Title of Invention	Apparatus and Method for Fabricating Optical Fiber Preform
Abstract	A rotary seal 20 capable to joining a stationary member 21 with the rotating member 22 is provided, wherein the rotary comprises a body 23 having a central hole 24 therethrough, wherein the central hole 24 has stepwise reducing diameter so as to form two part 25 and 26 central hole 24, wherein front part of 25 of said central hole 24 has higher inner diameter and rear part 26 of said central hole 24 has lower inner diameter, wherein said front part 25 forms a seat 27 with said rear part 26 of said central hole 24, wherein said front part 25 of said central hole 24 is capable of accommodating at least one sealing member 28, and said sealing member 28 is made to sit on to said seat 27 by a screwing member 31, which when fixed onto body 23 fits in to front part 25 of said central hole 24, and said rear part 26 of said central hole 24, is capable of accommodating connecting member 32 of said stationary member 21. An apparatus comprising the rotary seal 20 is also provided.

Title of Invention

Apparatus and Method for Fabricating Optical Fiber Preform

Abstract

A rotary seal 20 capable to joining a stationary member 21 with the rotating member 22 is provided, wherein the rotary comprises a body 23 having a central hole 24 therethrough, wherein the central hole 24 has stepwise reducing diameter so as to form two part 25 and 26 central hole 24, wherein front part of 25 of said central hole 24 has higher inner diameter and rear part 26 of said central hole 24 has lower inner diameter, wherein said front part 25 forms a seat 27 with said rear part 26 of said central hole 24, wherein said front part 25 of said central hole 24 is capable of accommodating at least one sealing member 28, and said sealing member 28 is made to sit on to said seat 27 by a screwing member 31, which when fixed onto body 23 fits in to front part 25 of said central hole 24, and said rear part 26 of said central hole 24, is capable of accommodating connecting member 32 of said stationary member 21. An apparatus comprising the rotary seal 20 is also provided.

Full Text	FORM 2 THE PATENTS ACT, 1970(39 of 1970) & THE PATENTS RULES, 2003 PROVISIONAL PATENT SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention:- Apparatus and Method for fabricating Optical fiber preform. 2. Applicant(s):- (a) Name: STERLITE OPTICAL TECHNOLOGIES LTD. (b) Nationality: An Indian Company (c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA 3. Preamble to the Description- Provisional Patent Specification: The following specification describes the Invention. STER/PA-H Field of the Invention: 5 The present invention relates to an apparatus and method for fabricating optical fiber preform. Particularly it relates to an apparatus and method for fabricating optical fiber preform and optical fiber therefrom, which has low attenuation in the wavelength range from 1300 nm to 1625 nm. 10 Background of Invention: Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. The primary object of telecommunication industry is to transmit greater amount of information, over longer distances, in shorter period of time. This object can be fulfilled with the optical fibers provided it has low optical 15 attenuation loss in the wavelength ranges varying from about 1300 nm to about 1625 nm. Optical fiber is drawn from optical fiber preform. The optical fiber preform generally comprises a central core and an outer cladding. The core rod itself comprises a core and part of cladding of the fiber preform. Conventionally, the core rod can be prepared by any known method, for example by Atmospheric Chemical Vapor 20 Deposition (ACVD) method, wherein the soot is deposited during the deposition process step on the cylindrical member (also referred as target rod or mandrel) to form soot porous body. In this method, the soot deposition is accomplished by traverse motion of the target rod over the burners or vice versa. The initial soot deposition comprises dopant chemicals to increase refractive index of the core and dopant chemicals are terminated 25 after desired core diameter is obtained. The deposition process continues until the required dimension of the soot porous body is attained for meeting desired core diameter in the optical fiber preform and the desired core-clad diameter ratio in the fiber. A typical single mode optical fiber may have core of about 8-10 um in diameter and clad of about 125 µm in diameter. After completion of soot deposition, the target rod is removed from 30 the soot porous body to form hollow cylindrical soot porous body defining a capillary at the center [herein after referred to as hollow soot porous body]. A glass plug is inserted 2 STER/PA-H into the end remote from the handle of the hollow soot porous body. The hollow soot porous body is moved into a sintering furnace, wherein this hollow soot porous body is first dehydrated and then sintered (also known as vitrification or consolidation) in a chlorine and helium atmosphere to form optical fiber preform at about 1500°C. 5 The dehydration and sintering processes can be carried out by any method known in the art. Preferably, it can be carried out inside specially built furnaces that are equipped with one or more heating elements and gas input mechanisms. The dehydration and sintering processes comprise inserting the hollow cylindrical soot porous body into a sintering furnace and subjecting it to a temperature regime under controlled chemical 10 environment to form sintered glass preform. The chemical environment necessary for dehydration is provided with the help of gases that promote dehydration. The chemical environment that is necessary for sintering is provided with gases that are inert and have high thermal conductivity. The sintered glass preform with capillary is subjected to, after removal from the 15 sintering furnace to a process step of rod draw to form a plurality of core rods having predetermined diameter. During this process step, vacuum is applied to the capillary to result in collapsing of the capillary, which gets closed due to the glass surface tension; relative viscosities of core and cladding. The application of vacuum increases the speed of collapsing step and it also facilitates removal of gases formed in the preform capillary 20 during the rod draw step. The core rod drawn may be, if desired, subjected to a process step of overcladding by depositing soot over the core rod to form soot porous body having solid core rod. The soot porous body may be subjected to process steps of dehydration and consolidation to form optical fiber preform (called daughter preform). Accordingly, the optical fiber can 25 be drawn either directly from the mother preform or the daughter preform. As described hereinabove the sintered preform with uncollapsed capillary therein is removed from the furnace and then in the separate step of rod draw, it is collapsed under negative pressure or vacuum. This step of collapsing capillary in rod draw step increases the chances of capillary contamination. The preform capillary forms the center 30 of optical fiber core, through which most of the light travels. The performance of the optical fiber used for transmission is primarily determined by the optical attenuation loss 3 STER/PA-H and dispersion in the optical fiber. The contamination or deformation of any kind of the capillary has a severe negative impact on the optical fiber attenuation drawn therefrom. It is desired that the capillary should not be exposed to the ambient atmosphere at all. The U.S. Patent 4,251,251 discloses a method so that the capillary is not exposed 5 to the ambient atmosphere at all. According to '251, the process steps of making the preform are carried out in the order namely: 1. Deposition; 2. Removal of target rod; 3. Sintering and collapsing the capillary to get solid glass preform without any 10 capillary therein; 4. Rod drawing from the so formed solid glass preform; prevents exposure of capillary to the ambient atmosphere. The capillary is collapsed simultaneously with the sintering of hollow soot porous body or during the step of sintering without removing it from the sintering furnace. Vacuum is applied to the 15 capillary of the hollow soot porous body. The capillary collapses during the sintering step due to glass surface tension; relative viscosities of core and cladding layer. The U.S. Patent '251 also discloses that in order to collapse the capillary in sintering step itself, higher temperature is required than that needed for only sintering the hollow soot porous body. This temperature is higher than the softening point of glass. 20 Even at the softening point of glass, the glass gets deformed and elongated due to gravity. The elongation distorts the preform shape and, hence the refractive index profile. This deformation of refractive index profile and shape results in unwanted attenuation increase or other waveguide parameters of the optical fiber. For example, the clad to core diameter ratio may vary if the shape of the resulting preform is deformed in sintering. 25 This results in variations in the core diameter of the resulting optical fiber drawn therefrom. The variations in core diameter results in change of cutoff wavelength value from the desired value. The variation in core diameter result in elevated attenuation in the optical fiber. The U.S. Patent '251 does not provide any remedy for these problems. It is observed that the above-mentioned problem is solved by rotating the hollow 30 soot porous body about its longitudinal axis at a predetermined rotational speed. The centrifugal acceleration due to rotation nullifies the gravity effect and eliminate the 4 STER/PA-H distortion. The hollow soot porous preform rotation about its longitudinal axis of symmetry allows for proper collapse of the capillary and other problems related with unsymmetrical collapse are taken care of. The hollow soot porous body with capillary, which is suspended in the sintering 5 furnace, is rotated along its longitudinal axis of symmetry and also the soot porous preform capillary is connected to a stationary vacuum pump through a pipe. A connector [herein after referred to as rotary sealing mechanism] for connecting a stationary pipe from the vacuum pump and the rotating soot porous preform capillary is needed. The U.S patent 4,347,069 discloses a rotary seal at the junction of stationary 10 member connecting the supply source of vapor stream and the rotating preform tube. The rotary seal according to this patent consists of locating an end of the rotating member inside an end of another rotating member and positioning two washers or O-rings between the two ends of the rotating members. The another end of another rotating member is then made to sit in an end cap provided with two inlets, one inlet for receiving 