement of either the carriage 32 or the headstock 25 and tailstock 26 of the lathe 24 is needed.
In the preferred embodiment, the plasma source 40 is a plasmatron torch having a dry plasma gas introduced into it through a first gas line 42 and a source gad introduced into it through a second gas line 44.
The plasma gas is substantially comprised of nitrogen and oxygen in an appropriate, predetermined proportion. Air may serve as the plasma gas. In such case, filtered air first passes through a first dryer 46 to remove moisture before entering the first gas line 42. This ensures that the hydroxy1 concentration of the plasma gas is low, on the order of 2.0 ppm, or less. The total volume of gas being delivered will be regulated by a mass flow controller (HFC) 80 or by a flowmeter, as an alternative.
The source gas comprises a source chemical such as SiCl4, and at least one carrier gas, such as oxygen 0, or nitrogen N2. The carrier gases enter the Asecond dryer 48 to remove moisture. This ensures that the hydroxyl concentration of the source gas is also very low, on the order of 0.5 ppm. After the carrier gases are dried, they proceed to a MFC 81 before entering a bubbler 50 to pick up the source chemical. Depending upon the characteristics of the MFC, it is also possible to use it downstream of the bubbler. The gas stream comprising carrier gases laden with the source chemical then proceeds to the second gas line 44. Optionally, by opening valve 52, a dopant gas may be introduced into the gas stream before it reaches the plasmatron torch.

In the preferred embodiment, the source chemical Is SiCl4. This chemical is chosen for its retctive properties in a plasma. Specifically, the SiCl2 serves as a source of Si to form SiO2 which is deposited on the target 30. The dopant can be a fluorine dopant gas in the form of SiF4 or SiF2. Fluorine dopants will lower the index of refraction and also change the viscosity of the quartz. In addition, fluorine dopants result in increased design flexibility for optical fiber preforms. As is well known, however, if one wishes to increase the index of refraction, Geo, or other equivalent substance may be used as the dopant.
In the preferred embodiment, the source chemical for GeO2 is GeCl4. This chemical is chosen for its purity because of its having similar physical ami chemical properties SiCl4. The delivery of the GeCl4 will be similar to SiCl4. The carrier gas from the dryer 48, can be split to another branch where it will be regulated by a MFC 82, before proceeding to a bubbler 83 to pick up the source chemical GeCl4. Similar to the control of chemical SiClf4 the HFC can also be located downstream of the bubbler. This gas stream can feed into the gas line 44 and form a mixture before entering the plasmatron torch. It is also possible to directly introduce the GeCl4 gas stream by a separate line 84 to the plasmatron torch. One advantage of using the separated delivery lines is to minimize the competing chemical reactions between GeCl^ and SiCl4. Other source chemicals that can be used for doping instead of germanium oxide (Geo,) or co-doping with germanium oxide are materials such as POCI3, pels, and other similar index increasing dopants such as Aluminum and Titanium containing chemicals.
Fig. 2 shows a cutaway side view of the plasmatron torch 40 positioned below the target 30. The plasmatron torch 40 comprises a substantially tubular torch housing 50 formed from quartz. The housing has a diameter of 60 mm and a height of 220 mm. However, diameters ranging

from 40-80 mm and heights between 180-400 mm may also be used.
A copper induction coil 52 is provided around the upper portion of the housing 50. The coil 52 comprises a plurality of windings 54 having a diameter of approximately 72 mm and spaced apart from each other by 6 mm. A gap between the housing and the coil can be between 2-10 mm. The uppermost portion of the coil 52, as indicated by uppermost winding 54", is separated from the outer surface of the tubular member 34 by a spacing designated by L, which is on the order of 30-55 mm.
As the quartz glass is deposited, its outer diameter increases. However, the spacing L is maintained by" adjusting the height of a support stand 56 on which the plasma torch 40 is placed. Support stand 56, in turn, is mounted to carriage 32, and moves laterally therewith. Initially, the support stand 56 is set at a predetermined height, and this height is reduced as the diameter of the deposited material increases during deposition. This maintains a predetermined distance between the plasma torch 40 and the deposited material. An optical or other sensor mounted on the carriage 32 and connected to a controller may be used to gauge the distance of the radially growing tubular member 34 from the carriage, and adjust the height of the support stand 56, accordingly.
On either side of the uppermost portion of the housing 50 is a plasma stabilizer bar 58. Each stabilizer bar is formed from quartz and comprises a U-shaped gutter extending laterally from the rim of the housing 50. The stabilizer bars 58 have a diameter of 60 mm and extend 20 mm on diametrically opposite sides of the housing rim, although diameters in the range of 40-80 mm and lengths of 15-40 mm may also be used. When the plasmatron torch 40 is in use, the stabilizer bars 58 are aligned parallely to the target. This arrangement helps spread the reactive source chemicals being deposited onto the growing tubular member 34.

