Title of Invention	METHOD OF MANUFACTURING OPTICAL FIBER PREFORM HAVING UNIFORMLY DEPOSITED SHOOT PARTICLES
Abstract	A method for producing an optical fiber preform having uniformly deposited soot particles comprising depositing soot particles by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction is provided.

Title of Invention

METHOD OF MANUFACTURING OPTICAL FIBER PREFORM HAVING UNIFORMLY DEPOSITED SHOOT PARTICLES

Abstract

A method for producing an optical fiber preform having uniformly deposited soot particles comprising depositing soot particles by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction is provided.

Full Text	FORM 2THE PATENTS ACT, 1970 (39 of 1970)& THE PATENTS RULES, 2003PROVIS1ONAL/COMPLETE SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention: - Optical Fiber Preform having Uniform Soot DepoS1tion & its Method of Preparation 2. Applicant(s):-(a) Name: STERLITE OPTICAL TECHNOLOGIES LTD.(b) Nationality: An Indian Company(c) Address: E 2, MIDC, Waluj, Aurangabad - 431136, Maharashtra, INDIA 3. Preamble to the Description:- Complete Specification:The following specification particularly describes the Invention and the manner in which it is to be performed. ProviS1onal Specification? The following specification describes the Invention Field of the Invention: The present invention relates to an optical fiber preform having uniform soot depoS1tion and its method of preparation. Particularly it relates to an optical fiber preform having uniform soot particles depoS1tion and its method of preparation, particularly it relates to method for depoS1ting soot particles on a target rod to form an optical fiber preform having reduced tapering effect and the optical fiber preform produced thereby. Background of the Invention: Optical fibers are inherently versatile as a transmisS1on medium for all forms of information, be it voice, video or data. The optical fibers are drawn from an optical fiber preform. The optical fiber of predetermined dimenS1on is drawn from the optical fiber preform by subjecting one end of the preform to a high temperature, for example above 2000°C. Under such a high temperature, the tip of the preform softens, from which a thin fiber of deS1red dimenS1on is drawn. The different methods employed in manufacture of these preforms are described in the literature. The optical fiber preform can be manufactured by different methods of chemical vapour depoS1tion (CVD). The optical fiber preform manufacturing process primarily involves a step of preparing the core rod conS1sting of core of the fiber and part of clad which may be followed by over-cladding. The core rod can be prepared by methods known in the art, such as modified chemical vapour depoS1tion (MCVD), plasma chemical vapour depoS1tion (PCVD), outS1de vapour depoS1tion (OVD), vapour axial depoS1tion (VAD) etc. The over-cladding of the core rod can also be carried out by various methods, such as glass tube jacketing, OVD soot over-cladding, VAD soot over-cladding, plasma over-cladding etc. The optical fiber preform can be manufactured by any combination of the core rod manufacturing methods and the over-cladding preparation methods. 2 The most commonly used manufacturing process is the MCVD process which as compared to OVD is S1mpler in nature. However, due to wide flexibility, the OVD process has been successfully industrialized and is now preferred over MCVD for manufacturing wide range of optical fibers, such as S1ngle-mode fibers, long-distance, graded-index multimode fibers, short-distance fibers etc. In the OVD method, a flame hydrolyS1s burner fabricates the soot glass particles on the surface of the core rod conS1sting of core. The chemical reactants involved in the formation of soot particles are S1licon tetrachloride [S1Cl4], oxygen [O2] and oxyhydrogen burner gases. S1Cl4 vapours reacts with O2 with the help of oxyhydrogen flame to form sub micrometer S1zed S1O2 glass particles in a reaction which is expressed as follows: S1Cl4(g) + 2H2(g) + O2(g) ===→ S1O2(s) + 4HCl(g) The above mentioned depoS1tion process occurs till the deS1red amount of glass particles have been depoS1ted and then the soot porous body is moved into S1ntering furnace, where the depoS1ted soot layer is dried with chlorine atmosphere and then S1ntered to form a solid glass preform in a helium atmosphere at about 1500°C. The S1ntered preform is drawn into an optical fiber, which conS1sts of core and clad. One of the known methods employing principles of OVD method for manufacturing the optical fiber preform having soot depoS1ted involves glass forming material are supplied into a burner to form soot particles over the core rod. In this method, for adjusting the distance between the burner and the soot depoS1ted, the burner is adapted to move towards or away from the target rod. The major drawback of depoS1ting the soot is that it results in varying outer diameter at both the ends of the preform formed, which in-turn results in formation of poor quality optical fiber having irregularities in hardness and thermal 3 expanS1on mismatch between the target rod and soot depoS1t causing cracking in the soot. The main reason for this drawback has been asS1gned to rise in temperature around both the ends of the target rod due to round trip of the burner. The rise in temperature at both the ends is conS1dered primarily due to constant heating power of the burner at both the ends of the target rod. Further, with the constant flow rate of the gases, the surface temperature of the soot depoS1t increases further adding to the problem. To overcome above problem, a method for soot depoS1tion has been reported having varying heating power of the burner and varying gas flow rate at both the ends of the target rod. In this method, the distance between burner and target rod is increased at both the ends and reduced at the middle portion having constant diameter of the soot depoS1t, and the flow rate of the gases is maintained constant at both the ends and increased at the middle portion of the target rod. The problem of above method is that the constant gas flow rate at both the ends cannot work in all circumstances. Under such circumstances, it is either increased or decreased depending on the case. Further, the increased gas flow rate at the middle portion of the rod is also decided depending upon the case. In some circumstances, it is required to be increased progresS1vely. Therefore, this method does not resolve the problem completely and leaves the user to decide on the gas flow rate depending upon the case. Still another problem of the above method is that it requires precise control of flow rates and relative concentration of oxygen and/or hydrogen gases. Further, it needs constant watch on approaching end portions to retreat the burners to increase the distance between the burner and the end portions of the target rod. It also needs careful monitoring of temperature at all times to adjust flow rate and distance constantly, that is this method cannot work at a given flow rate of the gases and a given distance of the burner. Further, to avoid increase in 4 volume of the varying diameter end portions of the preform, the supply of the soot forming material is also required to be adjusted. Further, in case the above method is carried out with plurality of burners, then the heating power of individual burners will have to be adjusted in accordance with the changes in the flow rates of soot forming materials that occur in the end portions of the soot depoS1tion end portion adding to further complexity of the method. It has also been observed that above method is not suitable, that is the change of burner gas flow is not achieved with respect to poS1tion of length when the speed of traverse is about 1500 to about 12000 mm/min or more. The methods known in the art as elaborated hereinabove, only partly overcome above problems by alternatively or additionally adjusting flow rate of the soot forming materials and/or gases. These methods not only adjust the flow rate of the soot forming materials and/or gases differently at middle portion and end portions, but also within the middle portion and/or end portions. Another method of soot depoS1tion employs a system elaborated in accompanying Figure 1, wherein glass soot is depoS1ted on the core rod 101 to form soot porous body 100. In this method the soot depoS1tion is accomplished by traverse motion 110 of the core rod over the burner as shown in Figure 1 by arrow 110. In the methods employing this system, the movement of burner is not towards and away from the target rod, but is along the axis of the target rod. In the method known in the art employing the system of Figure 1, the maximum traverse speed of the forward and reverse motions is kept same [Figure 4]. In this method the soot depoS1tion takes place during both the motions - forward and reverse motions. When the end portion of the target rod reaches the burner in a forward motion, the traverse speed is reduced from its maximum value S1 to zero with a predetermined deceleration dl. During the reverse motion, the traverse speed increases 5 from zero to its maximum value S1 with an acceleration al, and again reduces with a deceleration dl to zero at the end of the traverse and so on. The soot depoS1tion process continues in the same pattern until the required dimenS1on (diameter) of the soot depoS1tion is achieved. After attaining the required dimenS1on of the soot depoS1tion, the target rod with soot depoS1ted thereon is moved to a S1ntering furnace to dry and S1nter it to form an optical fiber preform which can be drawn to an optical fiber. However, the optical fiber preform formed by the above method also has constant outer diameter in the middle portion, but the varying diameter towards the end portions as shown in Figure 6. It has been observed that even with a system of Figure 1, during a normal process of soot depoS1tion, there is a relative movement between burner and the target rod taking place in each traverse, each traverse compriS1ng a forward and a reverse motion. When the forward and reverse motions are kept constant, at the time of change of direction of the movement of the burner on reaching end portion of the rod or at the time of change of direction of the movement of the rod on its one end portion reaching the burner, as the case may be, it causes the end portions of the rod to come in contact with the heat provided by the burner, twice in quick and immediate succesS1on in a short period of time and once each during the forward and reverse motions of any traverse, as compared to the middle portion of the rod. This results in the outer surface of the rod near its end portions attaining relatively higher surface temperatures. Accordingly, this method also suffers from the problem of elevation of temperature towards and at the end portions of the rod. It has been observed that this method results in increase in denS1ty of the soot depoS1ted, but reduces the volume of the soot depoS1ted on the end portions of the rod. Consequently, the diameter at the end portions of the rod is smaller than the diameter of the middle portion of the rod [Figure 6]. 6 Accordingly, the main drawback of this method is that the outer diameter of the soot depoS1ted on the rod is non-uniform, that is, it is reduced towards both the ends resulting in a tapering effect. The reasoning for tapering effect is believed to be relatively higher surface temperature of the ends of the rod, which results in increase in denS1ty, but decrease in volume of soot depoS1ted on the ends. Another drawback of this method is that the thickness of the optical fiber drawn from the preform prepared by this method will have unacceptable variance depending upon the S1ze and the extend of the taper. To avoid the unacceptable variance, the tapered portion of the preform will have to be rejected, which enhances cost of production of optical fiber. The tapered portion of the optical fiber preform produced by this method is, therefore, not used for fiber drawing which leads to increase in the ineffective portion and decrease in useable or effective portion of the preform, thereby leading to wastage and enhancement of cost of production of the optical fiber. This effect known as tapering effect is a major drawback of this method employing a system of Figure 1. Need of the Invention: Accordingly, there is a need of a method for producing the optical fiber preform which should not only have uniform soot depoS1tion with uniform outer diameter on the entire length of the target rod or at least on its major length so as to avoid losses due to tapering effect, but should also be of a better quality having uniform bulk denS1ty with least radial fluctuation and cracking so as to result in formation of a better quality optical fiber at the reasonably low cost of production. Objects of the Invention: The main object of the present invention is to provide a method for producing the optical fiber preform which should not only have uniform soot depoS1tion with uniform outer diameter on the entire length of the target rod or at least on its major length so as to avoid losses due to tapering effect, but should also be of a better quality having uniform bulk 7 denS1ty with least radial fluctuation and cracking so as to result in formation of a better quality optical fiber at the reasonably low cost of production. It is an object of the present invention to provide a method for producing an optical fiber preform not only having reduced tapering effect, but also having reduced bulk denS1ty fluctuations and cracking in the soot depoS1ted on the target rod. It is still an object of the present invention to provide a method for producing an optical fiber preform which can overcome at least some of the problems and drawbacks of the prior art and is still S1mpler and economical to produce the optical fiber preform at reasonably low cost in higher yield. It is further an object of the present invention to provide an optical fiber preform and an optical fiber produced from the optical fiber preform. Brief Description of the Invention: The prior art methods have made an attempt to overcome problems of the tapering effect, fluctuations in bulk denS1ty and cracking in soot by either adjusting distance of burner with respect to the rod and/or by controlling flow rate of fuel gases and/or by controlling contents of the soot forming materials. In case only one parameter is controlled, then the results are neither encouraging nor as deS1red. However, in case two or more parameters are controlled then the method becomes more complex requiring precise control of flow rates and relative concentration of oxygen and/or hydrogen gases, constant watch on approaching end portions to retreat the burners to increase the distance between the burner and the end portions of the target rod, and careful monitoring of temperature at all times to adjust flow rate and distance constantly. In some circumstances, even the supply of the soot forming material is also required to be adjusted. 