15 the tube connected to supply source of vapor stream and another inlet for receiving the tube connected to supply source of oxygen. This rotary seal overcomes problem of early wear out of washers or O-rings by connecting one end of rotating member connected to rotating preform tube with one end of another rotating member. However, another end of said another rotating member, 20 which constitutes an additional part of the apparatus is still located onto one end of stationary member connected to supply source of vapor stream which is fitted inside said another end of said another rotating member [an additional part of the apparatus] in the manner known in the prior art. This particular rotary seal as disclosed in the '069 can be used for connecting the 25 stationary vacuum tube and the rotating hollow soot porous body. However, the inventors of the present invention faced limitations described below. Even the rotary seal of above-referred patent suffers from the main problem of early wear out of washers or O-rings. As the amount of heat to which the rotary seal will be exposed during the 30 simultaneous sintering and collapsing of the soot porous body is quite high than of the amount of heat during the fabrication of preform in accordance with the method 5 STER/PA-H described in the said document '069. Therefore, it has been observed by the inventors of the present invention that the O-rings wear out during a single operation also itself, because the O-rings are positioned inside the rotary seal. The wearing out of O-rings during the simultaneous sintering and collapsing of the soot porous body is observed to 5 cause following problems:- 1. Certain particles are generated as the O-rings wear out due to heat and friction against the stationary/rotating members (vacuum tube/ soot porous body handle) which plunge into the capillary of the soot porous body thereby resulting in contamination of the capillary and, hence increase in the attenuation level of the resulted optical fiber drawn 10 therefrom. 2. Further, wearing of O-rings also lead to leakage of vacuum in the soot porous body capillary thereby causing contamination present in the ambient atmosphere to enter in the capillary of the soot porous body which in-turn further contaminates it. This contamination results in further increase in the attenuation level and thereby makes the 15 resulting optical fiber, drawn therefrom, unsuitable for any use. 3. Still further, the vacuum leakage due to rupture or wearing out of the O-rings has been observed to result in partial or incomplete closure of capillary and/or formation of bubbles therein. 4. Still another problem of the rotary seal of said patent '069 is that it calls for additional 20 supply of oxygen gas that's too at a very high pressure at least higher than the pressure of the vapor stream. This additional supply of oxygen gas at a very high pressure is provided to achieve pressure differential in the cavity created between stationary end cap and said another [additional] rotating member to avoid or inhibit leakage of toxic vapor stream in the environment as and when there is a leakage due to early wear out 25 of washer or O-rings provided between said another end of said another rotating member which constitutes an additional part of the apparatus and the stationary member connected to source of supply of vapor stream. Therefore, it is clear from the foregoing description that even the rotary seal provided by said US patent '069 suffers from various disadvantages, drawbacks and 30 limitation, and in-addition it also requires provision of two additional parts - a) said another rotating member and b) said end cap, and additionally it also requires i) 6 STER/PA-H additional supply source of oxygen, ii) additional supply of oxygen gas and iii) that's too at a very high pressure thereby resulting in increase in cost of apparatus and cost of production of optical fiber preform, but it still suffers from the main problem of leakage of rotary seal at the junction of rotating member and stationary member thereby making 5 the rotary seal of said patent '069 and the apparatus comprising the same and the method employing the same highly un-satisfactory and highly un-economical in-addition to being complicated due to precise provision of said end cap to achieve the pressure differential. Further, in case of leakage, not only the toxic vapour stream will come in contact with the environment, but also the oxygen gas, that's too at a very high pressure, which 10 in-turn even in PPM level will cause damage to the skilled persons operating the system and in the surrounding area in-addition to very high risk of fire due to leakage of oxygen at a very high pressure. It has also been observed that due to leakage of commonly available washers or O-rings, the oxygen which is supplied at a very high pressure, dilutes the vapour stream 15 entering the rotating preform tube thereby resulting in production of preform having undesired characteristics. It has also been observed that the rotary seal of US patent '069 is not suitable for employing it in the collapsing process step, because the principle of pressure differential does not work when the stationary member is connected to source of vacuum. 20 Therefore, the rotary seal as disclosed in '069 cannot be used for connecting the stationary vacuum pump and hollow soot porous body capillary, particularly when simultaneous sintering of the hollow soot porous body and collapsing of the capillary of the hollow soot porous body is desired. 25 Need of the Invention: Accordingly, there is still a need to have an improved rotary sealing mechanism which is suitable for collapsing process step for causing collapse of capillary in the hollow soot porous body and particularly when simultaneous sintering of the hollow soot porous body and collapsing of the capillary of the hollow soot porous body is desired, 30 and at the same time it does not suffer at least from majority of above described disadvantages, drawbacks and limitations of the prior art, and also it does not suffer from 7 STER/PA-H the main problem of leakage of rotary sealing mechanism, as it has been observed in the rotary sealing mechanism of the prior art due to O-ring seal rupture at the junction of rotating member and stationary member, thereby making the rotary sealing mechanism thus developed and the apparatuses comprising the same and the methods employing the 5 same highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for producing the optical fiber preform. Objects and Advantages of the Invention: 10 The main object of the present invention is to provide an improved rotary sealing mechanism which is suitable for collapsing process step for causing collapse of capillary in the hollow soot porous body and particularly in process where sintering of the hollow soot porous body and collapsing of the capillary of the hollow soot porous body are performed simultaneously, and at the same time it neither suffers, at least, from majority 15 of above described disadvantages, drawbacks and limitations of the prior art, nor from the main problem of leakage of rotary sealing mechanism thereby making the presently disclosed rotary sealing mechanism and the apparatuses comprising the same and the methods employing the same highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for 20 producing the optical fiber preform. The another main object of the present invention is to provide an improved rotary sealing mechanism suitable for avoiding and inhibiting leakage of vacuum or vapor stream and/or mixing of environmental gases with the vapor stream and/or mixing of toxic gases in the environment at the junction of stationary member connected to the 25 vacuum pump or to supply source of vapor stream and the rotating member connected to the rotating hollow soot porous body preform or rotating preform tube thereby making the process more safer and more economical. Another object of this invention is to provide an improved rotary seal suitable for a) depositing the soot particles inside the rotating preform tube and also for b) collapsing 30 process step for causing collapse of capillary in the soot porous preform and at the same time does not suffer from above described disadvantages, drawbacks and limitations of 8 STER/PA-H the prior art thereby making the disclosed rotary sealing mechanism versatile and widely applicable. Still another object of this invention is to provide an improved rotary seal which does not require additional rotating member, additional supply source of oxygen and 5 additional oxygen supply and at the same time does not suffer from the main problem of leakage of the seal at the junction of rotating member and stationary member thereby making the rotary seal and the apparatus comprising the same and the method employing the same highly productive, highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for 10 producing the optical fiber preform. Yet another object of this invention is to provide an improved rotary seal which does not suffer from the problem of early wear out of sealing member due to constant rotation of rotating member against stationary member thereby avoiding and inhibiting leakage of vacuum to make the process more effective and more productive. 15 Yet another object of the present invention is to fabricate optical fiber preform by the ACVD method and using the rotary sealing mechanism in accordance with the present invention. The optical fiber obtained from the optical fiber preform so fabricated is characterized in that it can be used in the wavelength range varying from about 1300 nm to about 1625 nm, 20 Still another object of the present invention is to fabricate an optical fiber preform and optical fiber there from such that the optical fiber has an absorption loss less than or equal to 0.35 dB/Km at wavelength ranges varying from about 1300 nm to about 1625 nm. The other objects and advantages of the present invention will be more apparent 25 when the following description is read in conjunction with the accompanying drawings which are not intended to limit the scope of this invention. Brief Description of the Accompanying Figures: Figure 1A is a schematic representation of deposition process in accordance with 30 known Atmospheric Chemical Vapor Deposition (ACVD) method. 9 STER/PA-H Figure IB is a schematic representation of the hollow soot porous body with capillary therethrough connected to preform handle in accordance with the present invention. Figure 1C shows cross-sectional view of the hollow soot porous body produced in 5 accordance with known Atmospheric Chemical Vapor deposition method. Figure ID shows a preform handle for the fabrication of hollow soot porous body shown in Figure 1B in accordance with the present invention. Figure 2A shows exploded cross-sectional view of the rotary sealing mechanism in accordance with one of the preferred embodiments of the present invention. 