A pair of injection ports 60 connect the second gas line 44 carrying the source chemicals to the plasmatron torch 40. The injection ports 60 enter the housing at substantially the same height along the housing 50, at a point between the uppermost windings 54" of the coil 52 and the stabilizer bars 58. The injection ports comprise quartz tubing having a diameter of 5 mm, although tubing diameters on the order of 3-10 mm may be used with the plasmatron torch 40 of the present invention. In the preferred embodiment, a pair of injection ports 60 enter the housing 50 at the same height and are positioned diametrically across from each other, instead of just two such ports, however, three or even more ports, symmetrically arranged, may be provided. In Pig. 2, the two injection ports 60 are shown to be directly beneath the stabilizer bars. This, however, is not an absolute necessity, and the injection ports 60 may be angularly offset from the stabilizer bars 58, in a top view of the plasmatron torch, as shown in Fig. 3.
A pair of plasma gas inlets 62 connect the first gas line 42 carrying the plasma gases to the plasmatron torch 40. The plasma gas inlets 62 enter the housing at substantially the same height, proximate to the base of the housing. These inlets 62 comprise stainless steel tubing having a diameter of 5 mm, although a range of diameters may suffice for this purpose.
The plasmatron torch 40 is also provided with a coolant inlet 64 and outlet 66. During use, a coolant, such as water, passes through the inlet 64, circulates within the outer wall of the housing 50, and exits through the outlet 66. The coolant inlet and outlet are formed from stainless steel and have a diameter of 5 mm. As with the plasma gas inlet and the injection port, this diameter may also vary.
The plasma gas inlets 62, the coolant inlet 64 and the coolant outlet 66 are all formed in a stainless steel chamber 68. The chamber 68 is a stainless steel square block 80 mm on a side, and having a heigh of

approximately 40 mm. The chamber 68 is mounted onto the support stand 56 which, in turn, is mounted on the carriage 32 for movement along the target 30.
A high frequency generator (not shown) is electrically connected to the coil 52, powering the latter with a variable power output up to 80 kW at a frequency of approximately 5.0 MHz. In the preferred embodiment, the generator is Model No. T-80-3MC from Lepel Corporation. This generator is driven with a 60 Hz, 3-phase 460 V power supply to energize the plasmatron torch 40. As an alternative, a Model No. IG 60/5000 generator is available from Fritz Huttinger Electronic GmbH of Germany.
Fig. 4 depicts the plasma jet 70 forred within the plasmatron torch 40 when the dry plasma gas is fed through the inlets 62 and converted into a plasma. The plasma jet 70 is substantially symmetric about the torch"s longitudinal axis A". The position of the injection ports 60 is such that the source chemicals are introduced into the plasma just above a point V where the vertical velocity of said plasma is zero. This provides the needed structure of hydrodynamic and thermal flow of the source chemical jet into the border layers to realize efficient deposition onto the growing tubular member 34. And while the preferred embodiment has the injection ports entering laterally into the housing, this is not an absolute requirement. Instead, the source gases may introduced into the center of the plasma jet 70 by a water cooled probe extending along the longitudinal axis A" of the plasmatron torch 40.
Fig. 5 illustrates a well-known procedure which can performed with a lathe 124, such as Model No. PFH842XXLS Precision Quartz and Glass Working Lathe, manufactured by Heathway. The headstock 125 and tailstock 126 of the lathe 124 can move longitudinally relative to one another. This allows for easy loading and unloading of a finished workpiece 130 of length L3 which has been leposited atop an initial target. More significantly, it