8 One method could overcome above problems, partly, but still suffers from drawback of tapering effects due to relatively higher surface temperature of the outer surface of the rod near its end portions. Accordingly, a need has been felt to have a method which can control the temperature over the entire length of the target rod, including the end portions, in a S1mpler and eaS1er manner, preferably by controlling only one parameter, not only to have uniform soot depoS1tion on the target rod, but also to have uniform bulk denS1ty of the soot depoS1ted on the target rod. In-addition, the optical fiber perform produced thereby having no or reduced cracking in the soot depoS1ted with or without dopants. The inventors have modified the system of Figure 1 and observed that it is now posS1ble to have the core rod stationary and only rotating on its own axis and burner traverS1ng in the forward and backward motions as shown by arrow 112 in Figure 2. The forward and reverse motions referred hereto are in reference to the motion of either the target rod or the burner and both cases rod rotates on its own axis. Accordingly, it is immaterial whether the rod is fitted horizontally as shown in Figure 1 or Figure 2 or vertically as in the prior art discussed hereinabove, what is required is that either the burner is movable in forward and reverse motions along the axis 112 [Figure 2] of the rod or the rod is movable along its own axis 110 in forward and reverse motion [Figure 1] and rotates on its own axis as shown by arrow 111 as shown in Figure 1 and Figure 2. Now it is also posS1ble to modify the system of Figure 1 to have the core rod traverS1ng along its own axis as shown by arrow 110 and rotating on its own axis as shown by arrow 111 and burner traverS1ng in the forward and backward motions as shown by arrow 112 in Figure 3. The forward and reverse motions referred hereto are in reference to the motion of either the target rod or the burner and in this case also the rod rotates on its own axis. Accordingly, in this case also it is immaterial 9 whether the rod is fitted horizontally as shown in Figure 3 or vertically as in the prior art discussed hereinabove, what is required is that the burner is movable in forward and reverse motions along the axis 112 of the rod and the rod is movable along its own axis 110 in forward and reverse motion and rotates also on its own axis as shown by arrow 111 in Figure 3. It has been surpriS1ng observed by the inventors that when the speed of the rod, given that the rod is moving in forward and reverse directions [Figure 1], is changed during one direction of the traverse the tapering effect on the end portions of the target rod is greatly reduced as well as the uniform bulk denS1ty is achieved with reduced cracking in the soot. It has also been observed that when the speed is increased during the reverse motion, the tapering effect is reduced further as well as the uniform bulk denS1ty is enhanced further with almost no cracking in the soot. The change in speed of the rod results in change in temperature around the rod and the increase in speed of the rod results in decrease in temperature around the rod over its entire length including the end portions of the rod where the rise in temperature has been observed under ordinary conditions thereby resulting in reduced tapering at the end portions of the rod. It has been observed that the reduced temperature also results in reduced fluctuations in bulk denS1ty of the soot depoS1ted and in reduced cracking thereby overcoming the problems of tapering effect, fluctuations in bulk denS1ty and cracking in soot. In accordance with the preferred embodiment of the present invention, the speed of the rod is increased during the reverse motion. The speed is increased to such an extent that the amount of depoS1tion is very less due to very high speed of the rod. As during the reverse motion of depoS1tion, the speed of the rod is kept very high thereby resulting in very low soot depoS1tion, the supply of the reactant gases, if deS1red may be terminated during the reverse motion to save on the cost of the raw 10 material. The time losses are very less when compared with the improvement in effective length of soot depoS1tion on the targeting rod. The method of the present invention can also be performed when the rod only rotates on its own axis and the soot forming burner is made to move along the axis of the target rod in forward and reverse directions [Figure 2]. The method of the present invention can also be performed when the rod rotates on its own axis and moves along its own axis in forward and reverse directions, and the soot forming burner is made to move along the axis of the target rod in forward and reverse directions in such a manner that the rod and the burner move in oppoS1te directions to each other [Figure 3]. Accordingly, the present invention relates to a method for producing an optical fiber preform having uniformly depoS1ted soot particles with reduced tapering effect, bulk denS1ty fluctuations and cracking in the soot depoS1ted by depoS1ting the soot particles around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction. In one embodiment of the present invention, the soot particles are depoS1ted around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in the forward direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in the reverse direction. In another embodiment of the present invention, the soot particles are depoS1ted around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in the reverse direction with reference to the speed 11 of the target rod and/or the soot forming burner in traverse motion in the forward direction. In accordance with preferred embodiment of this invention the speed of the target rod and/or the soot forming burner in traverse motion in forward direction is selected to be less than the speed of the target rod and/or the soot forming burner in the traverse motion in reverse direction. In accordance with more preferred embodiment of this invention the speed of the target rod and/or the soot forming burner in traverse motion in forward direction is selected to be varying from about 1500 to about 12000 mm/min and the speed of the target rod and/or the soot forming burner in traverse motion in reverse direction is selected to be varying from about 20000 to about 30000 mm/min. In one embodiment of the present invention, the rod traverses (moves) in longitudinal direction and as well as it rotates on its own axis and burner is kept stationary [Figure 1]. In another embodiment of the present invention, the rod only rotates onto its own axis and the burner moves along the axis of the rod in forward and reverse directions [Figure 2]. In still another embodiment of the present invention, the rod and burner both moves in forward and reverse directions, but in oppoS1te directions with respect to each other, and the rod is also made to rotate onto its own axis [Figure 3]. In case the rod and the burner will move in same directions then the soot depoS1tion will not take place along the entire length of the target rod. In case the rod does not rotate around its own axis then the soot depoS1tion will take place around the entire diameter of the target rod. It is immaterial at what speed the rod or burner moves [Figure 1 / Figure 2], or at what relative speed the rod and burner moves when both are moving in oppoS1te direction to each other [Figure 3], what is required 12 is that the speed of the rod and/or burner in one direction should be different than the speed of the rod and/or burner in another direction, preferably the speed in the reverse direction of the rod and/or burner should be different than the speed in the forward direction, more preferably the speed in the forward direction should be less than the speed in the reverse direction. The other embodiments and advantages of the present invention will be more apparent when the following description is read in conjunction with the accompanying drawings, which are intended only for the better understanding of the invention and not for restricting its scope. Brief Description of the Accompanying Drawings: Figure 1 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its one of the embodiments. Figure 2 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its another embodiment. Figure 3 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its still another embodiment. Figure 4 is a schematic representation of traverse speed in a conventional method. Figure 5 shows a schematic representation of the traverse speed in accordance with the presentation invention. Figure 6 shows the soot porous body of optical fiber preform produced by the conventional method depicting the tapering effect. 13 Figure 7 shows the soot porous body of an optical fiber preform produced by the method of the present invention. Figure 8 shows the traverse speed motion over the core rod. Detailed Description of the Invention: Now referring to Figure 1, the target rod 101, which may also be referred as core rod or a rod or a substrate is mounted on a glass-working lathe 102 between the chucks 103 and 104 over one or more of soot forming burners 105 placed at close proximity to each other in direction traverse to the length of the target rod. The glass-working lathe 102 is enclosed inS1de the gas cabinet 106. Both ends of the target rod are attached with handle rods 107 in order to help mounting of target rod 101 between chucks 103 and 104. An exhaust duct 108 is provided in a gas cabinet 106 in order to remove un-depoS1ted soot particles and unused reactant gases. The target rod is capable of rotating around its own axis as shown by arrow 111. In this embodiment of the present invention the apparatus for soot depoS1tion on a target rod is selected to be one as shown in Figure 1, wherein the glass forming lathe is placed or mounted horizontally. In this embodiment, the target rod 101 rotates about its own axis as shown by arrow 111 and traverses along its own axis as shown by arrow 110. In this embodiment, the burner 105 is kept stationary. The movement of the rod 101 is posS1ble due to movement of lathe assembly 102 along the axis of the rod 101 as shown by arrow 110 in Figure 1. The rod 101 may also be said to moving or traverS1ng in upward and downward directions if the glass forming lathe 102 is mounted in the vertical direction (not shown in Figures). The advantage of the present method is that it can also be carried out in an apparatus for soot depoS1tion on a target rod wherein the target rod is capable of moving or traverS1ng along its own axis in forward and reverse directions if it is placed horizontally [Figure 2 / Figure 3] or in 14 upward and downward directions if it is placed in vertical direction [not shown in Figures]. During the traverse passes, S1Cl4 vpours are supplied from the burner along with the oxyhydrogen fuel from the delivery line 109 so that S1Cl4 reacts with O2 to form soot particles for depoS1tion on the target rod 101 resulting into a soot porous body 100. Now referring to Figure 5, in accordance with the present invention the respective traverse speeds of the moving part which is either target rod or soot forming burner or both during the forward and reverse motions are different thereby resulting in change in temperature along the length of the target rod. The change in temperature is observed to be inversely proportional to the change in speed of the moving part. In according with the present invention, the set of traverse may begin either from S1de SL or from S1de SR. The speed S1 is the speed in the forward direction and the speed S2 is the speed in the reverse direction which are measured in mm/min. When the traverse begins from S1de SL, at the start of the forward motion of a traverse, the traverse speed of the moving part in the forward motion is raised from zero to its maximum value S1 with acceleration al mm/min2 and the traverse continues towards the S1de SR at the maximum speed S1. At the end of the forward motion, that is after the moving part reaches the end SR, the traverse speed of the moving part is reduced from its maximum value S1 to zero with deceleration dl mm/min2. To start the traverse in the reverse direction from the end SR towards the end SL, the speed of the moving part is once again raised from zero value, but this time to its maximum value S2 with acceleration a2 mm/min2 and the traverse continues towards the S1de SL at the maximum speed S2 which in accordance with the present invention is different from the speed S1, preferably higher than the speed S1. At the end of the reverse motion, that is after the moving part reaches the end SL, the traverse speed of the moving part in the reverse motion is reduced 15 from its maximum value S2 to zero with deceleration d2 mm/min2. This is followed repeatedly till K number of traverses are completed. After completion of K number of traverses, the moving part is moved from end SL to SR at the speed S1 in the forward direction by following above steps. This movement is referred as switching start poS1tion of forward and reverse motions. At the end of this movement, the new traverse starts from another end SR. Now when the traverse begins from S1de SR, at the start of the forward motion of a traverse, the traverse speed of the moving part in the forward motion is raised from zero to its maximum value S1 with acceleration al mm/min2 and the traverse continues towards the S1de SL at the maximum speed S1. At the end of the forward motion, that is after the moving part reaches the end SL, the traverse speed of the moving part is reduced from its maximum value S1 to zero with deceleration dl mm/min2. To start the traverse in the reverse direction from the end SL towards the end SR, the speed of the moving part is once again raised from zero value, but this time to its maximum value S2 with acceleration a2 mm/min2 and the traverse continues towards the S1de SR at the maximum speed S2 which in accordance with the present invention is different from the speed S1, preferably higher than the speed S1. At the end of the reverse motion, that is after the moving part reaches the end SR, the traverse speed of the moving part in the reverse motion is reduced from its maximum value S2 to zero with deceleration d2 mm/min2. This is followed repeatedly till K number of traverses are completed. The above process is continued till the deS1red amount or thickness of the soot is depoS1ted on the target rod. In accordance with the present invention, when the rod is rotating on its own axis, and either the rod or the burner is traverS1ng along the axis of the rod, hereinafter referred to as moving part, and another part, either burner or the rod is kept stationary, hereinafter referred to as 16 stationary part [Figure 1 or Figure 2], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:- a) mounting a core rod over a movable lathe; b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; wherein said method is characterized by following sequence of steps:- e) increasing the speed of the moving part in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; f) continuing said forward motion of said moving part at said first maximum speed S1 of step - e) from end SL till said moving part reaches another end portion SR with respect to said stationary part; g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; h) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till said moving part reaches another end portion SL with respect to said stationary part; j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; 17 k) repeating each of above said steps e) to j) for a predetermined number of times K; 1) carrying out each of above said steps e) to g) in forward motion so that said moving part reaches another end SR with respect to said stationary part; m) beginning a new set of traverses with forward motion commencing from said end SR of said moving part with respect to said stationary part by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of said moving part with respect to said stationary part at said first maximum speed S1 so that said moving part reaches said end SL with respect to said stationary part; n) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; o) continuing said reverse motion of said moving part at said second maximum speed S2 of said step - n) from end SL till said moving part reaches said end SR with respect to said stationary part; p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; q) increasing the speed of said moving part in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till said moving part reaches said end SL with respect to said stationary part; s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; 18 t) repeating each of above said steps n) to s) for a predetermined number of times K; and u) continuing all the above steps till the required amount or thickness of soot is depoS1ted on the core rod. In accordance with one embodiment of this invention, when the rod rotates on its own axis and also traverses along its own axis, and the burner is kept stationary [Figure 1], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:- a) mounting a core rod over a movable lathe; b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; wherein said method is characterized by following sequence of steps:- e) increasing the speed of the rod in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; f) continuing said forward motion of the rod at said first maximum speed S1 of step - e) from end SL till the rod reaches another end portion SR with respect to stationary burner; g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; h) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till the rod reaches another end portion SL with respect to stationary burner; 19 j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; k) repeating each of above said steps e) to j) for a predetermined number of times K; 1) carrying out each of above said steps e) to g) in forward motion so that the rod reaches another end SR with respect to stationary burner; m) beginning a new set of traverses with forward motion commencing from said end SR of the core rod with respect to stationary burner by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod with respect to stationary burner at said first maximum speed S1 so that the rod reaches said end SL with respect to stationary burner; n) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; o) continuing said reverse motion of the rod at said second maximum speed S2 of said step - n) from end SL till the rod reaches said end SR with respect to stationary burner; p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; q) increasing the speed of the rod in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till the rod reaches said end SL with respect to stationary