10 Figure 2B shows exploded cross-sectional view of the rotary sealing mechanism in accordance with another preferred embodiment of the present invention, wherein a cooling fluid cools the rotary sealing mechanism. Figure 2C shows exploded cross-sectional view of the rotary sealing mechanism in accordance with still another preferred embodiment of the present invention. 15 Figure 2D shows exploded cross-sectional view of the rotary sealing mechanism in accordance with yet another preferred embodiment of the present invention. Figure 3A is schematic overall and cross sectional view of one type of lip seal in accordance with one of the preferred embodiments of the present invention. Figure 3B is schematic overall and cross sectional view of another type of lip seal 20 in accordance with another preferred embodiment of the present invention. Figure 4A is a schematic representation of the apparatus having rotary sealing mechanism for simultaneous sintering and collapsing of the hollow soot porous preform in accordance with one of preferred embodiments of this invention. Figure 4B shows enlarged cross-sectional view of the rotary sealing mechanism 25 connected to rotating member on one end and stationary member on another end in accordance with one of the preferred embodiments of this invention. Description of the Invention: Reference will now be made in detail to the present preferred embodiments of the 30 invention, examples of which are illustrated in the accompanying drawings which are not 10 STER/PA-H intended to limit scope of the present invention. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. In accordance with the invention, the hollow soot porous body 2, as exemplarily illustrated in [Figure IB], is preferably formed by chemically reacting at least some of the 5 glass-forming compounds of a moving fluid mixture in an oxidizing medium (a hydroxy flame) to form a silica-based reaction product. At least a portion of this reaction product is directed toward a target rod, to form a soot porous body. The method of forming soot porous body described above is called the Atmospheric Chemical Vapor Deposition method or just simply ACVD. 10 As shown in [Figure 1A] a target rod 3 is inserted through a glass body such as hollow or tubular handle 1 [Figure ID] and mounted on a lathe. The lathe is designed to rotate (by use of rotating means 6) and translate either target rod 3 and/or the soot-generating burner 5 in close proximity with each other. As target rod 3 is rotated and translated, silica-based reaction product 4, known as soot, is directed toward target rod 3. 15 At least a portion of soot 4 is deposited on target rod 3 and on a portion of handle 1 to form a body 2 thereon. The soot 4 may contain some concentration of a suitable dopant (generally GeCL4 used to increase the refractive index of the glass. Once the desired quantity of soot is deposited on the target rod 3, soot deposition is terminated and the target rod 3 is removed from soot porous body 2 to result in the 20 formation of a soot porous body having a capillary 9. In accordance with the present invention and as depicted in Figure 1A, upon removal of target rod 3, soot porous body 2 defines a soot porous body having a capillary 9 [Figure IB and Figure 1C] passing axially therethrough. The hollow soot porous body 2 with capillary 9 [herein after referred to as hollow soot porous body] 2 is suspended by 25 handle 1 in a sintering furnace 31 [Figure 4A]. The end 11 of the hollow soot porous preform capillary 9 remote from handle 1 is preferably fitted with a bottom glass plug 12 prior to positioning hollow soot porous body 2 within sintering furnace 31. The bottom glass plug 12 is positioned and held in place with respect to hollow soot porous body 2 by friction fit. Bottom glass plug 12 is further preferably tapered to facilitate entry and to 30 allow at least temporary affixing and at least loosely within the hollow soot porous body 2. 11 STER/PA-H The hollow soot porous body 2 is preferably chemically dried, for example, by exposing hollow soot porous body 2 to a chlorine-helium containing atmosphere at elevated temperature within sintering furnace 31. The chlorine-helium containing atmosphere removes water and other impurities from hollow soot porous body 2, which 5 otherwise have an undesirable effect on the properties of optical waveguide fiber manufactured from the hollow soot porous body 2. Following the chemical drying step, the temperature of the sintering furnace 31 is elevated to a temperature sufficient to sinter the hollow soot porous body 2 into a sintered glass preform. In accordance with the present invention, the soot porous body capillary 9 10 is collapsed simultaneously during the sintering step, which avoid additional process step thereby resulting in savings of overall production time of the solid glass preform. Simultaneous sintering of the hollow soot porous body 2 and collapsing the hollow soot porous body capillary 9 also eliminate another disadvantage that occurs when the hollow soot porous body 2 is chemically dried and sintered, and following chemical drying and 15 sintering, the resulting sintered glass preform is exposed to ambient environment, such as ambient atmosphere, for example, when the sintered glass preform is removed from the sintering furnace 31 and moved to a rod draw furnace for further processing steps. The optical fibers manufactured using such preforms have been observed to exhibit high optical attenuation in the ranges varying from about 1310 nm to about 1625 nm. This 20 high attenuation is largely due to contamination of the portion of the sintered glass preform surrounding the capillary 9 prior to capillary collapse in rod draw step. The greater the exposure period, the greater the amount of contamination adsorbed by the glass and is proportional to the time period for which the capillary 9 is exposed to the ambient atmosphere. Thus, exposure to ambient atmosphere is desired to be strictly 25 avoided in the present invention. In a preferred embodiment of the method of the present invention, exposure of the capillary 9 to ambient atmosphere is reduced or prevented by collapsing the capillary 9 simultaneously in the sintering step. In another preferred embodiment of the method of the present invention, the 30 simultaneous sintering and collapsing of the hollow soot porous body 2 capillary 9 is done under the application of negative pressure or vacuum. 12 STER/PA-H In accordance with present invention, in order to prevent distortion in the shape and refractive index profile of the hollow soot porous body 2 during the step of simultaneous sintering and collapsing, not only vacuum is applied, but also the hollow soot porous body 2 is rotated about its longitudinal axis of symmetry 29 [Figure 4A]. 5 As elaborated hereinabove, in order to rotate while avoiding disadvantages, drawbacks and limitations of the prior art, the hollow soot porous body (herein after may also be referred to as rotating member) 2 and to connect the hollow soot porous body 2 capillary to vacuum pump (herein after may also be referred to as stationary) it is necessary to use rotary sealing mechanism. 10 Figure 2A shows a cross sectional view of the rotary sealing mechanism in accordance with first embodiment of the present invention. The drawing is not drawn to scale. After completion of deposition, the soot porous body 2 is detached from the lathe along with the handle 1 [Figure IB and Figure 1C]. Thereafter, the target rod 3 is removed from the soot porous body 2, forming a capillary 9 there through as described 15 above [Figure 1C]. Thus obtained hollow soot porous body 2 having a capillary 9 is then transferred to the sintering furnace 31 [Figure 4A] along with the handle 1. In the sintering furnace the hollow soot porous body 2 is sintered and collapsed simultaneously to give a solid glass preform. 20 The hollow soot porous body handle 1 is made to fit into or onto the member 34 depending upon the relative diameter of these two members and the hollow soot porous body 2 is suspended in the sintering furnace 31. Preferably the member 34 is made of a non-corrosive and has very high melting point material, typically more than 1800°C. The member 34 has two ends, 34A fitting into the handle 1 of the hollow soot porous body 2 25 and the other 34B fitting through the screw hole 23, passing onto the lip seal 18, covering the lip seals 18. Thus the lip seals 18 do not show up inside the rotary sealing mechanism 35 interiors [Figure 3B]. Thus the lip seals 18 remain outside. The lip seals 18 are positioned in the same manner as shown in the figure 2A, 2B, 2C, 2D, and 4B. The bifurcated top of the lip seal holds both the ends tightly with which they contact. 30 The lip seal 18 described above is the one generally used in the automobile industry as oil seal in automobile engines. There are numerous types of lip seals available 13 STER/PA-H in the market. Two such types of lip seals are shown in the figures 3A and 3B. In accordance with one of the preferred embodiments of the present invention, a lip seal 18 can be made of a rubber material or any material having suitable elasticity. In accordance with a preferred embodiment of the present invention, the lip seal 18 comprises a ring 5 shape having top surface 18B bifurcated into two ends 18B1 and 18B2, and the bottom surface 18A of such lip seal 18 is preferably substantially flat surface as illustrated in the accompanying Figures 3A and 3B. The bifurcated top surface 18B is placed in such a manner that the pressure of the fluid is on this top surface. Due to the pressure, the ends 18B1 and 18B2 of the bifurcated top surface move apart from each other and grip the 10 surfaces of the body in contact more tightly. The fluid is selected from a gas or vacuum or a liquid, or a mixture of gases or liquids. These types of lip seals are used in the rotary sealing mechanism of the present invention to provide the sealing. This type of the rotary sealing mechanism has the following advantage over the O-rings (solid rubber rings): 1. The O-rings hold only one end or one contacting surface tightly on, which 15 it is fitted whereas, in contrast lip seal (bifurcated end contacts both the contacting surfaces tightly). 2. As the pressure on the lip seal bifurcated end increases the lip seal 18 holds both the contacting surfaces more tightly. The connection between the hollow soot porous body handle 1 and the member 20 end 34A is made rigid and leak proof, either by threading or use of sealant suitable for operating in corrosive and high temperature environments. The screw 25, the lip seals 18 are housed in a main body 26. The main body 26 and the screw 25 are made of a material that is typically non-corrosive and high temperature resistant. In one preferred embodiment of the present invention, the main 25 body 26 and the screw 25 is made of stainless steel. In another preferred embodiment of the present invention, the material can be a ceramic or glass. In yet another embodiment of the present invention, it may be made of polyurethane or polyethylene with cooling systems. [Figure 2B] In one embodiment of the present invention, the member 34 is made of stainless 30 steel of high quality. In another embodiment of the present invention, the member 34 is made of glass. In still another embodiment of the present invention, the member 34 is STER/PA-H made of ceramic with high melting point. In yet another embodiment of the present invention, the member 34 is made of polyurethane with cooling fluid circulation [not shown in the figure 4B]. The other end 17 of the rotary sealing mechanism 35 is connected to a vacuum 5 pump through a pipe 36. The material of the pipe can be which can withstand high temperatures and is corrosion free. The connection of the rotary sealing mechanism 35 and the pipe 36 can be made in usual way, either by threading or any other known leak proof method. As already described the capillary end 11 remote from the handle 1 is closed with 10 a glass plug 12. With this the setup [Figure 4A] comprising of hollow soot porous body 2, member 34, rotary sealing mechanism 35 and vacuum pump (not shown in the figure 4A) is ready. The hollow soot porous body 2 with glass plug 12 at the end 11 is inserted into the sintering furnace 31 at a predetermined speed. The hollow soot porous body 2 is chemically dried at a first drying temperature as already discussed. Thereafter the 15 temperature of the sintering furnace 31 is increased and the tip of the hollow soot porous body 2 is sintered, so that the hollow soot porous body 2 contracts and engages glass plug 12 fitted at the end 11 of the hollow soot porous body 2. After this step of engaging the glass plug 12, the vacuum is applied to the capillary 9 of the hollow soot porous body 2. Sintering furnace temperature is maintained high enough to soften the glass, which is 20 suitable to collapse the hollow soot porous body capillary 9. Typically this temperature is in the range varying from about 1550 to 1650°C. The care is taken that this temperature is higher than the temperature which is required just for sintering the hollow soot porous body. To prevent deformation in shape and refractive index profile of the hollow soot 25 porous body 2, in accordance with the present invention, the hollow soot porous body, is rotated at predetermined speed [Figure 4A]. In one of the preferred embodiments of the present invention, the hollow soot porous body 2, the member 34 rotates while the rotary sealing mechanism 35 remains stationary. As discussed herein above, the end 34B of the member 34 slides over the lip seals 18. The end 34B outside surface (which is in contact 30 with the lip seals) is preferably smooth finished to avoid friction. The screw 25 of the rotary sealing mechanism 35 is tightened in such a way that the lip seals 18 expands on to 15 STER/PA-H the tubular part 34C of the member 34, so as to allow sliding of the tubular part 34Cof the member 34, but at the same time it is leak proof. In accordance with one of the preferred embodiments of the present invention, the rotary sealing mechanism 35 is provided with a cooling system [Figure 2B]. As the 5 process size increases the temperature for simultaneous sintering and collapse also increases, the rotary sealing mechanism 35 in such cases is provided with cooling systems as illustrated in the accompanying Figure 2B. The rotary sealing mechanism 35 as shown in Figure 2A has been modified to accommodate the cooling system [Figure 2B]. The Figure 2B shows one such possible modification of Figure 2A. The holes 28 10 and 27 are provided for flowing cooling fluid therethrough, to cool the rotary sealing mechanism 35. The rest of the working of the rotary sealing mechanism 35 of the preferred embodiment of the present invention as illustrated in accompanying Figure 2B is same as described for the preferred embodiment of the present invention illustrated in accompanying Figure 2A. 15 In accordance with another preferred embodiment of the present invention, the rotary sealing mechanism 35 may comprise a plurality of lip seals 18. For example, it may comprise two lip seals 18 as illustrated in the accompanying Figure 2C, which shows that in accordance with one of the preferred embodiments of the present invention, the rotary sealing mechanism may comprise two lip seals 18. It has been observed that 20 plurality of lip seals 18 helps in increasing the leak proofness and also the reliability of the rotary sealing mechanism 35. It is to be understood that the number of lip seals 18 may vary depending upon the time duration of the process, temperature and corrosion considerations. In accordance with still another preferred embodiment of the present invention, 25 the rotary sealing mechanism 35 may comprise lip seals 18 which are guided by guiding bushes 38, which have been surprisingly observed to prevent the brushing of the top and bottom ends or surfaces of the lip seals 18 against the main body 16 and the screw 25 of the rotary seal, especially the bifurcated part which is thin and delicate. In accordance with one of the preferred embodiments of the present invention, the 30 guiding bushes 38 may be provided on one or both ends of the lip seals 18, preferably the 16 STER/PA-H guiding bushes 38 are provided on both ends of lip seals 18 as illustrated in accompanying Figure 2D. Examples: 5 Example 1: A hollow soot porous body with outer diameter of 145 mm and having a capillary at the center of about 4 mm (after removing the target rod) was fabricated by ACVD process. This hollow soot porous body was fitted with glass plug at the capillary end remote from the handle 1. The hollow soot porous body handle 1 was fitted the rotary sealing mechanism in accordance with the present invention (without any 10 cooling system) [Figure 2A, Figure 4A and Figure 4B]. It was dried in an atmosphere of chlorine and helium at a temperature of about 1000°C. Then the end of the capillary 9 remote from the handle 1 of the preform fitted with a glass plug 12 was heated to a temperature of about 1550°C to engage the glass plug 12. Thereafter a vacuum of about 400 mm Hg was applied to the capillary 9, through the rotary sealing mechanism 35 in 15 accordance with the present invention. The hollow soot porous body 2 was rotated at a speed of 7 rpm (revolutions per minute). The hollow soot porous body 2 was inserted at an insertion speed of 6 mm/min in the sintering furnace. The hollow soot porous body 2 was sintered into a solid glass preform. The glass preform (mother preform) was then drawn into five number of rods. These rods were further processed to form daughter 20 preforms. A fiber was drawn from one of the daughter preforms. The attenuation values for the optical fiber thus drawn are shown in table I below: Table I Sr.No. Wavelength (nm) Loss (dB/Km) 1 1310 0.332 2 1550 0.191 25 The attenuation losses for the optical fiber were observed to be well below the desired values of 0.34 dB/Km at 1310 nm and 0.20 dB/Km at 1550 nm. The rotary sealing mechanism 35 [Figure 4A and Figure 4B] was observed to work well without any leakage of vacuum during the whole process of simultaneous sintering and collapsing the 17 STER/PA-H hollow soot porous body 2. Further, no vacuum loss was observed during the entire process. Further, after completion of the process the lip seals 18 were also checked and were found to be in good condition. Example 2: A hollow soot porous body 2 with outer diameter of 148 mm and 5 having a capillary at the center of about 4 mm (after removing the target rod 3) was fabricated by ACVD process. This hollow soot porous body was fitted with glass plug at the capillary end remote from the handle 1. The hollow soot porous body handle 1 was fitted the rotary sealing mechanism of the U.S. patent '069. It was dried in an atmosphere of chlorine and helium at a temperature of about 1000°C. Then the end of the capillary 9 10 remote from the handle 1 of the preform fitted with a glass plug 12 was heated to a temperature of about 1550°C to engage the glass plug 12. Thereafter, a vacuum of about 400 mm Hg was applied to the capillary 9. The hollow soot porous body 2 was rotated at a speed of 7 rpm (revolutions per minute). The hollow soot porous body 2 was inserted at an insertion speed of 6 mm/min in the sintering furnace. During the sintering step the 0- 15 rings of the rotary seal were observed to get ruptured due to heat. The vacuum loss was also observed to occur and the capillary was also observed to get contaminated with the ambient atmospheric gases. The capillary of the hollow soot porous body, therefore, could not be collapsed, though it was sintered. Further processing of the sintered preform was, therefore, abandoned. Therefore, the rotary sealing mechanism of the US patent 20 ' 069 could not be used for collapsing of the capillary of the hollow soot porous body. Further, it could also not be used for a process employing simultaneous collapsing and sintering process steps. The same model of rotary sealing mechanism as described in conjunction with figure 2A, 2B, 2C, 2D, 3A and 3B has been used for connecting the stationary vapor 25 supply tube and the rotary deposition tube in modified chemical vapor deposition (MCVD) process as well. The deposition silica glass tube was connected instead of member 34 in the above-described process, whereas the chemical delivery tube is connected to that end of rotary sealing mechanism, which has been observed to hold the vacuum connection satisfactorily. It was observed that no leakages of chemical vapour 30 occurred during the completed process of fabrication of the core rod by the modified chemical vapour deposition method. 18 STER/PA-H It may be noted that the rotary sealing mechanism of the present invention can be used for collapsing the capillary during sintering and also for the rod draw process steps. It may also be noted that the rotary sealing mechanism may be modified to suit any other process for fabricating an optical fiber preform without deviating from the 5 scope of the present invention. Therefore, such modifications of the presently disclosed rotary sealing mechanism are intended within the scope of present invention. Dated this 3rd day of March, 2006. 10 [Dr. Ramesh Kr. Mehta] Patent Attorney for the Applicants Of Mehta & Mehta Associates 15 19

Full Text

FORM 2
THE PATENTS ACT, 1970(39 of 1970)
&
THE PATENTS RULES, 2003
PROVISIONAL PATENT SPECIFICATION
(See Section 10 and Rule 13)
1. Title of the Invention:-
Apparatus and Method for fabricating Optical fiber preform.
2. Applicant(s):-
(a) Name: STERLITE OPTICAL TECHNOLOGIES LTD.
(b) Nationality: An Indian Company
(c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA
3. Preamble to the Description-
Provisional Patent Specification:
The following specification describes the Invention.

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Field of the Invention:
5 The present invention relates to an apparatus and method for fabricating optical
fiber preform. Particularly it relates to an apparatus and method for fabricating optical fiber preform and optical fiber therefrom, which has low attenuation in the wavelength range from 1300 nm to 1625 nm.
10 Background of Invention:
Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. The primary object of telecommunication industry is to transmit greater amount of information, over longer distances, in shorter period of time. This object can be fulfilled with the optical fibers provided it has low optical
15 attenuation loss in the wavelength ranges varying from about 1300 nm to about 1625 nm.
Optical fiber is drawn from optical fiber preform. The optical fiber preform
generally comprises a central core and an outer cladding. The core rod itself comprises a
core and part of cladding of the fiber preform. Conventionally, the core rod can be
prepared by any known method, for example by Atmospheric Chemical Vapor
20 Deposition (ACVD) method, wherein the soot is deposited during the deposition process step on the cylindrical member (also referred as target rod or mandrel) to form soot porous body. In this method, the soot deposition is accomplished by traverse motion of the target rod over the burners or vice versa. The initial soot deposition comprises dopant chemicals to increase refractive index of the core and dopant chemicals are terminated
25 after desired core diameter is obtained. The deposition process continues until the required dimension of the soot porous body is attained for meeting desired core diameter in the optical fiber preform and the desired core-clad diameter ratio in the fiber. A typical single mode optical fiber may have core of about 8-10 um in diameter and clad of about 125 µm in diameter. After completion of soot deposition, the target rod is removed from
30 the soot porous body to form hollow cylindrical soot porous body defining a capillary at the center [herein after referred to as hollow soot porous body]. A glass plug is inserted
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into the end remote from the handle of the hollow soot porous body. The hollow soot
porous body is moved into a sintering furnace, wherein this hollow soot porous body is
first dehydrated and then sintered (also known as vitrification or consolidation) in a
chlorine and helium atmosphere to form optical fiber preform at about 1500°C.
5 The dehydration and sintering processes can be carried out by any method known
in the art. Preferably, it can be carried out inside specially built furnaces that are equipped with one or more heating elements and gas input mechanisms. The dehydration and sintering processes comprise inserting the hollow cylindrical soot porous body into a sintering furnace and subjecting it to a temperature regime under controlled chemical
10 environment to form sintered glass preform. The chemical environment necessary for dehydration is provided with the help of gases that promote dehydration. The chemical environment that is necessary for sintering is provided with gases that are inert and have high thermal conductivity.
The sintered glass preform with capillary is subjected to, after removal from the
15 sintering furnace to a process step of rod draw to form a plurality of core rods having predetermined diameter. During this process step, vacuum is applied to the capillary to result in collapsing of the capillary, which gets closed due to the glass surface tension; relative viscosities of core and cladding. The application of vacuum increases the speed of collapsing step and it also facilitates removal of gases formed in the preform capillary
20 during the rod draw step.
The core rod drawn may be, if desired, subjected to a process step of overcladding by depositing soot over the core rod to form soot porous body having solid core rod. The
soot porous body may be subjected to process steps of dehydration and consolidation to form optical fiber preform (called daughter preform). Accordingly, the optical fiber can
25 be drawn either directly from the mother preform or the daughter preform.
As described hereinabove the sintered preform with uncollapsed capillary therein is removed from the furnace and then in the separate step of rod draw, it is collapsed under negative pressure or vacuum. This step of collapsing capillary in rod draw step increases the chances of capillary contamination. The preform capillary forms the center
30 of optical fiber core, through which most of the light travels. The performance of the optical fiber used for transmission is primarily determined by the optical attenuation loss
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and dispersion in the optical fiber. The contamination or deformation of any kind of the capillary has a severe negative impact on the optical fiber attenuation drawn therefrom. It is desired that the capillary should not be exposed to the ambient atmosphere at all.
The U.S. Patent 4,251,251 discloses a method so that the capillary is not exposed
5 to the ambient atmosphere at all. According to '251, the process steps of making the preform are carried out in the order namely:
1. Deposition;
2. Removal of target rod;
3. Sintering and collapsing the capillary to get solid glass preform without any 10 capillary therein;
4. Rod drawing from the so formed solid glass preform;
prevents exposure of capillary to the ambient atmosphere. The capillary is collapsed
simultaneously with the sintering of hollow soot porous body or during the step of sintering without removing it from the sintering furnace. Vacuum is applied to the
15 capillary of the hollow soot porous body. The capillary collapses during the sintering step due to glass surface tension; relative viscosities of core and cladding layer.
The U.S. Patent '251 also discloses that in order to collapse the capillary in sintering step itself, higher temperature is required than that needed for only sintering the hollow soot porous body. This temperature is higher than the softening point of glass.
20 Even at the softening point of glass, the glass gets deformed and elongated due to
gravity. The elongation distorts the preform shape and, hence the refractive index profile. This deformation of refractive index profile and shape results in unwanted attenuation increase or other waveguide parameters of the optical fiber. For example, the clad to core diameter ratio may vary if the shape of the resulting preform is deformed in sintering.
25 This results in variations in the core diameter of the resulting optical fiber drawn therefrom. The variations in core diameter results in change of cutoff wavelength value from the desired value. The variation in core diameter result in elevated attenuation in the optical fiber. The U.S. Patent '251 does not provide any remedy for these problems.
It is observed that the above-mentioned problem is solved by rotating the hollow
30 soot porous body about its longitudinal axis at a predetermined rotational speed. The centrifugal acceleration due to rotation nullifies the gravity effect and eliminate the
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distortion. The hollow soot porous preform rotation about its longitudinal axis of symmetry allows for proper collapse of the capillary and other problems related with unsymmetrical collapse are taken care of.
The hollow soot porous body with capillary, which is suspended in the sintering
5 furnace, is rotated along its longitudinal axis of symmetry and also the soot porous
preform capillary is connected to a stationary vacuum pump through a pipe. A connector
[herein after referred to as rotary sealing mechanism] for connecting a stationary pipe
from the vacuum pump and the rotating soot porous preform capillary is needed.
The U.S patent 4,347,069 discloses a rotary seal at the junction of stationary
10 member connecting the supply source of vapor stream and the rotating preform tube. The rotary seal according to this patent consists of locating an end of the rotating member inside an end of another rotating member and positioning two washers or O-rings between the two ends of the rotating members. The another end of another rotating member is then made to sit in an end cap provided with two inlets, one inlet for receiving
15 the tube connected to supply source of vapor stream and another inlet for receiving the tube connected to supply source of oxygen.
This rotary seal overcomes problem of early wear out of washers or O-rings by connecting one end of rotating member connected to rotating preform tube with one end of another rotating member. However, another end of said another rotating member,
20 which constitutes an additional part of the apparatus is still located onto one end of stationary member connected to supply source of vapor stream which is fitted inside said another end of said another rotating member [an additional part of the apparatus] in the manner known in the prior art.
This particular rotary seal as disclosed in the '069 can be used for connecting the
25 stationary vacuum tube and the rotating hollow soot porous body. However, the inventors of the present invention faced limitations described below.
Even the rotary seal of above-referred patent suffers from the main problem of early wear out of washers or O-rings.
As the amount of heat to which the rotary seal will be exposed during the
30 simultaneous sintering and collapsing of the soot porous body is quite high than of the
amount of heat during the fabrication of preform in accordance with the method
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described in the said document '069. Therefore, it has been observed by the inventors of the present invention that the O-rings wear out during a single operation also itself, because the O-rings are positioned inside the rotary seal. The wearing out of O-rings during the simultaneous sintering and collapsing of the soot porous body is observed to
5 cause following problems:-
1. Certain particles are generated as the O-rings wear out due to heat and friction against
the stationary/rotating members (vacuum tube/ soot porous body handle) which plunge
into the capillary of the soot porous body thereby resulting in contamination of the
capillary and, hence increase in the attenuation level of the resulted optical fiber drawn
10 therefrom.
2. Further, wearing of O-rings also lead to leakage of vacuum in the soot porous body
capillary thereby causing contamination present in the ambient atmosphere to enter in
the capillary of the soot porous body which in-turn further contaminates it. This
contamination results in further increase in the attenuation level and thereby makes the
15 resulting optical fiber, drawn therefrom, unsuitable for any use.
3. Still further, the vacuum leakage due to rupture or wearing out of the O-rings has been
observed to result in partial or incomplete closure of capillary and/or formation of
bubbles therein.
4. Still another problem of the rotary seal of said patent '069 is that it calls for additional
20 supply of oxygen gas that's too at a very high pressure at least higher than the pressure
of the vapor stream. This additional supply of oxygen gas at a very high pressure is
provided to achieve pressure differential in the cavity created between stationary end
cap and said another [additional] rotating member to avoid or inhibit leakage of toxic
vapor stream in the environment as and when there is a leakage due to early wear out
25 of washer or O-rings provided between said another end of said another rotating
member which constitutes an additional part of the apparatus and the stationary member connected to source of supply of vapor stream.
Therefore, it is clear from the foregoing description that even the rotary seal
provided by said US patent '069 suffers from various disadvantages, drawbacks and
30 limitation, and in-addition it also requires provision of two additional parts - a) said
another rotating member and b) said end cap, and additionally it also requires i)
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additional supply source of oxygen, ii) additional supply of oxygen gas and iii) that's too at a very high pressure thereby resulting in increase in cost of apparatus and cost of
production of optical fiber preform, but it still suffers from the main problem of leakage
of rotary seal at the junction of rotating member and stationary member thereby making
5 the rotary seal of said patent '069 and the apparatus comprising the same and the method
employing the same highly un-satisfactory and highly un-economical in-addition to being
complicated due to precise provision of said end cap to achieve the pressure differential. Further, in case of leakage, not only the toxic vapour stream will come in contact
with the environment, but also the oxygen gas, that's too at a very high pressure, which
10 in-turn even in PPM level will cause damage to the skilled persons operating the system
and in the surrounding area in-addition to very high risk of fire due to leakage of oxygen
at a very high pressure.
It has also been observed that due to leakage of commonly available washers or
O-rings, the oxygen which is supplied at a very high pressure, dilutes the vapour stream 15 entering the rotating preform tube thereby resulting in production of preform having
undesired characteristics.
It has also been observed that the rotary seal of US patent '069 is not suitable for
employing it in the collapsing process step, because the principle of pressure differential
does not work when the stationary member is connected to source of vacuum.
20 Therefore, the rotary seal as disclosed in '069 cannot be used for connecting the
stationary vacuum pump and hollow soot porous body capillary, particularly when
simultaneous sintering of the hollow soot porous body and collapsing of the capillary of
the hollow soot porous body is desired.
25 Need of the Invention:
Accordingly, there is still a need to have an improved rotary sealing mechanism which is suitable for collapsing process step for causing collapse of capillary in the hollow soot porous body and particularly when simultaneous sintering of the hollow soot porous body and collapsing of the capillary of the hollow soot porous body is desired,
30 and at the same time it does not suffer at least from majority of above described disadvantages, drawbacks and limitations of the prior art, and also it does not suffer from
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the main problem of leakage of rotary sealing mechanism, as it has been observed in the rotary sealing mechanism of the prior art due to O-ring seal rupture at the junction of rotating member and stationary member, thereby making the rotary sealing mechanism thus developed and the apparatuses comprising the same and the methods employing the
5 same highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for producing the optical fiber preform.
Objects and Advantages of the Invention:
10 The main object of the present invention is to provide an improved rotary sealing
mechanism which is suitable for collapsing process step for causing collapse of capillary in the hollow soot porous body and particularly in process where sintering of the hollow soot porous body and collapsing of the capillary of the hollow soot porous body are performed simultaneously, and at the same time it neither suffers, at least, from majority
15 of above described disadvantages, drawbacks and limitations of the prior art, nor from the main problem of leakage of rotary sealing mechanism thereby making the presently disclosed rotary sealing mechanism and the apparatuses comprising the same and the methods employing the same highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for
20 producing the optical fiber preform.
The another main object of the present invention is to provide an improved rotary sealing mechanism suitable for avoiding and inhibiting leakage of vacuum or vapor stream and/or mixing of environmental gases with the vapor stream and/or mixing of toxic gases in the environment at the junction of stationary member connected to the
25 vacuum pump or to supply source of vapor stream and the rotating member connected to the rotating hollow soot porous body preform or rotating preform tube thereby making the process more safer and more economical.
Another object of this invention is to provide an improved rotary seal suitable for a) depositing the soot particles inside the rotating preform tube and also for b) collapsing
30 process step for causing collapse of capillary in the soot porous preform and at the same
time does not suffer from above described disadvantages, drawbacks and limitations of
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the prior art thereby making the disclosed rotary sealing mechanism versatile and widely applicable.
Still another object of this invention is to provide an improved rotary seal which does not require additional rotating member, additional supply source of oxygen and
5 additional oxygen supply and at the same time does not suffer from the main problem of leakage of the seal at the junction of rotating member and stationary member thereby making the rotary seal and the apparatus comprising the same and the method employing the same highly productive, highly satisfactory and highly economical in-addition to being easy to fabricate and easy to employ in the presently available apparatuses for
10 producing the optical fiber preform.
Yet another object of this invention is to provide an improved rotary seal which does not suffer from the problem of early wear out of sealing member due to constant rotation of rotating member against stationary member thereby avoiding and inhibiting leakage of vacuum to make the process more effective and more productive.
15 Yet another object of the present invention is to fabricate optical fiber preform by
the ACVD method and using the rotary sealing mechanism in accordance with the present invention. The optical fiber obtained from the optical fiber preform so fabricated is characterized in that it can be used in the wavelength range varying from about 1300 nm to about 1625 nm,
20 Still another object of the present invention is to fabricate an optical fiber preform
and optical fiber there from such that the optical fiber has an absorption loss less than or equal to 0.35 dB/Km at wavelength ranges varying from about 1300 nm to about 1625 nm.
The other objects and advantages of the present invention will be more apparent
25 when the following description is read in conjunction with the accompanying drawings which are not intended to limit the scope of this invention.
Brief Description of the Accompanying Figures:
Figure 1A is a schematic representation of deposition process in accordance with
30 known Atmospheric Chemical Vapor Deposition (ACVD) method.
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Figure IB is a schematic representation of the hollow soot porous body with capillary therethrough connected to preform handle in accordance with the present invention.
Figure 1C shows cross-sectional view of the hollow soot porous body produced in 5 accordance with known Atmospheric Chemical Vapor deposition method.
Figure ID shows a preform handle for the fabrication of hollow soot porous body shown in Figure 1B in accordance with the present invention.
Figure 2A shows exploded cross-sectional view of the rotary sealing mechanism
in accordance with one of the preferred embodiments of the present invention.
10 Figure 2B shows exploded cross-sectional view of the rotary sealing mechanism
in accordance with another preferred embodiment of the present invention, wherein a cooling fluid cools the rotary sealing mechanism.
Figure 2C shows exploded cross-sectional view of the rotary sealing mechanism
in accordance with still another preferred embodiment of the present invention.
15 Figure 2D shows exploded cross-sectional view of the rotary sealing mechanism
in accordance with yet another preferred embodiment of the present invention.
Figure 3A is schematic overall and cross sectional view of one type of lip seal in
accordance with one of the preferred embodiments of the present invention.
Figure 3B is schematic overall and cross sectional view of another type of lip seal 20 in accordance with another preferred embodiment of the present invention.
Figure 4A is a schematic representation of the apparatus having rotary sealing mechanism for simultaneous sintering and collapsing of the hollow soot porous preform in accordance with one of preferred embodiments of this invention.
Figure 4B shows enlarged cross-sectional view of the rotary sealing mechanism 25 connected to rotating member on one end and stationary member on another end in accordance with one of the preferred embodiments of this invention.
Description of the Invention:
Reference will now be made in detail to the present preferred embodiments of the 30 invention, examples of which are illustrated in the accompanying drawings which are not
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intended to limit scope of the present invention. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
In accordance with the invention, the hollow soot porous body 2, as exemplarily illustrated in [Figure IB], is preferably formed by chemically reacting at least some of the
5 glass-forming compounds of a moving fluid mixture in an oxidizing medium (a hydroxy flame) to form a silica-based reaction product. At least a portion of this reaction product is directed toward a target rod, to form a soot porous body. The method of forming soot porous body described above is called the Atmospheric Chemical Vapor Deposition method or just simply ACVD.
10 As shown in [Figure 1A] a target rod 3 is inserted through a glass body such as
hollow or tubular handle 1 [Figure ID] and mounted on a lathe. The lathe is designed to rotate (by use of rotating means 6) and translate either target rod 3 and/or the soot-generating burner 5 in close proximity with each other. As target rod 3 is rotated and translated, silica-based reaction product 4, known as soot, is directed toward target rod 3.
15 At least a portion of soot 4 is deposited on target rod 3 and on a portion of handle 1 to form a body 2 thereon. The soot 4 may contain some concentration of a suitable dopant (generally GeCL4 used to increase the refractive index of the glass.
Once the desired quantity of soot is deposited on the target rod 3, soot deposition is terminated and the target rod 3 is removed from soot porous body 2 to result in the
20 formation of a soot porous body having a capillary 9.
In accordance with the present invention and as depicted in Figure 1A, upon
removal of target rod 3, soot porous body 2 defines a soot porous body having a capillary 9 [Figure IB and Figure 1C] passing axially therethrough. The hollow soot porous body 2
with capillary 9 [herein after referred to as hollow soot porous body] 2 is suspended by
25 handle 1 in a sintering furnace 31 [Figure 4A]. The end 11 of the hollow soot porous preform capillary 9 remote from handle 1 is preferably fitted with a bottom glass plug 12 prior to positioning hollow soot porous body 2 within sintering furnace 31. The bottom glass plug 12 is positioned and held in place with respect to hollow soot porous body 2 by friction fit. Bottom glass plug 12 is further preferably tapered to facilitate entry and to
30 allow at least temporary affixing and at least loosely within the hollow soot porous body
2.
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The hollow soot porous body 2 is preferably chemically dried, for example, by exposing hollow soot porous body 2 to a chlorine-helium containing atmosphere at
elevated temperature within sintering furnace 31. The chlorine-helium containing atmosphere removes water and other impurities from hollow soot porous body 2, which
5 otherwise have an undesirable effect on the properties of optical waveguide fiber manufactured from the hollow soot porous body 2.
Following the chemical drying step, the temperature of the sintering furnace 31 is elevated to a temperature sufficient to sinter the hollow soot porous body 2 into a sintered glass preform. In accordance with the present invention, the soot porous body capillary 9
10 is collapsed simultaneously during the sintering step, which avoid additional process step thereby resulting in savings of overall production time of the solid glass preform. Simultaneous sintering of the hollow soot porous body 2 and collapsing the hollow soot porous body capillary 9 also eliminate another disadvantage that occurs when the hollow soot porous body 2 is chemically dried and sintered, and following chemical drying and
15 sintering, the resulting sintered glass preform is exposed to ambient environment, such as ambient atmosphere, for example, when the sintered glass preform is removed from the sintering furnace 31 and moved to a rod draw furnace for further processing steps. The optical fibers manufactured using such preforms have been observed to exhibit high optical attenuation in the ranges varying from about 1310 nm to about 1625 nm. This
20 high attenuation is largely due to contamination of the portion of the sintered glass preform surrounding the capillary 9 prior to capillary collapse in rod draw step. The greater the exposure period, the greater the amount of contamination adsorbed by the glass and is proportional to the time period for which the capillary 9 is exposed to the ambient atmosphere. Thus, exposure to ambient atmosphere is desired to be strictly
25 avoided in the present invention.
In a preferred embodiment of the method of the present invention, exposure of the capillary 9 to ambient atmosphere is reduced or prevented by collapsing the capillary 9 simultaneously in the sintering step.
In another preferred embodiment of the method of the present invention, the
30 simultaneous sintering and collapsing of the hollow soot porous body 2 capillary 9 is done under the application of negative pressure or vacuum.
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In accordance with present invention, in order to prevent distortion in the shape and refractive index profile of the hollow soot porous body 2 during the step of
simultaneous sintering and collapsing, not only vacuum is applied, but also the hollow
soot porous body 2 is rotated about its longitudinal axis of symmetry 29 [Figure 4A].
5 As elaborated hereinabove, in order to rotate while avoiding disadvantages,
drawbacks and limitations of the prior art, the hollow soot porous body (herein after may also be referred to as rotating member) 2 and to connect the hollow soot porous body 2 capillary to vacuum pump (herein after may also be referred to as stationary) it is necessary to use rotary sealing mechanism.
10 Figure 2A shows a cross sectional view of the rotary sealing mechanism in
accordance with first embodiment of the present invention. The drawing is not drawn to scale. After completion of deposition, the soot porous body 2 is detached from the lathe along with the handle 1 [Figure IB and Figure 1C]. Thereafter, the target rod 3 is removed from the soot porous body 2, forming a capillary 9 there through as described
15 above [Figure 1C].
Thus obtained hollow soot porous body 2 having a capillary 9 is then transferred to the sintering furnace 31 [Figure 4A] along with the handle 1. In the sintering furnace the hollow soot porous body 2 is sintered and collapsed simultaneously to give a solid glass preform.
20 The hollow soot porous body handle 1 is made to fit into or onto the member 34
depending upon the relative diameter of these two members and the hollow soot porous body 2 is suspended in the sintering furnace 31. Preferably the member 34 is made of a non-corrosive and has very high melting point material, typically more than 1800°C. The member 34 has two ends, 34A fitting into the handle 1 of the hollow soot porous body 2
25 and the other 34B fitting through the screw hole 23, passing onto the lip seal 18, covering the lip seals 18. Thus the lip seals 18 do not show up inside the rotary sealing mechanism 35 interiors [Figure 3B]. Thus the lip seals 18 remain outside. The lip seals 18 are positioned in the same manner as shown in the figure 2A, 2B, 2C, 2D, and 4B. The bifurcated top of the lip seal holds both the ends tightly with which they contact.
30 The lip seal 18 described above is the one generally used in the automobile
industry as oil seal in automobile engines. There are numerous types of lip seals available
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in the market. Two such types of lip seals are shown in the figures 3A and 3B. In accordance with one of the preferred embodiments of the present invention, a lip seal 18 can be made of a rubber material or any material having suitable elasticity. In accordance with a preferred embodiment of the present invention, the lip seal 18 comprises a ring
5 shape having top surface 18B bifurcated into two ends 18B1 and 18B2, and the bottom surface 18A of such lip seal 18 is preferably substantially flat surface as illustrated in the accompanying Figures 3A and 3B. The bifurcated top surface 18B is placed in such a manner that the pressure of the fluid is on this top surface. Due to the pressure, the ends 18B1 and 18B2 of the bifurcated top surface move apart from each other and grip the
10 surfaces of the body in contact more tightly. The fluid is selected from a gas or vacuum or a liquid, or a mixture of gases or liquids. These types of lip seals are used in the rotary sealing mechanism of the present invention to provide the sealing. This type of the rotary sealing mechanism has the following advantage over the O-rings (solid rubber rings):
1. The O-rings hold only one end or one contacting surface tightly on, which
15 it is fitted whereas, in contrast lip seal (bifurcated end contacts both the contacting
surfaces tightly).
2. As the pressure on the lip seal bifurcated end increases the lip seal 18
holds both the contacting surfaces more tightly.
The connection between the hollow soot porous body handle 1 and the member
20 end 34A is made rigid and leak proof, either by threading or use of sealant suitable for operating in corrosive and high temperature environments.
The screw 25, the lip seals 18 are housed in a main body 26. The main body 26 and the screw 25 are made of a material that is typically non-corrosive and high temperature resistant. In one preferred embodiment of the present invention, the main
25 body 26 and the screw 25 is made of stainless steel. In another preferred embodiment of the present invention, the material can be a ceramic or glass. In yet another embodiment of the present invention, it may be made of polyurethane or polyethylene with cooling systems. [Figure 2B]
In one embodiment of the present invention, the member 34 is made of stainless
30 steel of high quality. In another embodiment of the present invention, the member 34 is
made of glass. In still another embodiment of the present invention, the member 34 is
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made of ceramic with high melting point. In yet another embodiment of the present invention, the member 34 is made of polyurethane with cooling fluid circulation [not
shown in the figure 4B].