also allows one to draw down a portion of a workpiece into a secondary rod of a reduced diameter comparable to that of the original target. This is accomplished by-keeping the headstock 125 stationary and moving the tailstock 126 away from the headstock 125 while the plasma source 140 is moved in a direction opposite to that of tailstock 126. Alternatively, this can also be accomplished by placing a plasma source 140, or other heat source, at one end of the workpiece 130 to soften it. Then the headstock 125 and tailstock 126 are moved in the same direction, but with different speeds by distances L5, L4, respectively, to the positions shown in phantom 125", 126". The result is a thin, secondairy rod 132, which can (but need not) have the same diameter as the original target. As is known to those skilled in the art, the secondary rod has the same cross-sectional composition as the workpiece from which it is derived, as so has a center whose consistency along is substantially similar to that of the original target, and outer layer substantially similar to the materials deposited atop the target during the formation of the workpiece.
The lathe 124 allows the headstock 125 and tailstock 126 to be moved far enough longitudinally to stretch the secondary rod to a distance L4, which is substantially the same as the length L3 of the workpiece from which it is derived. The secondary rod 132 may be cut from the workpiece, mounted on the lathe 124 in place of the workpiece 130, and used as a target for subsequent deposition with the plasma source 140. Thus, the original, or first-generation, target is used to create a first-generation workpiece, from which a secondary rod can be drawn to be used as a second-generation target. Deposition atop this second-generation target can will thereby form a second-generation workpiece, and so on. This iterative process of plasma deposition on a target to form a workpiece, stretching one end of the workpiece to form a reduced-diameter rod, and using this reduced-

diameter rod as a subsequent target for further deposition can be repeated an arbitrary number of times. If the material being deposited atop the target is unchanged through the iterations, the result of N iterative steps is an N-th generation rod having a very small center which is substantially identical in composition to the original target, and an annular layer reflective of the materials deposited atop the target. For instance, if the original target has a diameter Dl and the finished workpiece has a diameter D2 = M x Dl, then the proportion of the original target material in the first-generation workpiece is approximately 1/M2. If a second-generation target of diameter Dl is drawn from this workpiece and material sufficient to form a second-generation workpiece of diameter D2 is deposited thereon, the proportion of the original target material in the second generation workpiece is approximately 1/M2. Thus, it can be seen that one may readily form a workpiece having a predetermined proportion of the original target material therein by controlling M during deposition, along with the total number of iterations.
A method for forming a multimode optical fiber preform using the aforementioned iterative technique will now be described. In order to provide a more detailed explanation, some dimensions are given. However, it must be noted that in the actual process, many different value are possible.
The method begins by providing a first generation target, horizontally mounted on a lathe, such as that shown in Fig. 5. The target is preferably formed from pure silica, in which case it may be purchased from a commercial vendor, such as Product no. F300, available from Heraeus Amersil of Georgia. Alternatively, the first-generation target may be an Nth-generation doped silica rod formed using the current process. In the preferred embodiment, the first-generation target has a length of one meter and a diameter Dl = 6 mm.

silica doped with GeO2 is deposited atop the first-generation target using the plasma source described above. The dopant concentration for the Geo2, depends on the desired numerical aperture (NA) of the multimode optical fiber being produced. For instance, to form a fiber with a NA of 0.2, the maximum GeO2 dopant concentration is approximately 10%. And to form a fiber with a NA of 0.275, the maximum GeO2 dopant concentration will be approximately 18%.
The dopant concentration may be held at the same level during deposition, in which case a stepped layer, is formed. Alternatively, the dopant concentration may be gradually varied to form a graded layer. This is done by automatically controlling, by means of. -i microprocessor or like, an adjustable flow meter through which the dopant is introduced. It should be noted that stepped and graded layers may succeed one another in subsequent generations of workpieces, and that layers having different, constant doping concentrations may succeed one another, as well. Thus, a graded layer may be deposited on the first-generation target, and a stepped layer may be deposited atop the second-generation target formed after drawing down the first-generation workpiece. Similarly, one may deposit a stepped layer atop a graded layer, which has been deposited atop an original first-generation target. Also, a first stepped layer, having a first dopant concentration, may be deposited atop a target, and a second stepped layer, having a second dopant concentration, deposited atop the next generation target. Additional layers, either graded or stepped, may be deposited atop any of the above structure.
In the preferred embodiment, silica doped with 18 % GeO2 is deposited as a stepped layer atop the 6 mm diameter first generation target until a workpiece having a length of one meter and a diameter of D2 = 48 mm is formed (i.e., M = 8). This resulting first-generation workpiece has approximately 64 times the cross-sectional