burner; 20 s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; t) repeating each of above said steps n) to s) for a predetermined number of times K; and u) continuing all the above steps till the required amount or thickness of soot is depoS1ted on the core rod. In accordance with another embodiment of this invention, when the rod only rotates on its own axis and the burner is moving along the longitudinal axis of the rod [Figure 2], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:- a) mounting a core rod over a movable lathe; b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; wherein said method is characterized by following sequence of steps: e) increasing the speed of the burner in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; f) continuing said forward motion of the burner at said first maximum speed S1 of step - e) from end SL till the burner reaches another end portion SR of said core rod; g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; h) increasing the speed of the burner in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; 21 i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till the burner reaches another end portion SL of said core rod; j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; k) repeating each of above said steps e) to j) for a predetermined number of times K; 1) carrying out each of above said steps e) to g) in forward motion so that the burner reaches another end SR; m) beginning a new set of traverses with forward motion commencing from said end SR of the core rod by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod at said first maximum speed S1 so that the burner reaches said end SL of core rod; n) increasing the speed of the burner in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; o) continuing said reverse motion of the burner at said second maximum speed S2 of said step - n) from end SL till the burner reaches said end SR of said core rod; p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; q) increasing the speed of the burner in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till the burner reaches said end SL of said core rod; 22 s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; t) repeating each of above said steps n) to s) for a predetermined number of times K; and u) continuing all the above steps till the required amount or thickness of soot is depoS1ted on the core rod. As stated hereinabove, the first maximum speed S1 is different from the second maximum speed S2. The first maximum speed S1 is less than the second maximum speed S2. The first maximum speed S1 varies from about 1500 to about 12000 mm/min, preferably from about 1500 to about 2000 mm/min, more preferably from about 1500 to about 1600 mm/min. The second maximum speed S2 varies from about 20000 to about 30000 mm/min, preferably it is kept around the value of about 20000 mm/min. The accelerations al and a2, and decelerations dl and d2 are selected so as to further reduce the tapering effect. It has been surpriS1ngly observed that when the accelerations al and a2, and decelerations dl and d2 are varied depending upon the motion speeds S1 and S2, the tapering effect is further reduced. The reduction is tapering effect has been also been observed when the distance of the traverse motion from which the speed increases from 0 to S1 mm/min is same when the speed decreases from S2 to 0 mm/min. In accordance with the present invention, the acceleration al and deceleration dl during the forward motion speed S1 are kept in the range of about 270 to 290 mm/sec2. The acceleration a2 and deceleration d2 during the reverse motion S1 are kept in the range of about 500 to about 600 mm/sec2. 23 The first maximum speed S1 and the second maximum speed S2 can be optionally changed in every set of passes. The ends SL and SR are the left and right ends of the target rod where in the conventional methods tapering effects have been observed. However, when following the method of the present invention the tapering effect has been greatly reduced. In one embodiment of this invention, when the target rod is moving, in-addition to rotation on its own axis, with respect to the soot forming burner, that is the burner is kept stationary [Figure 1], then at the end of the motion, the ends SL and SR of the moving rod reach the stationary burner. In another embodiment of this invention, when the rod only rotates on its own axis and the burner is moving in forward and reverse directions [Figure 2], then at the end of the motion, the moving burner reaches the ends SL and SR of the rod. In still another embodiment of this invention, when the rod and burner both are moving in the oppoS1te directions [Figure 3], then at the end of the motion, the ends SL and SR of the moving rod reach the moving burner. The value of K is 16 or less than 16, preferably 12 or less than 12. The value of K may be same or different in set traverses starting from end SL and end SR. Accordingly, for example, in accordance with the present invention, the K in one set of traverses may be 12 to 16 and in following set of traverse it may be 12 or less than 12. The reactant gases are for example SiCl4, GeCl4, BCl3, and other dopant gases. Oxygen and other fuel gas are used along with reactant gases to form the soot. The contents and the flow rate of the reactant gases and the fuel gases can be maintained as known in the art. The flow rate of the reactant gases can be maintained in the range of about 180 to about 220 Gram/min. The flow rate of oxygen and hydrogen as fuel 24 gases can be maintained in the range of about 120 to about 180 slpm and 50 to 85 slpm respectively. In one embodiment of the present invention, the soot forming burners 105 are preferably two. The target rod 101 rotates about its own axis with a rotational speed preferably of about 60 rpm or more. The above description elaborates a case when the traverse in forward and/or reverse directions continue towards the oppoS1te ends of the rod at respective maximum speed of S1 and/or S2 respectively. However, the speeds S1 and S2 need not necessarily be constant during the respective traverse. These may be constantly increasing or decreasing. In one embodiment, the present invention provides an optical fiber preform produced by the present method. The optical fiber preform produced according to the method of the present invention has effective length improved approximately by 20 to 25% as compared to the conventional methods known in the art, and the diameter variation within the preform is well controlled which is approximately 3mm and other characteristics of the optical fiber preform produced by the present method are also well within the limits to obtain the deS1red parameters, of the optical fiber to be drawn from the optical fiber preform produced by this method. An exemplary optical fiber drawn from the optical fiber preform produced by the present method has been observed to have following characteristics: Attention at 1310 Attention at 1550 Cutoff wavelength 1160 to 1300 nm Mode Field Diameter 8.9 to 9.5 μm Clad Diameter 125+/- 1 μm 25 We Claim: 1. A method for producing an optical fiber preform having uniformly deposited soot particles depoS1ted by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction. 2. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner is changed in the forward direction with reference to the speed of said target rod and/or said soot forming burner in traverse motion in the reverse direction. 3. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner is changed in the reverse direction with reference to the speed of said target rod and/or said soot forming burner in traverse motion in the forward direction. 4. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner in traverse motion in forward direction is selected to be less than the speed of said target rod and/or said soot forming burner in the traverse motion in reverse direction. 5. A method as claimed in any one of preceding claims, wherein the speed of said target rod and/or said soot forming burner in traverse motion in forward direction is selected to be varying from about 1500 to about 12000 mm/min, 6. A method as claimed in any one of preceding claims, wherein the speed of said target rod and/or said soot forming burner in traverse motion in reverse direction is selected to be varying from about 20000 to about 30000 mm/min. 7. A method for producing an optical fiber preform having uniform soot deposition comprises steps of:- a) mounting a core rod over a movable lathe; 26 b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; wherein said method is characterized by following sequence of steps :- e) increasing the speed of the moving part in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; f) continuing said forward motion of said moving part at said first maximum speed S1 of step - e) from end SL till said moving part reaches another end portion SR with respect to said stationary part; g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; h) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till said moving part reaches another end portion SL with respect to said stationary part; j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; 27 k) repeating each of above said steps e) to j) for a predetermined number of times K; 1) carrying out each of above said steps e) to g) in forward motion so that said moving part reaches another end SR with respect to said stationary part; m) beginning a new set of traverses with forward motion commencing from said end SR of said moving part with respect to said stationary part by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of said moving part with respect to said stationary part at said first maximum speed S1 so that said moving part reaches said end SL with respect to said stationary part; n) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; o) continuing said reverse motion of said moving part at said second maximum speed S2 of said step - n) from end SL till said moving part reaches said end SR with respect to said stationary part; p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; q) increasing the speed of said moving part in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till said moving part reaches said end SL with respect to said stationary part; 28 s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; t) repeating each of above said steps n) to s) for a predetermined number of times K; and u) continuing all the above steps till the required amount or thickness of soot is deposited on the core rod. 8. A method for producing an optical fiber preform having uniform soot depoS1tion comprises steps of:- a) mounting a core rod over a movable lathe; b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; wherein said method is characterized by following sequence of steps:- e) increasing the speed of the rod in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; f) continuing said forward motion of the rod at said first maximum speed S1 of step - e) from end SL till the rod reaches another end portion SR with respect to stationary burner; g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; h) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; 29 i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till the rod reaches another end portion SL with respect to stationary burner; j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; k) repeating each of above said steps e) to j) for a predetermined number of times K; 1) carrying out each of above said steps e) to g) in forward motion so that the rod reaches another end SR with respect to stationary burner; m) beginning a new set of traverses with forward motion commencing from said end SR of the core rod with respect to stationary burner by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod with respect to stationary burner at said first maximum speed S1 so that the rod reaches said end SL with respect to stationary burner; n) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; o) continuing said reverse motion of the rod at said second maximum speed S2 of said step - n) from end SL till the rod reaches said end SR with respect to stationary burner; p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; q) increasing the speed of the rod in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; 30 r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till the rod reaches said end SL with respect to stationary burner; s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; t) repeating each of above said steps n) to s) for a predetermined number of times K; and u) continuing all the above steps till the required amount or thickness of soot is deposited on the core rod. A method for producing an optical fiber preform having uniform soot depoS1tion comprises steps of:- a) mounting a core rod over a movable lathe; b) selecting a method of depoS1ting soot over said core rod; c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod; d) beginning traverse from one end (SL) to another end (SR) of core rod; e) wherein said method is characterized by following sequence of steps: f) increasing the speed of the burner in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; g) continuing said forward motion of the burner at said first maximum speed S1 of step - e) from end SL till the burner reaches another end portion SR of said core rod; h) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; 31 i) increasing the speed of the burner in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; j) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till the burner reaches another end portion SL of said core rod; k) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; 1) repeating each of above said steps e) to j) for a predetermined number of times K; m) carrying out each of above said steps e) to g) in forward motion so that the burner reaches another end SR; n) beginning a new set of traverses with forward motion commencing from said end SR of the core rod by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod at said first maximum speed S1 so that the burner reaches said end SL of core rod; o) increasing the speed of the burner in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value; p) continuing said reverse motion of the burner at said second maximum speed S2 of said step - n) from end SL till the burner reaches said end SR of said core rod; q) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value; r) increasing the speed of the burner in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value; 32 s) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till the burner reaches said end SL of said core rod; t) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value; u) repeating each of above said steps n) to s) for a predetermined number of times K; and v) continuing all the above steps till the required amount or thickness of soot is depoS1ted on the core rod. 10. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said first maximum speed S1 varies from about 1500 to about 12000 mm/min, preferably from about 1500 to about 2000 mm/min, more preferably from about 1500 to about 1600 mm/min. 11. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said second maximum speed S2 varies from about 20000 to about 30000 mm/min, preferably it is kept around the value of about 20000 mm/min. 12. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said acceleration al and deceleration dl during the forward motion speed S1 are kept in the range of about 270 to 290 mm/sec2 and said acceleration a2 and deceleration d2 during the reverse motion S1 are kept in the range of about 500 to about 600 mm/sec2. 13. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said K is 16 or less than 16, preferably 12 or less than 12. 14. A method for producing an optical fiber preform having uniformly depoS1ted soot particles substantially as herein before described and illustrated with the help of the accompanying drawings. 33 15. An optical fiber preform as and when manufactured by the process as claimed in any one or more of the preceding claims. 16. An optical fiber as and when manufactured from the optical fiber preform as claimed in claim 15. 17. An optical fiber as claimed in claim 16, wherein said fiber has following characteristics: Attention at 1310 Attention at 1550 Cutoff wavelength 1160 to 1300 nm Mode Field Diameter 8.9 to 9.5 μm Clad Diameter 125+/- 1 μm 18. An optical fiber produced from the optical fiber preform substantially as herein before described and illustrated with the help of the accompanying drawings. Dated this 12th day of August, 2005. [Dr. Ramesh Kr. MEHTA] Patent Attorney for the Applicants Mehta & Mehta Associates 34 ABSTRACT A method for producing an optical fiber preform having uniformly depoS1ted soot particles comprising depositing soot particles by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction is provided. Figures 1 & 7 35

Full Text

FORM 2THE PATENTS ACT, 1970 (39 of 1970)& THE PATENTS RULES, 2003PROVIS1ONAL/COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
1. Title of the Invention: - Optical Fiber Preform having Uniform Soot DepoS1tion & its Method of Preparation
2. Applicant(s):-(a) Name: STERLITE OPTICAL TECHNOLOGIES LTD.(b) Nationality: An Indian Company(c) Address: E 2, MIDC, Waluj, Aurangabad - 431136, Maharashtra, INDIA
3. Preamble to the Description:-
Complete Specification:The following specification particularly describes the Invention and the manner in which it is to be performed.
ProviS1onal Specification?
The following specification describes the Invention

Field of the Invention:
The present invention relates to an optical fiber preform having uniform soot depoS1tion and its method of preparation. Particularly it relates to an optical fiber preform having uniform soot particles depoS1tion and its method of preparation, particularly it relates to method for depoS1ting soot particles on a target rod to form an optical fiber preform having reduced tapering effect and the optical fiber preform produced thereby.
Background of the Invention:
Optical fibers are inherently versatile as a transmisS1on medium for all forms of information, be it voice, video or data. The optical fibers are drawn from an optical fiber preform. The optical fiber of predetermined dimenS1on is drawn from the optical fiber preform by subjecting one end of the preform to a high temperature, for example above 2000°C. Under such a high temperature, the tip of the preform softens, from which a thin fiber of deS1red dimenS1on is drawn. The different methods employed in manufacture of these preforms are described in the literature.
The optical fiber preform can be manufactured by different methods of chemical vapour depoS1tion (CVD). The optical fiber preform manufacturing process primarily involves a step of preparing the core rod conS1sting of core of the fiber and part of clad which may be followed by over-cladding. The core rod can be prepared by methods known in the art, such as modified chemical vapour depoS1tion (MCVD), plasma chemical vapour depoS1tion (PCVD), outS1de vapour depoS1tion (OVD), vapour axial depoS1tion (VAD) etc. The over-cladding of the core rod can also be carried out by various methods, such as glass tube jacketing, OVD soot over-cladding, VAD soot over-cladding, plasma over-cladding etc. The optical fiber preform can be manufactured by any combination of the core rod manufacturing methods and the over-cladding preparation methods.
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The most commonly used manufacturing process is the MCVD process which as compared to OVD is S1mpler in nature. However, due to wide flexibility, the OVD process has been successfully industrialized and is now preferred over MCVD for manufacturing wide range of optical fibers, such as S1ngle-mode fibers, long-distance, graded-index multimode fibers, short-distance fibers etc.
In the OVD method, a flame hydrolyS1s burner fabricates the soot glass particles on the surface of the core rod conS1sting of core. The chemical reactants involved in the formation of soot particles are S1licon tetrachloride [S1Cl4], oxygen [O2] and oxyhydrogen burner gases. S1Cl4 vapours reacts with O2 with the help of oxyhydrogen flame to form sub micrometer S1zed S1O2 glass particles in a reaction which is expressed as follows:
S1Cl4(g) + 2H2(g) + O2(g) ===→ S1O2(s) + 4HCl(g)
The above mentioned depoS1tion process occurs till the deS1red amount of glass particles have been depoS1ted and then the soot porous body is moved into S1ntering furnace, where the depoS1ted soot layer is dried with chlorine atmosphere and then S1ntered to form a solid glass preform in a helium atmosphere at about 1500°C. The S1ntered preform is drawn into an optical fiber, which conS1sts of core and clad.
One of the known methods employing principles of OVD method
for manufacturing the optical fiber preform having soot depoS1ted involves glass forming material are supplied into a burner to form soot particles over the core rod. In this method, for adjusting the distance between the burner and the soot depoS1ted, the burner is adapted to move towards or away from the target rod. The major drawback of depoS1ting the soot is that it results in varying outer diameter at both the ends of the preform formed, which in-turn results in formation of poor quality optical fiber having irregularities in hardness and thermal
3

expanS1on mismatch between the target rod and soot depoS1t causing cracking in the soot. The main reason for this drawback has been asS1gned to rise in temperature around both the ends of the target rod due to round trip of the burner. The rise in temperature at both the ends is conS1dered primarily due to constant heating power of the burner at both the ends of the target rod. Further, with the constant flow rate of the gases, the surface temperature of the soot depoS1t increases further adding to the problem.
To overcome above problem, a method for soot depoS1tion has been reported having varying heating power of the burner and varying gas flow rate at both the ends of the target rod. In this method, the distance between burner and target rod is increased at both the ends and reduced at the middle portion having constant diameter of the soot depoS1t, and the flow rate of the gases is maintained constant at both the ends and increased at the middle portion of the target rod.
The problem of above method is that the constant gas flow rate at both the ends cannot work in all circumstances. Under such circumstances, it is either increased or decreased depending on the case. Further, the increased gas flow rate at the middle portion of the rod is also decided depending upon the case. In some circumstances, it is required to be increased progresS1vely. Therefore, this method does not resolve the problem completely and leaves the user to decide on the gas flow rate depending upon the case.
Still another problem of the above method is that it requires precise control of flow rates and relative concentration of oxygen and/or hydrogen gases. Further, it needs constant watch on approaching end portions to retreat the burners to increase the distance between the burner and the end portions of the target rod. It also needs careful monitoring of temperature at all times to adjust flow rate and distance constantly, that is this method cannot work at a given flow rate of the gases and a given distance of the burner. Further, to avoid increase in
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volume of the varying diameter end portions of the preform, the supply of the soot forming material is also required to be adjusted. Further, in case the above method is carried out with plurality of burners, then the heating power of individual burners will have to be adjusted in accordance with the changes in the flow rates of soot forming materials that occur in the end portions of the soot depoS1tion end portion adding to further complexity of the method.
It has also been observed that above method is not suitable, that is the change of burner gas flow is not achieved with respect to poS1tion of length when the speed of traverse is about 1500 to about 12000 mm/min or more.
The methods known in the art as elaborated hereinabove, only partly overcome above problems by alternatively or additionally adjusting flow rate of the soot forming materials and/or gases. These methods not only adjust the flow rate of the soot forming materials and/or gases differently at middle portion and end portions, but also within the middle portion and/or end portions.
Another method of soot depoS1tion employs a system elaborated in accompanying Figure 1, wherein glass soot is depoS1ted on the core rod 101 to form soot porous body 100. In this method the soot depoS1tion is accomplished by traverse motion 110 of the core rod over the burner as shown in Figure 1 by arrow 110. In the methods employing this system, the movement of burner is not towards and away from the target rod, but is along the axis of the target rod.
In the method known in the art employing the system of Figure 1, the maximum traverse speed of the forward and reverse motions is kept same [Figure 4]. In this method the soot depoS1tion takes place during both the motions - forward and reverse motions. When the end portion of the target rod reaches the burner in a forward motion, the traverse speed is reduced from its maximum value S1 to zero with a predetermined deceleration dl. During the reverse motion, the traverse speed increases
5

from zero to its maximum value S1 with an acceleration al, and again reduces with a deceleration dl to zero at the end of the traverse and so on. The soot depoS1tion process continues in the same pattern until the required dimenS1on (diameter) of the soot depoS1tion is achieved. After attaining the required dimenS1on of the soot depoS1tion, the target rod with soot depoS1ted thereon is moved to a S1ntering furnace to dry and S1nter it to form an optical fiber preform which can be drawn to an optical fiber.
However, the optical fiber preform formed by the above method also has constant outer diameter in the middle portion, but the varying diameter towards the end portions as shown in Figure 6.
It has been observed that even with a system of Figure 1, during a normal process of soot depoS1tion, there is a relative movement between burner and the target rod taking place in each traverse, each traverse compriS1ng a forward and a reverse motion. When the forward and reverse motions are kept constant, at the time of change of direction of the movement of the burner on reaching end portion of the rod or at the time of change of direction of the movement of the rod on its one end portion reaching the burner, as the case may be, it causes the end portions of the rod to come in contact with the heat provided by the burner, twice in quick and immediate succesS1on in a short period of time and once each during the forward and reverse motions of any traverse, as compared to the middle portion of the rod. This results in the outer surface of the rod near its end portions attaining relatively higher surface temperatures. Accordingly, this method also suffers from the problem of elevation of temperature towards and at the end portions of the rod. It has been observed that this method results in increase in denS1ty of the soot depoS1ted, but reduces the volume of the soot depoS1ted on the end portions of the rod. Consequently, the diameter at the end portions of the rod is smaller than the diameter of the middle portion of the rod [Figure 6].
6

Accordingly, the main drawback of this method is that the outer diameter of the soot depoS1ted on the rod is non-uniform, that is, it is reduced towards both the ends resulting in a tapering effect. The reasoning for tapering effect is believed to be relatively higher surface temperature of the ends of the rod, which results in increase in denS1ty, but decrease in volume of soot depoS1ted on the ends. Another drawback of this method is that the thickness of the optical fiber drawn from the preform prepared by this method will have unacceptable variance depending upon the S1ze and the extend of the taper. To avoid the unacceptable variance, the tapered portion of the preform will have to be rejected, which enhances cost of production of optical fiber. The tapered portion of the optical fiber preform produced by this method is, therefore, not used for fiber drawing which leads to increase in the ineffective portion and decrease in useable or effective portion of the preform, thereby leading to wastage and enhancement of cost of production of the optical fiber. This effect known as tapering effect is a major drawback of this method employing a system of Figure 1.
Need of the Invention:
Accordingly, there is a need of a method for producing the optical fiber preform which should not only have uniform soot depoS1tion with uniform outer diameter on the entire length of the target rod or at least on its major length so as to avoid losses due to tapering effect, but should also be of a better quality having uniform bulk denS1ty with least radial fluctuation and cracking so as to result in formation of a better quality optical fiber at the reasonably low cost of production. Objects of the Invention:
The main object of the present invention is to provide a method for producing the optical fiber preform which should not only have uniform soot depoS1tion with uniform outer diameter on the entire length of the target rod or at least on its major length so as to avoid losses due to tapering effect, but should also be of a better quality having uniform bulk
7

denS1ty with least radial fluctuation and cracking so as to result in formation of a better quality optical fiber at the reasonably low cost of production.
It is an object of the present invention to provide a method for producing an optical fiber preform not only having reduced tapering effect, but also having reduced bulk denS1ty fluctuations and cracking in the soot depoS1ted on the target rod.
It is still an object of the present invention to provide a method for producing an optical fiber preform which can overcome at least some of the problems and drawbacks of the prior art and is still S1mpler and economical to produce the optical fiber preform at reasonably low cost in higher yield.
It is further an object of the present invention to provide an optical fiber preform and an optical fiber produced from the optical fiber preform. Brief Description of the Invention:
The prior art methods have made an attempt to overcome problems of the tapering effect, fluctuations in bulk denS1ty and cracking in soot by either adjusting distance of burner with respect to the rod and/or by controlling flow rate of fuel gases and/or by controlling contents of the soot forming materials. In case only one parameter is controlled, then the results are neither encouraging nor as deS1red. However, in case two or more parameters are controlled then the method becomes more complex requiring precise control of flow rates and relative concentration of oxygen and/or hydrogen gases, constant watch on approaching end portions to retreat the burners to increase the distance between the burner and the end portions of the target rod, and careful monitoring of temperature at all times to adjust flow rate and distance constantly. In some circumstances, even the supply of the soot forming material is also required to be adjusted.
8

One method could overcome above problems, partly, but still suffers from drawback of tapering effects due to relatively higher surface temperature of the outer surface of the rod near its end portions.
Accordingly, a need has been felt to have a method which can control the temperature over the entire length of the target rod, including the end portions, in a S1mpler and eaS1er manner, preferably by controlling only one parameter, not only to have uniform soot depoS1tion on the target rod, but also to have uniform bulk denS1ty of the soot depoS1ted on the target rod. In-addition, the optical fiber perform produced thereby having no or reduced cracking in the soot depoS1ted with or without dopants.
The inventors have modified the system of Figure 1 and observed that it is now posS1ble to have the core rod stationary and only rotating on its own axis and burner traverS1ng in the forward and backward motions as shown by arrow 112 in Figure 2. The forward and reverse motions referred hereto are in reference to the motion of either the target rod or the burner and both cases rod rotates on its own axis. Accordingly, it is immaterial whether the rod is fitted horizontally as shown in Figure 1 or Figure 2 or vertically as in the prior art discussed hereinabove, what is required is that either the burner is movable in forward and reverse motions along the axis 112 [Figure 2] of the rod or the rod is movable along its own axis 110 in forward and reverse motion [Figure 1] and rotates on its own axis as shown by arrow 111 as shown in Figure 1 and Figure 2.
Now it is also posS1ble to modify the system of Figure 1 to have the core rod traverS1ng along its own axis as shown by arrow 110 and rotating on its own axis as shown by arrow 111 and burner traverS1ng in the forward and backward motions as shown by arrow 112 in Figure 3. The forward and reverse motions referred hereto are in reference to the motion of either the target rod or the burner and in this case also the rod rotates on its own axis. Accordingly, in this case also it is immaterial
9

whether the rod is fitted horizontally as shown in Figure 3 or vertically as in the prior art discussed hereinabove, what is required is that the burner is movable in forward and reverse motions along the axis 112 of the rod and the rod is movable along its own axis 110 in forward and reverse motion and rotates also on its own axis as shown by arrow 111 in Figure 3.
It has been surpriS1ng observed by the inventors that when the speed of the rod, given that the rod is moving in forward and reverse directions [Figure 1], is changed during one direction of the traverse the tapering effect on the end portions of the target rod is greatly reduced as well as the uniform bulk denS1ty is achieved with reduced cracking in the soot. It has also been observed that when the speed is increased during the reverse motion, the tapering effect is reduced further as well as the uniform bulk denS1ty is enhanced further with almost no cracking in the soot. The change in speed of the rod results in change in temperature around the rod and the increase in speed of the rod results in decrease in temperature around the rod over its entire length including the end portions of the rod where the rise in temperature has been observed under ordinary conditions thereby resulting in reduced tapering at the end portions of the rod. It has been observed that the reduced temperature also results in reduced fluctuations in bulk denS1ty of the soot depoS1ted and in reduced cracking thereby overcoming the problems of tapering effect, fluctuations in bulk denS1ty and cracking in soot.
In accordance with the preferred embodiment of the present invention, the speed of the rod is increased during the reverse motion. The speed is increased to such an extent that the amount of depoS1tion is very less due to very high speed of the rod. As during the reverse motion of depoS1tion, the speed of the rod is kept very high thereby resulting in very low soot depoS1tion, the supply of the reactant gases, if deS1red may be terminated during the reverse motion to save on the cost of the raw
10

material. The time losses are very less when compared with the improvement in effective length of soot depoS1tion on the targeting rod.
The method of the present invention can also be performed when
the rod only rotates on its own axis and the soot forming burner is made to move along the axis of the target rod in forward and reverse directions [Figure 2].
The method of the present invention can also be performed when the rod rotates on its own axis and moves along its own axis in forward and reverse directions, and the soot forming burner is made to move along the axis of the target rod in forward and reverse directions in such a manner that the rod and the burner move in oppoS1te directions to each other [Figure 3].
Accordingly, the present invention relates to a method for producing an optical fiber preform having uniformly depoS1ted soot particles with reduced tapering effect, bulk denS1ty fluctuations and cracking in the soot depoS1ted by depoS1ting the soot particles around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction.
In one embodiment of the present invention, the soot particles are depoS1ted around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in the forward direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in the reverse direction.
In another embodiment of the present invention, the soot particles are depoS1ted around the target rod by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in the reverse direction with reference to the speed
11

of the target rod and/or the soot forming burner in traverse motion in the forward direction.
In accordance with preferred embodiment of this invention the speed of the target rod and/or the soot forming burner in traverse motion in forward direction is selected to be less than the speed of the target rod and/or the soot forming burner in the traverse motion in reverse direction.
In accordance with more preferred embodiment of this invention the speed of the target rod and/or the soot forming burner in traverse motion in forward direction is selected to be varying from about 1500 to about 12000 mm/min and the speed of the target rod and/or the soot forming burner in traverse motion in reverse direction is selected to be varying from about 20000 to about 30000 mm/min.
In one embodiment of the present invention, the rod traverses (moves) in longitudinal direction and as well as it rotates on its own axis and burner is kept stationary [Figure 1].
In another embodiment of the present invention, the rod only rotates onto its own axis and the burner moves along the axis of the rod in forward and reverse directions [Figure 2].
In still another embodiment of the present invention, the rod and burner both moves in forward and reverse directions, but in oppoS1te directions with respect to each other, and the rod is also made to rotate onto its own axis [Figure 3].
In case the rod and the burner will move in same directions then the soot depoS1tion will not take place along the entire length of the target rod.
In case the rod does not rotate around its own axis then the soot depoS1tion will take place around the entire diameter of the target rod.
It is immaterial at what speed the rod or burner moves [Figure 1 / Figure 2], or at what relative speed the rod and burner moves when both are moving in oppoS1te direction to each other [Figure 3], what is required
12

is that the speed of the rod and/or burner in one direction should be different than the speed of the rod and/or burner in another direction, preferably the speed in the reverse direction of the rod and/or burner should be different than the speed in the forward direction, more preferably the speed in the forward direction should be less than the speed in the reverse direction.
The other embodiments and advantages of the present invention will be more apparent when the following description is read in conjunction with the accompanying drawings, which are intended only for the better understanding of the invention and not for restricting its scope.
Brief Description of the Accompanying Drawings:
Figure 1 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its one of the embodiments.
Figure 2 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its another embodiment.
Figure 3 is a schematic representation of an apparatus for soot depoS1tion on a target rod to form an optical fiber preform which has also be employed in the present invention in accordance with its still another
embodiment.
Figure 4 is a schematic representation of traverse speed in a conventional method.
Figure 5 shows a schematic representation of the traverse speed in
accordance with the presentation invention.
Figure 6 shows the soot porous body of optical fiber preform
produced by the conventional method depicting the tapering effect.
13

Figure 7 shows the soot porous body of an optical fiber preform produced by the method of the present invention.
Figure 8 shows the traverse speed motion over the core rod.
Detailed Description of the Invention:
Now referring to Figure 1, the target rod 101, which may also be referred as core rod or a rod or a substrate is mounted on a glass-working lathe 102 between the chucks 103 and 104 over one or more of soot forming burners 105 placed at close proximity to each other in direction traverse to the length of the target rod. The glass-working lathe 102 is enclosed inS1de the gas cabinet 106. Both ends of the target rod are attached with handle rods 107 in order to help mounting of target rod 101 between chucks 103 and 104. An exhaust duct 108 is provided in a gas cabinet 106 in order to remove un-depoS1ted soot particles and unused reactant gases. The target rod is capable of rotating around its own axis as shown by arrow 111.
In this embodiment of the present invention the apparatus for soot depoS1tion on a target rod is selected to be one as shown in Figure 1, wherein the glass forming lathe is placed or mounted horizontally. In this embodiment, the target rod 101 rotates about its own axis as shown by arrow 111 and traverses along its own axis as shown by arrow 110. In this embodiment, the burner 105 is kept stationary. The movement of the rod 101 is posS1ble due to movement of lathe assembly 102 along the axis of the rod 101 as shown by arrow 110 in Figure 1. The rod 101 may also be said to moving or traverS1ng in upward and downward directions if the glass forming lathe 102 is mounted in the vertical direction (not shown in Figures).
The advantage of the present method is that it can also be carried out in an apparatus for soot depoS1tion on a target rod wherein the target rod is capable of moving or traverS1ng along its own axis in forward and reverse directions if it is placed horizontally [Figure 2 / Figure 3] or in
14

upward and downward directions if it is placed in vertical direction [not shown in Figures].
During the traverse passes, S1Cl4 vpours are supplied from the burner along with the oxyhydrogen fuel from the delivery line 109 so that S1Cl4 reacts with O2 to form soot particles for depoS1tion on the target rod 101 resulting into a soot porous body 100.
Now referring to Figure 5, in accordance with the present invention the respective traverse speeds of the moving part which is either target rod or soot forming burner or both during the forward and reverse motions are different thereby resulting in change in temperature along the length of the target rod. The change in temperature is observed to be inversely proportional to the change in speed of the moving part.
In according with the present invention, the set of traverse may begin either from S1de SL or from S1de SR. The speed S1 is the speed in the forward direction and the speed S2 is the speed in the reverse direction which are measured in mm/min.
When the traverse begins from S1de SL, at the start of the forward motion of a traverse, the traverse speed of the moving part in the forward motion is raised from zero to its maximum value S1 with acceleration al mm/min2 and the traverse continues towards the S1de SR at the maximum speed S1. At the end of the forward motion, that is after the moving part reaches the end SR, the traverse speed of the moving part is reduced from its maximum value S1 to zero with deceleration dl mm/min2. To start the traverse in the reverse direction from the end SR towards the end SL, the speed of the moving part is once again raised from zero value, but this time to its maximum value S2 with acceleration a2 mm/min2 and the traverse continues towards the S1de SL at the maximum speed S2 which in accordance with the present invention is different from the speed S1, preferably higher than the speed S1. At the end of the reverse motion, that is after the moving part reaches the end SL, the traverse speed of the moving part in the reverse motion is reduced
15

from its maximum value S2 to zero with deceleration d2 mm/min2. This is followed repeatedly till K number of traverses are completed.
After completion of K number of traverses, the moving part is moved from end SL to SR at the speed S1 in the forward direction by following above steps. This movement is referred as switching start poS1tion of forward and reverse motions. At the end of this movement, the new traverse starts from another end SR.
Now when the traverse begins from S1de SR, at the start of the forward motion of a traverse, the traverse speed of the moving part in the forward motion is raised from zero to its maximum value S1 with acceleration al mm/min2 and the traverse continues towards the S1de SL at the maximum speed S1. At the end of the forward motion, that is after the moving part reaches the end SL, the traverse speed of the moving part is reduced from its maximum value S1 to zero with deceleration dl mm/min2. To start the traverse in the reverse direction from the end SL towards the end SR, the speed of the moving part is once again raised from zero value, but this time to its maximum value S2 with acceleration a2 mm/min2 and the traverse continues towards the S1de SR at the maximum speed S2 which in accordance with the present invention is different from the speed S1, preferably higher than the speed S1. At the end of the reverse motion, that is after the moving part reaches the end SR, the traverse speed of the moving part in the reverse motion is reduced from its maximum value S2 to zero with deceleration d2 mm/min2. This is followed repeatedly till K number of traverses are completed.
The above process is continued till the deS1red amount or thickness of the soot is depoS1ted on the target rod.
In accordance with the present invention, when the rod is rotating on its own axis, and either the rod or the burner is traverS1ng along the axis of the rod, hereinafter referred to as moving part, and another part, either burner or the rod is kept stationary, hereinafter referred to as
16

stationary part [Figure 1 or Figure 2], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:-
a) mounting a core rod over a movable lathe;
b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod;
d) beginning traverse from one end (SL) to another end (SR) of core rod;
wherein said method is characterized by following sequence of steps:-
e) increasing the speed of the moving part in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
f) continuing said forward motion of said moving part at said first maximum speed S1 of step - e) from end SL till said moving part reaches another end portion SR with respect to said stationary part;
g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
h) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till said moving part reaches another end portion SL with respect to said stationary part;
j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of
a fourth predetermined value;
17

k) repeating each of above said steps e) to j) for a predetermined
number of times K;
1) carrying out each of above said steps e) to g) in forward motion so
that said moving part reaches another end SR with respect to said stationary part;
m) beginning a new set of traverses with forward motion commencing from said end SR of said moving part with respect to said stationary part by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of said moving part with respect to said stationary part at said first maximum speed S1 so that said moving part reaches said end SL with respect to said stationary part;
n) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
o) continuing said reverse motion of said moving part at said second maximum speed S2 of said step - n) from end SL till said moving part reaches said end SR with respect to said stationary part;
p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value;
q) increasing the speed of said moving part in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till said moving part reaches said end SL with respect to said stationary part;
s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
18

t) repeating each of above said steps n) to s) for a predetermined
number of times K; and u) continuing all the above steps till the required amount or thickness
of soot is depoS1ted on the core rod.
In accordance with one embodiment of this invention, when the rod rotates on its own axis and also traverses along its own axis, and the burner is kept stationary [Figure 1], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:-
a) mounting a core rod over a movable lathe;
b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod;
d) beginning traverse from one end (SL) to another end (SR) of core rod;
wherein said method is characterized by following sequence of steps:-
e) increasing the speed of the rod in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
f) continuing said forward motion of the rod at said first maximum speed S1 of step - e) from end SL till the rod reaches another end portion SR with respect to stationary burner;
g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
h) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
i) continuing said reverse motion of step - h) at said second
maximum speed S2 of said step - h) from end SR till the rod reaches another end portion SL with respect to stationary burner;
19

j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value;
k) repeating each of above said steps e) to j) for a predetermined
number of times K;
1) carrying out each of above said steps e) to g) in forward motion so
that the rod reaches another end SR with respect to stationary burner;
m) beginning a new set of traverses with forward motion commencing from said end SR of the core rod with respect to stationary burner by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod with respect to stationary burner at said first maximum speed S1 so that the rod reaches said end SL with respect to stationary burner;
n) increasing the speed of the rod in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
o) continuing said reverse motion of the rod at said second maximum
speed S2 of said step - n) from end SL till the rod reaches said end
SR with respect to stationary burner;
p) decreasing said second maximum speed S2 of said reverse motion
to 0 using an deceleration d2 measured in mm/sec2 of a fourth
predetermined value;
q) increasing the speed of the rod in the forward motion from 0 to
said first maximum speed S1 using an acceleration al measured in
mm/sec2 of first predetermined value;
r) continuing said forward motion at said first maximum speed S1 of
said step - q) from end SR till the rod reaches said end SL with
respect to stationary burner;
20

s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
t) repeating each of above said steps n) to s) for a predetermined
number of times K; and
u) continuing all the above steps till the required amount or thickness of soot is depoS1ted on the core rod.
In accordance with another embodiment of this invention, when the rod only rotates on its own axis and the burner is moving along the longitudinal axis of the rod [Figure 2], the present method for manufacturing an optical fiber preform having uniform soot depoS1tion comprises steps of:-
a) mounting a core rod over a movable lathe;
b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod;
d) beginning traverse from one end (SL) to another end (SR) of core rod;
wherein said method is characterized by following sequence of steps:
e) increasing the speed of the burner in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
f) continuing said forward motion of the burner at said first maximum speed S1 of step - e) from end SL till the burner reaches another end portion SR of said core rod;
g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
h) increasing the speed of the burner in the reverse motion from 0 to
second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
21

i) continuing said reverse motion of step - h) at said second maximum speed S2 of said step - h) from end SR till the burner reaches another end portion SL of said core rod;
j) decreasing said second maximum speed S2 of said reverse motion
of step - i) to 0 using an deceleration d2 measured in mm/sec2 of
a fourth predetermined value;
k) repeating each of above said steps e) to j) for a predetermined number of times K;
1) carrying out each of above said steps e) to g) in forward motion so that the burner reaches another end SR;
m) beginning a new set of traverses with forward motion commencing
from said end SR of the core rod by carrying out each of above said
steps e) to g) in forward motion from end SR to end SL of the core
rod at said first maximum speed S1 so that the burner reaches said
end SL of core rod;
n) increasing the speed of the burner in the reverse motion from 0 to
second maximum speed S2 using an acceleration a2 measured in
mm/sec2 of third predetermined value;
o) continuing said reverse motion of the burner at said second
maximum speed S2 of said step - n) from end SL till the burner
reaches said end SR of said core rod;
p) decreasing said second maximum speed S2 of said reverse motion
to 0 using an deceleration d2 measured in mm/sec2 of a fourth
predetermined value;
q) increasing the speed of the burner in the forward motion from 0 to
said first maximum speed S1 using an acceleration al measured in
mm/sec2 of first predetermined value;
r) continuing said forward motion at said first maximum speed S1 of
said step - q) from end SR till the burner reaches said end SL of
said core rod;
22

s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
t) repeating each of above said steps n) to s) for a predetermined
number of times K; and
u) continuing all the above steps till the required amount or thickness
of soot is depoS1ted on the core rod.
As stated hereinabove, the first maximum speed S1 is different from the second maximum speed S2.
The first maximum speed S1 is less than the second maximum speed S2.
The first maximum speed S1 varies from about 1500 to about 12000 mm/min, preferably from about 1500 to about 2000 mm/min, more preferably from about 1500 to about 1600 mm/min.
The second maximum speed S2 varies from about 20000 to about 30000 mm/min, preferably it is kept around the value of about 20000 mm/min.
The accelerations al and a2, and decelerations dl and d2 are selected so as to further reduce the tapering effect. It has been surpriS1ngly observed that when the accelerations al and a2, and decelerations dl and d2 are varied depending upon the motion speeds S1 and S2, the tapering effect is further reduced. The reduction is tapering effect has been also been observed when the distance of the traverse motion from which the speed increases from 0 to S1 mm/min is same when the speed decreases from S2 to 0 mm/min.
In accordance with the present invention, the acceleration al and deceleration dl during the forward motion speed S1 are kept in the range of about 270 to 290 mm/sec2. The acceleration a2 and deceleration d2 during the reverse motion S1 are kept in the range of about 500 to about 600 mm/sec2.
23

The first maximum speed S1 and the second maximum speed S2 can be optionally changed in every set of passes.
The ends SL and SR are the left and right ends of the target rod where in the conventional methods tapering effects have been observed. However, when following the method of the present invention the tapering effect has been greatly reduced.
In one embodiment of this invention, when the target rod is moving, in-addition to rotation on its own axis, with respect to the soot forming burner, that is the burner is kept stationary [Figure 1], then at the end of the motion, the ends SL and SR of the moving rod reach the stationary burner.
In another embodiment of this invention, when the rod only rotates on its own axis and the burner is moving in forward and reverse directions [Figure 2], then at the end of the motion, the moving burner reaches the ends SL and SR of the rod.
In still another embodiment of this invention, when the rod and burner both are moving in the oppoS1te directions [Figure 3], then at the end of the motion, the ends SL and SR of the moving rod reach the moving burner.
The value of K is 16 or less than 16, preferably 12 or less than 12. The value of K may be same or different in set traverses starting from end SL and end SR. Accordingly, for example, in accordance with the present invention, the K in one set of traverses may be 12 to 16 and in following set of traverse it may be 12 or less than 12.
The reactant gases are for example SiCl4, GeCl4, BCl3, and other dopant gases. Oxygen and other fuel gas are used along with reactant gases to form the soot. The contents and the flow rate of the reactant gases and the fuel gases can be maintained as known in the art. The flow rate of the reactant gases can be maintained in the range of about 180 to about 220 Gram/min. The flow rate of oxygen and hydrogen as fuel
24

gases can be maintained in the range of about 120 to about 180 slpm and 50 to 85 slpm respectively.
In one embodiment of the present invention, the soot forming burners 105 are preferably two. The target rod 101 rotates about its own axis with a rotational speed preferably of about 60 rpm or more.
The above description elaborates a case when the traverse in
forward and/or reverse directions continue towards the oppoS1te ends of the rod at respective maximum speed of S1 and/or S2 respectively. However, the speeds S1 and S2 need not necessarily be constant during the respective traverse. These may be constantly increasing or decreasing.
In one embodiment, the present invention provides an optical fiber preform produced by the present method. The optical fiber preform produced according to the method of the present invention has effective length improved approximately by 20 to 25% as compared to the conventional methods known in the art, and the diameter variation within the preform is well controlled which is approximately 3mm and other characteristics of the optical fiber preform produced by the present method are also well within the limits to obtain the deS1red parameters, of the optical fiber to be drawn from the optical fiber preform produced by this method.
An exemplary optical fiber drawn from the optical fiber preform produced by the present method has been observed to have following characteristics:
Attention at 1310 Attention at 1550 Cutoff wavelength 1160 to 1300 nm
Mode Field Diameter 8.9 to 9.5 μm
Clad Diameter 125+/- 1 μm
25

We Claim:
1. A method for producing an optical fiber preform having uniformly
deposited soot particles depoS1ted by changing the speed of the
target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction.
2. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner is changed in the forward direction with reference to the speed of said target rod and/or said soot forming burner in traverse motion in the reverse direction.
3. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner is changed in the reverse direction with reference to the speed of said target rod and/or said soot forming burner in traverse motion in the forward direction.
4. A method as claimed in claim 1, wherein the speed of said target rod and/or said soot forming burner in traverse motion in forward direction is selected to be less than the speed of said target rod and/or said soot forming burner in the traverse motion in reverse direction.
5. A method as claimed in any one of preceding claims, wherein the speed of said target rod and/or said soot forming burner in traverse motion in forward direction is selected to be varying from about 1500 to about 12000 mm/min,
6. A method as claimed in any one of preceding claims, wherein the speed of said target rod and/or said soot forming burner in traverse motion in reverse direction is selected to be varying from about 20000 to about 30000 mm/min.
7. A method for producing an optical fiber preform having uniform soot deposition comprises steps of:-
a) mounting a core rod over a movable lathe;
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b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow
rate of first predetermined level for depoS1ting soot over said
core rod;
d) beginning traverse from one end (SL) to another end (SR) of
core rod;
wherein said method is characterized by following sequence of steps :-
e) increasing the speed of the moving part in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
f) continuing said forward motion of said moving part at said first maximum speed S1 of step - e) from end SL till said moving part reaches another end portion SR with respect to said stationary part;
g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
h) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
i) continuing said reverse motion of step - h) at said second
maximum speed S2 of said step - h) from end SR till said moving part reaches another end portion SL with respect to said stationary part;
j) decreasing said second maximum speed S2 of said reverse
motion of step - i) to 0 using an deceleration d2 measured in
mm/sec2 of a fourth predetermined value;
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k) repeating each of above said steps e) to j) for a predetermined
number of times K;
1) carrying out each of above said steps e) to g) in forward motion so that said moving part reaches another end SR with respect to said stationary part;
m) beginning a new set of traverses with forward motion commencing from said end SR of said moving part with respect to said stationary part by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of said moving part with respect to said stationary part at said first maximum speed S1 so that said moving part reaches said end SL with respect to said stationary part;
n) increasing the speed of said moving part in the reverse motion from 0 to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
o) continuing said reverse motion of said moving part at said second maximum speed S2 of said step - n) from end SL till said moving part reaches said end SR with respect to said stationary part;
p) decreasing said second maximum speed S2 of said reverse motion to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value;
q) increasing the speed of said moving part in the forward motion from 0 to said first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
r) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till said moving part reaches said end SL with respect to said stationary part;
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s) decreasing said first maximum speed S1 of said reverse motion to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
t) repeating each of above said steps n) to s) for a
predetermined number of times K; and
u) continuing all the above steps till the required amount or thickness of soot is deposited on the core rod.
8. A method for producing an optical fiber preform having uniform soot depoS1tion comprises steps of:-
a) mounting a core rod over a movable lathe;
b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod;
d) beginning traverse from one end (SL) to another end (SR) of core rod;
wherein said method is characterized by following sequence of steps:-
e) increasing the speed of the rod in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
f) continuing said forward motion of the rod at said first maximum speed S1 of step - e) from end SL till the rod reaches another end portion SR with respect to stationary burner;
g) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
h) increasing the speed of the rod in the reverse motion from 0
to second maximum speed S2 using an acceleration a2 measured in mm/sec2 of third predetermined value;
29

i) continuing said reverse motion of step - h) at said second
maximum speed S2 of said step - h) from end SR till the rod reaches another end portion SL with respect to stationary burner;
j) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in
mm/sec2 of a fourth predetermined value;
k) repeating each of above said steps e) to j) for a predetermined number of times K;
1) carrying out each of above said steps e) to g) in forward
motion so that the rod reaches another end SR with respect to stationary burner;
m) beginning a new set of traverses with forward motion commencing from said end SR of the core rod with respect to stationary burner by carrying out each of above said steps e) to g) in forward motion from end SR to end SL of the core rod with respect to stationary burner at said first maximum
speed S1 so that the rod reaches said end SL with respect to
stationary burner;
n) increasing the speed of the rod in the reverse motion from 0
to second maximum speed S2 using an acceleration a2
measured in mm/sec2 of third predetermined value;
o) continuing said reverse motion of the rod at said second
maximum speed S2 of said step - n) from end SL till the rod
reaches said end SR with respect to stationary burner;
p) decreasing said second maximum speed S2 of said reverse
motion to 0 using an deceleration d2 measured in mm/sec2
of a fourth predetermined value;
q) increasing the speed of the rod in the forward motion from 0
to said first maximum speed S1 using an acceleration al
measured in mm/sec2 of first predetermined value;
30

r) continuing said forward motion at said first maximum speed
S1 of said step - q) from end SR till the rod reaches said end SL with respect to stationary burner;
s) decreasing said first maximum speed S1 of said reverse
motion to 0 using an deceleration dl measured in mm/sec2
of a second predetermined value;
t) repeating each of above said steps n) to s) for a
predetermined number of times K; and
u) continuing all the above steps till the required amount or
thickness of soot is deposited on the core rod.
A method for producing an optical fiber preform having uniform soot depoS1tion comprises steps of:-
a) mounting a core rod over a movable lathe;
b) selecting a method of depoS1ting soot over said core rod;
c) introducing reactant gases and oxyhydrogen gases at flow rate of first predetermined level for depoS1ting soot over said core rod;
d) beginning traverse from one end (SL) to another end (SR) of core rod;
e) wherein said method is characterized by following sequence of steps:
f) increasing the speed of the burner in the forward motion from 0 to first maximum speed S1 using an acceleration al measured in mm/sec2 of first predetermined value;
g) continuing said forward motion of the burner at said first maximum speed S1 of step - e) from end SL till the burner reaches another end portion SR of said core rod;
h) decreasing said first maximum speed S1 of said forward motion of said step - f) to 0 using an deceleration dl measured in mm/sec2 of a second predetermined value;
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i) increasing the speed of the burner in the reverse motion from 0 to second maximum speed S2 using an acceleration
a2 measured in mm/sec2 of third predetermined value;
j) continuing said reverse motion of step - h) at said second
maximum speed S2 of said step - h) from end SR till the burner reaches another end portion SL of said core rod;
k) decreasing said second maximum speed S2 of said reverse motion of step - i) to 0 using an deceleration d2 measured in mm/sec2 of a fourth predetermined value;
1) repeating each of above said steps e) to j) for a predetermined
number of times K;
m) carrying out each of above said steps e) to g) in forward motion so that the burner reaches another end SR;
n) beginning a new set of traverses with forward motion commencing from said end SR of the core rod by carrying out each of above said steps e) to g) in forward motion from end
SR to end SL of the core rod at said first maximum speed S1
so that the burner reaches said end SL of core rod;
o) increasing the speed of the burner in the reverse motion
from 0 to second maximum speed S2 using an acceleration
a2 measured in mm/sec2 of third predetermined value;
p) continuing said reverse motion of the burner at said second
maximum speed S2 of said step - n) from end SL till the
burner reaches said end SR of said core rod;
q) decreasing said second maximum speed S2 of said reverse
motion to 0 using an deceleration d2 measured in mm/sec2
of a fourth predetermined value;
r) increasing the speed of the burner in the forward motion
from 0 to said first maximum speed S1 using an acceleration
al measured in mm/sec2 of first predetermined value;
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s) continuing said forward motion at said first maximum speed S1 of said step - q) from end SR till the burner reaches said end SL of said core rod;
t) decreasing said first maximum speed S1 of said reverse
motion to 0 using an deceleration dl measured in mm/sec2
of a second predetermined value;
u) repeating each of above said steps n) to s) for a
predetermined number of times K; and
v) continuing all the above steps till the required amount or
thickness of soot is depoS1ted on the core rod.
10. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said first maximum speed S1 varies from about 1500 to about 12000 mm/min, preferably from about 1500 to about 2000 mm/min, more preferably from about 1500 to about 1600 mm/min.
11. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said second maximum speed S2 varies from about 20000 to about 30000 mm/min, preferably it is kept around the value of about 20000 mm/min.
12. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said acceleration al and deceleration dl during the forward motion speed S1 are kept in the range of about 270 to 290 mm/sec2 and said acceleration a2 and deceleration d2 during the reverse motion S1 are kept in the range of about 500 to about 600 mm/sec2.
13. A method as claimed in any one of the preceding claims 7, 8 or 9, wherein said K is 16 or less than 16, preferably 12 or less than 12.
14. A method for producing an optical fiber preform having uniformly depoS1ted soot particles substantially as herein before described
and illustrated with the help of the accompanying drawings.
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15. An optical fiber preform as and when manufactured by the process as claimed in any one or more of the preceding claims.
16. An optical fiber as and when manufactured from the optical fiber
preform as claimed in claim 15.
17. An optical fiber as claimed in claim 16, wherein said fiber has
following characteristics:
Attention at 1310 Attention at 1550 Cutoff wavelength 1160 to 1300 nm
Mode Field Diameter 8.9 to 9.5 μm
Clad Diameter 125+/- 1 μm
18. An optical fiber produced from the optical fiber preform
substantially as herein before described and illustrated with the
help of the accompanying drawings.
Dated this 12th day of August, 2005.

[Dr. Ramesh Kr. MEHTA]
Patent Attorney for the Applicants
Mehta & Mehta Associates
34

ABSTRACT
A method for producing an optical fiber preform having uniformly depoS1ted soot particles comprising depositing soot particles by changing the speed of the target rod and/or the soot forming burner moving along the axis of the target rod in traverse motion in one direction with reference to the speed of the target rod and/or the soot forming burner in traverse motion in another direction is provided.
Figures 1 & 7
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METHOD OF MANUFACTURING OPTICAL FIBER PREFORM HAVING UNIFORMLY DEPOSITED SHOOT PARTICLES

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