The other end 17 of the rotary sealing mechanism 35 is connected to a vacuum
5 pump through a pipe 36. The material of the pipe can be which can withstand high
temperatures and is corrosion free. The connection of the rotary sealing mechanism 35
and the pipe 36 can be made in usual way, either by threading or any other known leak
proof method.
As already described the capillary end 11 remote from the handle 1 is closed with
10 a glass plug 12. With this the setup [Figure 4A] comprising of hollow soot porous body
2, member 34, rotary sealing mechanism 35 and vacuum pump (not shown in the figure 4A) is ready. The hollow soot porous body 2 with glass plug 12 at the end 11 is inserted into the sintering furnace 31 at a predetermined speed. The hollow soot porous body 2 is chemically dried at a first drying temperature as already discussed. Thereafter the
15 temperature of the sintering furnace 31 is increased and the tip of the hollow soot porous body 2 is sintered, so that the hollow soot porous body 2 contracts and engages glass plug 12 fitted at the end 11 of the hollow soot porous body 2. After this step of engaging the glass plug 12, the vacuum is applied to the capillary 9 of the hollow soot porous body 2. Sintering furnace temperature is maintained high enough to soften the glass, which is
20 suitable to collapse the hollow soot porous body capillary 9. Typically this temperature is in the range varying from about 1550 to 1650°C. The care is taken that this temperature is higher than the temperature which is required just for sintering the hollow soot porous body.
To prevent deformation in shape and refractive index profile of the hollow soot
25 porous body 2, in accordance with the present invention, the hollow soot porous body, is rotated at predetermined speed [Figure 4A]. In one of the preferred embodiments of the present invention, the hollow soot porous body 2, the member 34 rotates while the rotary sealing mechanism 35 remains stationary. As discussed herein above, the end 34B of the member 34 slides over the lip seals 18. The end 34B outside surface (which is in contact
30 with the lip seals) is preferably smooth finished to avoid friction. The screw 25 of the rotary sealing mechanism 35 is tightened in such a way that the lip seals 18 expands on to
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the tubular part 34C of the member 34, so as to allow sliding of the tubular part 34Cof the member 34, but at the same time it is leak proof.
In accordance with one of the preferred embodiments of the present invention, the rotary sealing mechanism 35 is provided with a cooling system [Figure 2B]. As the
5 process size increases the temperature for simultaneous sintering and collapse also increases, the rotary sealing mechanism 35 in such cases is provided with cooling systems as illustrated in the accompanying Figure 2B. The rotary sealing mechanism 35 as shown in Figure 2A has been modified to accommodate the cooling system [Figure 2B]. The Figure 2B shows one such possible modification of Figure 2A. The holes 28
10 and 27 are provided for flowing cooling fluid therethrough, to cool the rotary sealing mechanism 35. The rest of the working of the rotary sealing mechanism 35 of the preferred embodiment of the present invention as illustrated in accompanying Figure 2B is same as described for the preferred embodiment of the present invention illustrated in accompanying Figure 2A.
15 In accordance with another preferred embodiment of the present invention, the
rotary sealing mechanism 35 may comprise a plurality of lip seals 18. For example, it may comprise two lip seals 18 as illustrated in the accompanying Figure 2C, which shows that in accordance with one of the preferred embodiments of the present invention, the rotary sealing mechanism may comprise two lip seals 18. It has been observed that
20 plurality of lip seals 18 helps in increasing the leak proofness and also the reliability of the rotary sealing mechanism 35. It is to be understood that the number of lip seals 18 may vary depending upon the time duration of the process, temperature and corrosion considerations.
In accordance with still another preferred embodiment of the present invention,
25 the rotary sealing mechanism 35 may comprise lip seals 18 which are guided by guiding bushes 38, which have been surprisingly observed to prevent the brushing of the top and bottom ends or surfaces of the lip seals 18 against the main body 16 and the screw 25 of the rotary seal, especially the bifurcated part which is thin and delicate.
In accordance with one of the preferred embodiments of the present invention, the
30 guiding bushes 38 may be provided on one or both ends of the lip seals 18, preferably the
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guiding bushes 38 are provided on both ends of lip seals 18 as illustrated in accompanying Figure 2D.
Examples:
5 Example 1: A hollow soot porous body with outer diameter of 145 mm and
having a capillary at the center of about 4 mm (after removing the target rod) was fabricated by ACVD process. This hollow soot porous body was fitted with glass plug at the capillary end remote from the handle 1. The hollow soot porous body handle 1 was fitted the rotary sealing mechanism in accordance with the present invention (without any
10 cooling system) [Figure 2A, Figure 4A and Figure 4B]. It was dried in an atmosphere of chlorine and helium at a temperature of about 1000°C. Then the end of the capillary 9 remote from the handle 1 of the preform fitted with a glass plug 12 was heated to a temperature of about 1550°C to engage the glass plug 12. Thereafter a vacuum of about 400 mm Hg was applied to the capillary 9, through the rotary sealing mechanism 35 in
15 accordance with the present invention. The hollow soot porous body 2 was rotated at a speed of 7 rpm (revolutions per minute). The hollow soot porous body 2 was inserted at an insertion speed of 6 mm/min in the sintering furnace. The hollow soot porous body 2 was sintered into a solid glass preform. The glass preform (mother preform) was then drawn into five number of rods. These rods were further processed to form daughter
20 preforms. A fiber was drawn from one of the daughter preforms. The attenuation values for the optical fiber thus drawn are shown in table I below:

Table I
Sr.No. Wavelength (nm) Loss (dB/Km)
1 1310 0.332
2 1550 0.191
25 The attenuation losses for the optical fiber were observed to be well below the
desired values of 0.34 dB/Km at 1310 nm and 0.20 dB/Km at 1550 nm. The rotary sealing mechanism 35 [Figure 4A and Figure 4B] was observed to work well without any leakage of vacuum during the whole process of simultaneous sintering and collapsing the
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hollow soot porous body 2. Further, no vacuum loss was observed during the entire process. Further, after completion of the process the lip seals 18 were also checked and were found to be in good condition.
Example 2: A hollow soot porous body 2 with outer diameter of 148 mm and
5 having a capillary at the center of about 4 mm (after removing the target rod 3) was fabricated by ACVD process. This hollow soot porous body was fitted with glass plug at the capillary end remote from the handle 1. The hollow soot porous body handle 1 was fitted the rotary sealing mechanism of the U.S. patent '069. It was dried in an atmosphere of chlorine and helium at a temperature of about 1000°C. Then the end of the capillary 9
10 remote from the handle 1 of the preform fitted with a glass plug 12 was heated to a temperature of about 1550°C to engage the glass plug 12. Thereafter, a vacuum of about 400 mm Hg was applied to the capillary 9. The hollow soot porous body 2 was rotated at a speed of 7 rpm (revolutions per minute). The hollow soot porous body 2 was inserted at an insertion speed of 6 mm/min in the sintering furnace. During the sintering step the 0-
15 rings of the rotary seal were observed to get ruptured due to heat. The vacuum loss was also observed to occur and the capillary was also observed to get contaminated with the ambient atmospheric gases. The capillary of the hollow soot porous body, therefore, could not be collapsed, though it was sintered. Further processing of the sintered preform was, therefore, abandoned. Therefore, the rotary sealing mechanism of the US patent
20 ' 069 could not be used for collapsing of the capillary of the hollow soot porous body. Further, it could also not be used for a process employing simultaneous collapsing and sintering process steps.
The same model of rotary sealing mechanism as described in conjunction with figure 2A, 2B, 2C, 2D, 3A and 3B has been used for connecting the stationary vapor
25 supply tube and the rotary deposition tube in modified chemical vapor deposition (MCVD) process as well. The deposition silica glass tube was connected instead of member 34 in the above-described process, whereas the chemical delivery tube is connected to that end of rotary sealing mechanism, which has been observed to hold the vacuum connection satisfactorily. It was observed that no leakages of chemical vapour
30 occurred during the completed process of fabrication of the core rod by the modified chemical vapour deposition method.
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It may be noted that the rotary sealing mechanism of the present invention can be used for collapsing the capillary during sintering and also for the rod draw process steps. It may also be noted that the rotary sealing mechanism may be modified to suit
any other process for fabricating an optical fiber preform without deviating from the
5 scope of the present invention. Therefore, such modifications of the presently disclosed rotary sealing mechanism are intended within the scope of present invention.
Dated this 3rd day of March, 2006.
10 [Dr. Ramesh Kr. Mehta]
Patent Attorney for the Applicants Of Mehta & Mehta Associates
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Apparatus and Method for Fabricating Optical Fiber Preform

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