the original target, two layers having substantially the same optical properties and fairly indistinguishable from one another, and a third, graded layer.
In the preferred embodiment, the 80 mm diameter third-generation workpiece is subject to additional processing to form a primary optical fiber preform. Specifically, a cladding, or barrier, layer is deposited atop the third-generation workpiece. The Thickness of the cladding layer depends on the type of finished optical fiber preform to be made. For a 62.5/125 fiber preform, the finished primary preform will have a final diameter of about 93 mm. For a 50/125 fiber preform, the finished primary preform will have a final diameter of about 96 mm. The cladding layer is formed by depositing silica doped at the seune concentration of GeO2 as the minimum doping concentration level used to form the third layer, i.e., 10% GeO2. This results in a structure having the original target material at the center, a constantly doped pair of second layers having the same optical properties, a graded layer having a dopant concentration varying from a maximum value to a minimum value, and a cladding layer comprising silica doped at the minimum value.
Once the cladding layer is applied, the finished primary preform must be stretched to form the final preforms. From a single, 1 meter long 62.5/125 preform having a diameter of 93 mm diameter, one can obtain eight, one-meter long preform pieces, each having an outer diameter of 32 mm. And from a single, 1 meter long 50/125 preform having a diameter of 96 mm diameter, one can obtain twelve, one-meter long pieces, each having an outer diameter of 27 mm.
A jacketing layer may be applied atop the cladding layer of these preform pieces. The jacketing layer preferably has the same index of refraction as pure silica. The jacket may be applied by plasma outside vapor deposition using pure silica. Alternatively, a tube or sheet of pure silica, having an appropriate

diameter or width, may be provided around a preform piece, and heat applied to fuse the jacket onto the preform piece to form the final optical fiber preform. In the preferred embodiment, the final optical preform has an outer diameter of about 56 mm. This final preform may then be drawn into approximately 200 Km of fiber having a diameter of 125 µM.
Although, for best performance, a cladding and then a jacketing layer is applied, it should be noted that one may dispose of the cladding step and directly apply a jacketing tube to the third-generation workpiece, once it has been stretched.
A similar method for making single moie optical fiber preform can be achieved by using the following procedure. The starting target can be a pure silica rod that can be either a F300 rod purchased from Heraeus or a pure s;.lica Nth-generation rod fabricated in house. Multiple fluorine doped silica layers with constant concentration are deposited on the target until it reaches a desired diameter. Single mode optical fibers can be drawn from this preform. There are many different glass index modifiers such as F, GeOj, P2O5, TiOj, AI2O3, etc., and in the proper combination, they can be used to make the doped core and/or doped cladding. In the preferred embodiment, the target is a Nth-generation GeOj doped rod with pure silica or doped silica cladding layers deposited on it. The preform is completed when the desired diameter is reached.
While the present invention has been disclosed with reference to certain preferred embodiments, these should not be considered to limit the present invention. One skilled in the art will readily recognize that variations of these embodiments are possible, each falling within the scope of the invention, as set forth in the claims below.

WE CLAIM:
1. A method for making an optical fiber perform, said method comprising steps of; (a) providing a target rod formed from a first material; (b) concurrently depositing atop said target rod, a first silica layer doped with a first dopant provided at a first concentration, said first silica layer being deposited and sintered to a predetermined first thickness; (c) drawing down said target rod with said first silica layer deposited thereon to a predetermined first diameter, thereby forming a doped silica rod; (d) repeating steps (b) and (c) (d 1) for a predetermined number of times, or (d2) until said first material comprises a predetermined proportion of said doped silica rod; (e) depositing on said doped silica rod a second layer comprising silica doped with a second dopant provided at a second concentration, said second silica layer being deposited to a predetermined second thickness to thereby form an intermediate structure; (f) depositing a third layer on said intermediate structure, said third layer being deposited to a predetermined third thickness to thereby form a preform structure; and, at least one of said steps (b), (e) and (f) is performed by plasma outside vapor deposition, which comprises steps of: providing a high-frequency plasma torch comprising a coil, said plasmatron being selectively positionable along a length of said target with a spacing of 30-55 mm separating the target from said coil; introducing a plasma gas having a hydroxyl content of less than 2 ppm into the plasmatron to form a plasma; injecting a source gas at least SiCU and a dopant into a region in communication with said plasma, said source gas having a hydroxyl content of less than 0.5 ppm; and depositing at least one reaction product of said plasma and said source gas onto the target while maintaining said spacing between the target and the coil.

2. The method as claimed in claim 1, comprising the step of; (g) applying a jacketing layer stop said perform structure, said jacketing layer consisting essentially of pure silica and being applied to a predetermined fourth thickness.
3. The method as claimed in claim 2, comprising the step of: drawing down said preform structure to a predetermined third diameter after step (f) and prior to step (g).
4. The method as claimed in claim 1, wherein the first material is selected from the group consisting of silica and silica doped with a dopant.
5. The method as claimed in claim 4, wherein the dopant is an index modifying material which is selected from the group consisting of F, Ge02, P2O5 Ti02 and AI2 O3.
6. The method as claimed in claim 1, comprises the step of varying the second concentration as the second silica layer is deposited.
7. The method as claimed in claim 6, wherein the second depoant is fluorine , and said second concentration is varied from a minimum value when said second silica layer is first being deposited, to a maximum value when deposited of said second silica a layer is nearing completion.
8. The method as claimed in claim 6, wherein said second concentration is varied from a maximum value when said second silica

layer is first being deposited, to a minimum value when deposition of said second silica is nearing completion.
9. The method as claimed in claim 8, wherein said maximum value of said second concentration is substantially the same as said first concentration.
10. The method as claimed in claim 8, wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with said second dopant at said minimum value.
11. The method as claimed in claim 8 wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with fluorine.
12. The method as claimed in claim 1, wherein the third layer deposited in step (f) is a cladding layer deposited by plasma outside vapor deposition, said cladding layer consisting essentially of silica doped with fluorine.
13. The method as claimed in claim 1, wherein the source gas is introduced just above a point in the plasmatron at which the vertical velocity of the plasma is zero.

coil comprises a plurality of windings, and the target is separated from a winding closest to the target by said spacing.
15. The method as claimed in claim 1, wherein the plasma gas is
dried before it is introduced into the plasmatron.
16. The method as claimed in claim 1, wherein the first and second
dopants and the first and second concentrations, are the same.
17. A method for making an optical fiber preform substantially as
herein above described with reference to the accompanying drawings.

Documents:

0421-mas-1999 abstract.pdf

0421-mas-1999 claims-duplicate.pdf

0421-mas-1999 claims.pdf

0421-mas-1999 correspondence-others.pdf

0421-mas-1999 correspondence-po.pdf

0421-mas-1999 description (complete)-duplicate.pdf

0421-mas-1999 description (complete).pdf

0421-mas-1999 drawings.pdf

0421-mas-1999 form-1.pdf

0421-mas-1999 form-19.pdf

0421-mas-1999 form-26.pdf

0421-mas-1999 form-4.pdf

0421-mas-1999 petition.pdf

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Patent Number

216650

Indian Patent Application Number

421/MAS/1999

PG Journal Number

17/2008

Publication Date

25-Apr-2008

Grant Date

17-Mar-2008

Date of Filing

15-Apr-1999

Name of Patentee

FIBERCORE INC

Applicant Address

253 WORCESTER ROAD, P.O BOX 180, CHARLTON,MA 01507,

Inventors:

#	Inventor's Name	Inventor's Address
1	MIKHAIL IVANOVICH GOUSKOV	ST IVANA FOMINA 13-1, APT.604, 194352 ST PETERSBURG,
2	MOHAMMAD AFZAL ASLAMI	7 LAUREL HILL DRIVE,STURBRIDGE, MASSACHUSETTS 01566,
3	EVGUENI BORISOVICH DANILOV	BOLSHEVIKOV PR.9-2 APT.11,193313, ST.PETERSBURG,
4	DAU WU	44 GILMORE ROAD, SOUTHBOROUGH, MASSACHUSETTS 01772,
5	JOHN EDWARD MATTISON	236 WEST MAIN STREET, WEST BROOKFIELD, MASSACHUSETTS 01585,

PCT International Classification Number

C03B 037/07

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA