Title of Invention	METHOD FOR PREPARATION OF CORE ROD ASSEMBLY FOR OVERCLADDING AND PREFORM AND FIBER PRODUCED FROM SUCH CORE ROD ASSEMBLY
Abstract	The present invention relates to method for preparation of core rod assembly for overcladding. Particularly,the present invention relates to a method for preparation of core rod assembly which is suitable for overcladding so as to form daughter preform which is suitable for fiber draw process.The present invention also relates to a core rod assembly for overcladding,and to the daughter preform and optical fiber produced from such core rod assembly.

Title of Invention

METHOD FOR PREPARATION OF CORE ROD ASSEMBLY FOR OVERCLADDING AND PREFORM AND FIBER PRODUCED FROM SUCH CORE ROD ASSEMBLY

Abstract

The present invention relates to method for preparation of core rod assembly for overcladding. Particularly,the present invention relates to a method for preparation of core rod assembly which is suitable for overcladding so as to form daughter preform which is suitable for fiber draw process.The present invention also relates to a core rod assembly for overcladding,and to the daughter preform and optical fiber produced from such core rod assembly.

Full Text	FORM 2 THE PATENTS ACT, 1970 (39 of 1970) 8r, THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention:- Method for preparation of core rod assembly for overcladding, and preform and fiber produced from such core rod assembly. 2. Applicant(s):- (a) Name: STERLITE OPTICAL TECHNOLOGIES LTD. (b) Nationality: An Indian Company (c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA 3. Preamble to the Description:- Complete Specification: The following specification particularly describes the invention and the manner in which it is to be performed. STER/P A/040 Field of the Invention: The present invention relates to method for preparation of core rod 5 assembly for overcladding. Particularly, the present invention relates to a method for preparation of core rod assembly which is suitable for overcladding so as to form daughter preform which is suitable for fiber draw process. The present invention also relates to a core rod assembly for overcladding, and to the daughter preform and optical fiber produced from such core rod assembly. 10 Background of the Invention: Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. The optical fibers are drawn 15 from an optical fiber preform. The optical fiber of predetermined dimension is drawn either from the solid glass preform [mother preform] or from sintered preform [daughter preform] by subjecting one end of the preform [mother preform or daughter preform] to a high temperature, for example above 2000°C. The sintered preform [daughter preform] is prepared from a core rod having soot 20 porous body [also referred as soot preform]. The daughter preform manufacturing process primarily involves a step of preparing the core rod comprising core of the fiber and part of clad which is followed by over-cladding. The core rod can be prepared by methods known in 25 the art, such as modified chemical vapour deposition (MCVD), plasma chemical vapour deposition (PCVD), Atmospheric chemical vapour deposition (ACVD), vapour axial deposition (VAD) etc. The over-cladding of the core rod can be carried out by various methods, such as glass tube jacketing, ACVD soot overcladding, VAD soot over-cladding, plasma over-cladding etc. Therefore, the 30 daughter preform can be manufactured by any combination of the core rod manufacturing methods and the over-cladding preparation methods. Typically, the daughter preform can be manufactured by any of the above-said conventional methods, for example by ACVD method, which is 35 described herein for reference. 2 STER/P A/040 In accordance with a typical ACVD process to manufacture a soot porous body, as illustrated in accompanying Figure 1, the preparation of soot porous 5 body 1 comprises the following steps. The glass-forming precursor compounds are oxidized and hydrolyzed to form porous silica based materials 2. The porous silica based materials 2 are deposited on a tapered cylindrical member referred as mandrel 3, which can be any commercially available mandrel with or without any specific preparation, preferably with specific preparation to remove the 10 contaminants therefrom which is provided with a handle rod 4 and fitted on a lathe 5 to form soot porous body 1. During the step of deposition, the mandrel 3 is rotated in a direction as illustrated by an arrow 6 and also moved along its length with reference to 15 burner 7 to deposit the soot particles 2 on the mandrel 3 for producing soot porous body 1. During the deposition process, the dopant chemicals for example GeCl4 may also be deposited to form the core of the preform and later the dopant chemicals may be terminated to form clad of the preform. The amount of deposition of the clad region 11 and core region 10 is achieved to 20 have any desired ratio diameter of clad region 11 to the diameter of core region 10. After completion of deposition, the soot porous body 1 is removed from lathe 5 along with mandrel 3 and handle rod 4, and the mandrel 3 is removed/ 25 detached, during the mandrel removal step, from the soot porous body 1 thereby resulting in formation of a hollow cylindrical soot porous body 8 (herein after referred to as hollow soot porous body) having a centerline 9 therethrough [Figure 2]. 30 The hollow soot porous body 8 thus formed comprises a core region 10 having a centerline hole 9 and a clad region 11 of the optical fiber preform [Figure 3], and said core region 10 has refractive index greater than that of the clad region 11. 3 STER/PA/040 After removal/detachment of mandrel 3 a centerline 9 is created inside the soot porous body 1. 5 Now referring to accompanying Figure 4, the prepared hollow soot porous body 101 is transferred to the sintering furnace 100 in order to achieve dehydration, and sintering of the hollow soot porous body 101 to form dehydrated and sintered hollow glass body. 10 The dehydrated and sintered hollow glass body is subjected to step of collapsing of the centerline 102 to form a solid glass preform 103 [Figure 5] with or without requiring any step of drilling or grinding or etching of the centerline 9/102 before steps of consolidation and collapsing. 15 Thus, the prepared hollow soot porous body 101 is dehydrated, sintered and collapsed to convert it into solid glass preform 103. In a typical embodiment of ACVD method, the hollow soot porous body 101, one end of which is provided with a plug 116 is inserted inside the furnace 20 100 with the help of the handle rod 106. The driving mechanism (not shown) facilitates lowering of the hollow soot porous body 101 into the furnace 100. The furnace 100 comprises a glass muffle tube 110 having a diameter sufficient to accommodate the hollow soot porous body 101 and to adequately provide the environment necessary for dehydration, sintering and collapsing. The muffle 25 tube 110 is suitable for heating to temperatures necessary for dehydration and simultaneous sintering and collapsing process steps with the heating means (not shown) which are suitably fitted to the sintering furnace 100. The heating means selected may be suitable to create three heat zones 30 inside the muffle tube 110 over a length. A thermocouple (not shown) provided in the furnace 100 measures the temperature of the hot zones inside the furnace created by the heating means, and the data measurement is fed to the temperature controller (not shown) that controls the temperature inside the muffle tube 110. 35 4 STER/P A/040 The furnace 100 is provided with an inlet port 115 located suitably on the furnace, preferably near the bottom of the muffle tube 110 for supplying desired gases in the furnace. The top end of the muffle tube 110 is suitably 5 closed with the lid 113 to achieve the preferred temperature profile inside the muffle tube 110 and to maintain the same during the dehydration, and simultaneous sintering and collapsing process steps, and to avoid leakage of gases from the muffle tube 110 to the outside environment. A suction port 114 is suitably provided near the top of muffle tube 110 to facilitate evacuation of 10 the gases from the muffle tube 110 as and when required or on completion of the process. In accordance with the known art methods, the mother preform produced is subjected, in a conventional manner, to a step of reducing the 15 diameter to form a core rod having reduced diameter, which is then subjected, in a conventional manner, to a step of overcladding to form a soot preform comprising soot porous body having core rod, also referred as soot preform, which is then subjected, in a conventional manner, to a sintering step to form a daughter preform, which is then subjected to a step of fiber draw to draw the 20 fiber. In accordance with methods known in the art, the opposite ends of the core rod having reduced diameter are first attached to glass rods by heating means before performing overcladding step thereon to form soot preform. This 25 process step is typically known as core rod assembly preparation step. Accordingly, a typical process for preparation of optical fiber from daughter preform comprises the steps of:- 30 a) preparing the mandrel; b) placing the mandrel over a lathe; c) depositing soot particles on the mandrel to prepare a soot porous body; d) removing the mandrel to form hollow soot porous body having capillary therethrough; 5 STER/P A/040 e) dehydrating the hollow soot porous body to form dehydrated soot porous body; f) performing sintering step on dehydrated soot porous body to form 5 sintered glass body; g) performing collapsing step to collapse the capillary of the sintered glass body to form solid glass preform; h) performing the step of reducing the diameter of the solid glass preform to form a core rod having reduced diameter; 10 i) performing core rod assembly preparation step to form a core rod assembly for the overcladding step; j) performing overcladding step on the core rod assembly to form soot preform comprising soot porous body having core rod; k) performing sintering step on the soot preform to form a daughter 15 preform; and 1) performing fiber draw step on the daughter preform having peform cone of desired shape and dimensions including diameter to draw the fiber therefrom. 20 It may be noted that for the ease of understanding the soot preform comprising soot porous body having core rod is referred as "soot preform" which may not be confused with "mother preform" or "daughter preform". As described herein, in accordance with the known art, the process of 25 core rod assembly preparation suitable for overcladding [above process step i) involves attaching or fixing or welding opposite ends of the core rod to glass rods by heating to a temperature for attaching or fixing or welding the gloss rods to the opposite ends of the core rod. 30 It has been observed that the known methods of core rod assembly preparation involve process step of heat treatment of opposite ends of the core rod to attach or fix or weld glass rods thereto. The known heat treatment process steps suffer from following drawbacks, disadvantages and limitations. 6 STER/PA/040 The main drawback of the known methods of core rod assembly preparation involving conventional process step of heat treatment of opposite ends of the core rod to attach or fix or weld [herein after for convenience 5 referred as to fix or fixing] glass rods thereto is that it causes stress formation at and around the joints formed between the glass rod and core rod. The stress formation in the core rod assembly has been observed to cause breakage of the core rod assembly during its handling and deposition of soot thereon to form the soot preform. 10 The another main drawback of the known methods of core rod assembly preparation involving conventional process step of heat treatment of opposite ends of the core rod to fix glass rods thereto is that it causes physical defects, for example cracks and breaks at and around the joints formed between the 15 glass rod and core rod, and such physical defects in the core rod assembly have been observed to cause bubbles and voids in the soot preform produced from such core rod assembly which in-turn results in breakage of the optical fiber during fiber drawing step, and hence, a loss of the productivity of the process. 20 The another problem of stress formation and formation of physical defects in the core rod assembly during conventional heat treatment process is that the breakage of the core rod assembly may take place at any time during the soot deposition which not only results in total loss of soot deposition, but also causes damage to the burner, because there is every possibility that the 25 core rod assembly will fall on the burner meaning thereby the conventional heat treatment process suffer from time loss, production loss and financial loss. It has been further observed that the stress formation and formation of physical defects in the core rod assembly also lead to transmission loss in the 30 resulting optical fiber or distortion of other optical parameters, for example, polarization mode dispersion, cutoff wavelength etc. If conventional heat treatment is performed by graphite resistance method it has been observed that due to relatively higher process time it causes 35 graphite oxidation which in-turn results in formation of oxidation products, for 7 STER/PA/040 example ash, graphite particles etc. which adheres to the core rod surface, and such contamination of the core rod with unwanted particles results in production of a daughter preform which will produce a fiber having increased 5 transmission loss and poor strength. Similarly, if conventional heat treatment is performed by lasers, such as carbon dioxide lasers, which are clean heat source to use, power consumption has been observed to be very high rendering the overall process very expensive. 10 Further, it has also been observed that large amount of thermal induced stress in the core rod at and around the joint of the core rod and glass rod area adversely effects joint strength which upon further processing may shatter the core rod. 15 Accordingly, the known methods of core rod assembly preparation are observed to be uneconomical for commercial applications. Need of the Invention: 20 Therefore, there is a need to have a method for preparation of core rod assembly for overcladding, particularly a method for preparation of core rod assembly which is suitable for overcladding so as to form daughter preform which is suitable for fiber draw process. 25 Objects of the Invention: Accordingly, the main object of the present invention is to provide a method for preparation of core rod assembly which is suitable for overcladding 30 so as to form daughter preform which in-turn is suitable for fiber draw process. The another main object of the present invention is to provide a method for preparation of core rod assembly wherein no stress is formed at and around the joints formed between the glass rod and core rod meaning thereby the core 8 STER/PA/040 rod assembly produced will not break, due to stress, during its handling and deposition of soot thereon to form the soot preform. 5 Still another main object of the present invention is to provide a method for preparation of core rod assembly wherein formation of physical defects, for example cracks and breaks at and around the joints formed between the glass rod and core rod is reduced, and hence, possibility of formation of bubbles and voids in the soot preform produced from such core rod assembly is reduced 10 meaning thereby possibility of breakage of the optical fiber, due to physical defects during fiber drawing step is reduced, and accordingly, possibility of loss of productivity of the process is reduced. Yet another main object of the present invention is to provide a method 15 for preparation of core rod assembly wherein possibility of stress formation and formation of physical defects in the core rod assembly is eliminated or greatly reduced, and hence, the possibility of breakage of the core rod assembly is eliminated or greatly reduced meaning thereby possibility of loss of soot deposition and damage of the burners is eliminated or greatly reduced. 20 Further object of the present invention is to provide a method for preparation of core rod assembly for overcladding wherein the core rod assembly is suitable for overcladding and to form suitable daughter preform. 25 One particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding which can overcome drawbacks, disadvantages and limitations of the prior art. Another particular object of the present invention is to provide a method 30 for preparation of core rod assembly for overcladding wherein no or reduced physical defects and stress is caused in the core rod, and hence the possibility of transmission loss in the resulting optical fiber or distortion of other optical parameters, for example, polarization mode dispersion, cutoff wavelength etc. is greatly reduced. 35 9 STER/P A/040 Still another particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding which even by performing heating by graphite resistance method does not cause graphite 5 oxidation, and hence, overcomes associated problems of formation of oxidation products, for example ash, graphite particles etc. which are known to adhere to the core rod surface. Accordingly, the present invention aims to provide a method for core rod assembly preparation wherein possibility of contamination of core rod with unwanted particles is greatly reduced, and hence, the daughter 10 preform produced from such core rod assembly will produce a fiber having decreased transmission loss and increased strength. Still further particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding which even if 15 heating is performed by lasers, such as carbon dioxide lasers, does not require higher power consumption meaning thereby which is relatively less expensive. Yet further particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding wherein 20 possibility of thermal induced stress in the core rod at and around the joint of the core rod and glass rod area is greatly reduced meaning thereby the joint strength is not adversely effected, and hence, the further processing of the produced core rod assembly will be safer and convenient. 25 It is also an object of the present invention to provide a core rod assembly which is suitable for overcladding to form a suitable daughter preform which will be capable of fiber draw process to produce suitable optical fiber. The other objects and advantages of the present invention will be 30 apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for the purpose of illustration of present invention and not to limit scope thereof. Brief Description of the Invention: 35 10 STER/P A/040 The inventors have surprisingly observed that if core rod is heated in a controlled manner to have its controlled heating after fixing the glass rods at its opposite ends and before performing step of overcladding, the problems 5 associated with known methods of core rod assembly preparation as described herein above can be overcome and a core rod assembly suitable for overcladding which in-turn is suitable for producing desired daughter preform. Accordingly, the present invention provides a method for preparation of 10 core rod assembly suitable for overcladding comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; 15 characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish. 20 The other embodiments and advantages of the present will be apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for illustration of preferred embodiments of the present invention and are not intended to limit scope thereof. 25 Brief Description of the Accompanying Drawings: Figure 1 illustrates a schematic representation of deposition process over a mandrel to produce a soot porous body. 30 Figure 2 illustrates a schematic representation of hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body. 11 STER/PA/040 Figure 3 illustrates a schematic cross-sectional view of hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body. 5 Figure 4 illustrates a schematic representation of hollow soot porous body inside the sintering furnace after removal of mandrel from the soot porous body. 10 Figure 5 illustrates a hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body which is subjected to steps of dehydration, sintering and collapsing to produce a solid glass preform. 15 Figure 6 illustrates a schematic representation of the mother preform comprising a core and a clad provided with handle rod on each of its opposite ends. Figure 6a illustrates a schematic representation of the daughter preform 20 comprising a core rod and overclad. Figure 7 illustrates a schematic representation of process step of fixing a glass rod at one of the two opposite ends of the core rod in accordance with method for preparing a core rod assembly according to one embodiment of the 25 present invention. Figure 8 illustrates a schematic representation of process step of fixing a glass rod at another opposite end of the core rod in accordance with method for preparing a core rod assembly according to one embodiment of the present 30 invention. Figure 9 illustrates a schematic representation of core rod assembly ready for controlled heat treatment in accordance with method of the present invention. 35 12 STER/P A/040 Figure 10 illustrates a schematic representation of process step of heat treatment of the core rod assembly in accordance with method for preparing a core rod assembly according to one embodiment of the present invention. 5 Figure 11 illustrates a schematic representation of the temperature profile of the hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention. 10 Figure 12 illustrates a schematic representation of the burner speed profile of the hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention. Figure 13 illustrates a schematic representation of temperature profile of 15 the soft fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention. Figure 14 illustrates a schematic representation of burner speed profile of the soft fire polish heat treatment process step of the core rod assembly in 20 accordance with one embodiment of the present invention. Detailed Description and preferred embodiments of the Invention: Accordingly, the present invention relates to a method for preparation of 25 core rod assembly suitable for overcladding comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; 30 characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish. 13 STER/P A/040 In accordance with one of the preferred embodiments of the present invention, the overcladding step is performed on the core rod assembly produced in above process step-c) to produce a core rod having overclad 5 referred as soot preform comprising soot porous body having core rod. In accordance with another preferred embodiment of the present invention, the soot preform comprising soot porous body having core rod produced from core rod having overclad which in-turn is produced produced 10 from core rod assembly produced in above process step-c) is subjected to sintering step to form a daughter preform, which is suitable for storage or immediate processing for fiber draw step. Accordingly, in one embodiment, the present invention relates to a 15 method for preparation of core rod having overcladding, that is, soot preform comprising soot porous body having core rod comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having 20 reduced diameter; characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein 25 first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod. 30 Accordingly, in another embodiment, the present invention relates to a method for preparation of daughter preform comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having 35 reduced diameter; 14 STER/P A/040 characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step 5 process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod; and 10 e) performing the sintering step on the soot preform produced in above process step-d) to form a daughter preform. Accordingly, in still another embodiment, the present invention relates to a method for preparation of optical fiber comprising the steps of:- 15 a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; characterized in that 20 c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in 25 above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod; e) performing the sintering step on the soot preform produced in above process step-d) to form a daughter preform; and f) performing fiber draw step on the daughter preform obtained in above 30 process step-e) to draw the fiber therefrom. Now referring to accompanying Figure 6, the mother preform 120 comprises a core 121 and a clad 122 and is provided with dummy glass handle 123 and 124 on each opposite ends 125 and 126 thereof for the purpose of 15 STER/P A/040 handling and holding the preform. The opposite end 125 [or 126] comprises a cone shape structure Cb [or Ct]. 5 The accompanying Figure 6a illustrates the daughter preform 130 comprising a core rod 131 and a overclad 132 wherein the extended parts 133 and 134 on the opposite ends 135 and 136 of the core rod 131 serve the purpose of handle rods on each of the opposite ends of the core rod 131 for handling and holding the preform. The opposite ends 135 and 136 comprise a 10 cone shape structure Cb [or Ct]. Now referring to accompanying Figure 7 illustrating a schematic representation of process step of fixing a glass rod at one of the two opposite ends of the core rod in accordance with method of the present invention, a glass 15 rod 201 is fixed onto one end of the core rod 202 after attaching the core rod over a lathe 203 having rotation means [not shown] to rotate the core rod and glass rod, and heating means 204 for heating the glass rod 201 and core rod 202 to form a joint 205 between the two [Figure 9]. 20 Now referring to accompanying Figure 8 illustrating a schematic representation of process step of fixing a second glass rod at another opposite end of the core rod in accordance with method of the present invention a second piece of glass rod 301 is fixed onto another end of the core rod 202 after attaching the core rod over a lathe 203 having rotation means [not shown] to 25 rotate the core rod and second glass rod, and heating means 204 for heating the glass rod 301 and core rod 202 to form a joint 305 between the two [Figure 9]. The core rod assembly prepared in accordance with present invention is 30 illustrated in accompanying Figure 9 which comprises one glass rod 201 fixed onto one end of the core rod 202 forming a joint 205 and second glass rod 301 fixed onto another end of the core rod 202 forming a joint 305. The points A, B, C and D are marked on the core rod before start of controlled heat treatment in accordance with present invention, wherein point A is marked at suitable end of 35 the one glass rod 201, and point D is marked at a suitable end of another glass 16 STER/P A/040 rod 301 in a manner that the glass rods are suitably held in the lathe for controlled heat treatment in accordance with present invention. The points B and C are the points of joint 205 formed between one glass rod 201 and core 5 rod 202 and joint 305 formed between second glass rod 301 and core rod 202 respectively. In accordance with present invention, the said first step of hard fire polish is performed by increasing the temperature from room temperature to a 10 temperature varying from about 1400°C to about 1800°C from point A to point B on the core rod, maintaining this temperature within said range varying from about 1400°C to about 1800°C from point B to point C on the core rod followed by reducing said temperature varying within said range varying from about 1400QC to about 1800°C to a temperature of about 3000C from point C to point 15 D on the core rod. The above temperature profile of the first hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 11. 20 In accordance with present invention, the said second step of soft fire polish is performed by increasing the temperature from a temperature of about 300°C to a temperature varying from about 1100°C to about 12500C from point D to point C on the core rod, maintaining this temperature within said range 25 varying from about 1100°C to about 1250°C from point C to point B on the core rod followed by reducing said temperature varying within said range varying from about 1100°C to about 1250°C to a temperature of about room temperature from point B to point A on the core rod. 30 The above temperature profile of the second soft fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 13. In accordance with present invention, the said first step of hard fire 35 polish is performed by changing said temperature from point A to D while 17 STER/PA/040 increasing the burner speed from a speed of zero to a speed varying from about 150 to about 250 mm/min from point A to point B on the core rod, maintaining said burner speed varying within said range varying from about 150 to about 5 250 mm/min from point B to point C on the core rod followed by reducing said burner speed varying within said range varying from about 150 to about 250 mm/min to a speed of about 50 mm/min from point C to point D on the core rod. 10 The above burner speed profile of the first hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 12. In accordance with present invention, the said second step of soft fire 15 polish is performed by changing said temperature from point D to A while increasing the burner speed from a speed of about 50 mm/min to a speed varying within a range varying from about 300 to about 350 mm/min from point D to point C on the core rod, maintaining said burner speed varying within said range varying from about 300 to about 350 mm/min from point C 20 to point B on the core rod followed by reducing said burner speed from said speed varying within said range varying from about 300 to about 350 mm/min to a zero speed from point C to point D on the core rod. The above burner speed profile of the first hard fire polish heat treatment 25 process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 14. It may be noted that the present invention is not restricted by performing the first hard fire polish step from point A to D and second soft fire polish step 30 from point D to A. One may also perform these steps in reverse direction, that is first hard fire polish step from point D to A and second soft fire polish step from point A to D. Accordingly, the present invention is restricted by performing second soft fire polish step in direction opposite to the direction of first hard fire polish step. 35 18 STER/P A/040 In accordance with one of the preferred embodiments of the present invention, the fire polish, that is, the controlled heat treatment of the core rod assembly is carried out while rotating the core rod assembly to achieve the 5 uniform heat treatment. Preferably, the rotation speed varies from about 50 to about 70 rpm. It has been found that on following above described controlled heat treatment method of the present invention no stress formation takes place at 10 and around the joints formed between the glass rod and core rod. Accordingly, no breaks have been observed in the core rod assembly produced by present method during its handling and deposition of soot thereon to form the soot preform. 15 It has also been found that on following above described controlled heat treatment method of the present invention, the formation of physical defects, for example cracks and breaks at and around the joints formed between the glass rod and core rod is greatly reduced, and hence, the possibility of formation of bubbles and voids in the soot preform produced from such core rod assembly is 20 greatly reduced meaning thereby the possibility of breakage of the optical fiber, due to physical defects during fiber drawing step is greatly reduced, and accordingly, the possibility of loss of productivity of the process is greatly reduced, which make the present method economically viable. 25 As the present method has been found to be free from stress formation and formation of physical defects in the core rod assembly, the possibility of breakage of the core rod assembly during soot deposition is eliminated or greatly reduced meaning thereby the possibility of loss of soot deposition and damage of the burners is eliminated or greatly reduced. 30 In accordance with present invention, the core rod with reduced diameter can be prepared by any method. As exemplary embodiment, the core rod having reduced diameter may be prepared in following manner:- 35 i) preparing the mandrel; 19 STER/P A/040 ii) placing the mandrel over a lathe; iii) depositing soot particles on the mandrel to prepare a soot porous body; iv) removing the mandrel to form hollow soot porous body having capillary 5 therethrough; v) dehydrating the hollow soot porous body to form dehydrated soot porous body; vi) performing sintering step on dehydrated soot porous body to form sintered glass body; 10 vii) performing collapsing step to collapse the capillary of the sintered glass body to form solid glass preform; and viii) performing the step of reducing the diameter of the solid glass preform to form a core rod having reduced diameter. 15 It is apparent from the foregoing description that the presently disclosed method has overcome disadvantages, limitations and drawbacks of the prior art. It may be noted that various terms, for example adjustable mandrel, soot 20 porous body, hollow soot porous body, capillary, dehydrated soot porous body, sintered glass body, solid glass preform, core rod having reduced diameter, soot porous body having core rod, core rod, mother preform, soot preform, daughter preform, preform end, preform cone, sintered core rod etc. as employed herein are merely intended to illustrate the present invention and are not intended to 25 restrict scope of the present invention. It is obvious for the persons skilled in the art that alternative terms may also be employed to describe the present method without deviating from the intended scope of the present invention. It may also be noted that the presently disclosed method has been 30 described with reference to ACVD method. However, the present method is suitable even for other alternative methods known for producing mother preform and daughter preform. 20 STER/P A/040 We claim:- 1. A method for preparation of core rod assembly suitable for overcladding comprising the steps of:- 5 a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above 10 process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish. 2. A method for preparation of soot preform comprising soot porous body 15 having core rod comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; characterized in that 20 c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in 25 above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod. 3. A method for preparation of daughter preform comprising the steps of:- a) preparing core rod having reduced diameter; 30 b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; characterized in that c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step 21 STER/PA/040 process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in 5 above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod; and e) performing the sintering step on the soot preform produced in above process step-d) to form a daughter preform. 10 4. A method for preparation of optical fiber comprising the steps of:- a) preparing core rod having reduced diameter; b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter; characterized in that 15 c) the core rod with fixed glass rods at its opposite ends obtained in above process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish; d) performing the overcladding step on the core rod assembly produced in 20 above process step-c) to produce a core rod having overcladding, that is soot preform comprising soot porous body having core rod; e) performing the sintering step on the soot preform produced in above process step-d) to form a daughter preform; and f) performing fiber draw step on the daughter preform obtained in above 25 process step-e) to draw the fiber therefrom. 5. A method as claimed in any of the preceding claims, wherein said first step of hard fire polish is performed by increasing the temperature from room temperature to a temperature varying from about 1400°C to about 1800°C from 30 point A to point B on the core rod, maintaining this temperature within said range varying from about 1400°C to about 1800°C from point B to point C on the core rod followed by reducing said temperature varying within said range varying from about 1400°C to about 1800°C to a temperature of about 300°C from point C to point D on the core rod. 35 22 STER/P A/040 6. A method as claimed in any one of the preceding claims 1 to 4, wherein said second step of soft fire polish is performed by increasing the temperature from a temperature of about 300°C to a temperature varying from about 1100°C 5 to about 1250°C from point D to point C on the core rod, maintaining this temperature within said range varying from about 1100°C to about 1250°C from point C to point B on the core rod followed by reducing said temperature varying within said range varying from about 1100°C to about 1250°C to a temperature of about room temperature from point B to point A on the core rod. 10 7. A method as claimed in any one of the preceding claims 1 to 4, wherein said first step of hard fire polish is performed by changing said temperature from point A to D while increasing the burner speed from a speed of zero to a speed varying from about 150 to about 250 mm/min from point A to point B on 15 the core rod, maintaining said burner speed varying within said range varying from about 150 to about 250 mm/min from point B to point C on the core rod followed by reducing said burner speed varying within said range varying from about 150 to about 250 mm/min to a speed of about 50 mm/min from point C to point D on the core rod. 20 8. A method as claimed in any one of the preceding claims 1 to 4, wherein said second step of soft fire polish is performed by changing said temperature from point D to A while increasing the burner speed from a speed of about 50 mm/min to a speed varying within a range varying from about 300 to about 25 350 mm/min from point D to point C on the core rod, maintaining said burner speed varying within said range varying from about 300 to about 350 mm/min from point C to point B on the core rod followed by reducing said burner speed from said speed varying within said range varying from about 300 to about 350 mm/min to a zero speed from point C to point D on the core rod. 30 9. A method as claimed in any one of the preceding claims, wherein said first step of hard fire polish and said second step of soft fire polish are carried in reverse directions. 23 STER/P A/040 10. A method as claimed in claim 5, wherein the temperature profile of Figure 11 is followed. 5 11. A method as claimed in claim 6, wherein the temperature profile of Figure 13 is followed. 12. A method as claimed in claim 7, wherein the burner speed profile of the Figure 12 is followed. 10 13. A method as claimed in claim 8, wherein the burner speed profile of the Figure 14 is followed. 14. A method as claimed in any of the preceding claims, wherein the fire 15 polish of the core rod assembly is carried out while rotating the core rod assembly. 15. A core rod assembly prepared in accordance with any one of the preceding claims 1 to 14. 20 16. A soot preform prepared from core rod assembly of claim 15. 17. A daughter preform prepared from soot preform of claim 16. 25 18. A fiber prepared from daughter preform of claim 17. 19. A method for preparation of core rod assembly substantially as herein described with reference to and as illustrated in the accompanying drawings. 30 Dated this 9th day of October, 2006. [Dr. Ramesh Kr. Mehta] Patent Attorney for the Applicants 35 Of Mehta & Mehta Associates 24

Full Text

FORM 2
THE PATENTS ACT, 1970
(39 of 1970) 8r,
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
1. Title of the Invention:-
Method for preparation of core rod assembly for overcladding, and
preform and fiber produced from such core rod assembly.
2. Applicant(s):-
(a) Name: STERLITE OPTICAL TECHNOLOGIES LTD.
(b) Nationality: An Indian Company
(c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136
Maharashtra, INDIA
3. Preamble to the Description:-
Complete Specification:
The following specification particularly describes the invention and the
manner in which it is to be performed.

STER/P A/040
Field of the Invention:

The present invention relates to method for preparation of core rod
5 assembly for overcladding. Particularly, the present invention relates to a method for preparation of core rod assembly which is suitable for overcladding so as to form daughter preform which is suitable for fiber draw process. The present invention also relates to a core rod assembly for overcladding, and to the daughter preform and optical fiber produced from such core rod assembly.
10
Background of the Invention:
Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. The optical fibers are drawn
15 from an optical fiber preform. The optical fiber of predetermined dimension is drawn either from the solid glass preform [mother preform] or from sintered preform [daughter preform] by subjecting one end of the preform [mother preform or daughter preform] to a high temperature, for example above 2000°C. The sintered preform [daughter preform] is prepared from a core rod having soot
20 porous body [also referred as soot preform].
The daughter preform manufacturing process primarily involves a step of preparing the core rod comprising core of the fiber and part of clad which is followed by over-cladding. The core rod can be prepared by methods known in
25 the art, such as modified chemical vapour deposition (MCVD), plasma chemical vapour deposition (PCVD), Atmospheric chemical vapour deposition (ACVD), vapour axial deposition (VAD) etc. The over-cladding of the core rod can be carried out by various methods, such as glass tube jacketing, ACVD soot overcladding, VAD soot over-cladding, plasma over-cladding etc. Therefore, the
30 daughter preform can be manufactured by any combination of the core rod manufacturing methods and the over-cladding preparation methods.
Typically, the daughter preform can be manufactured by any of the
above-said conventional methods, for example by ACVD method, which is
35 described herein for reference.
2

STER/P A/040
In accordance with a typical ACVD process to manufacture a soot porous body, as illustrated in accompanying Figure 1, the preparation of soot porous
5 body 1 comprises the following steps. The glass-forming precursor compounds are oxidized and hydrolyzed to form porous silica based materials 2. The porous silica based materials 2 are deposited on a tapered cylindrical member referred as mandrel 3, which can be any commercially available mandrel with or without any specific preparation, preferably with specific preparation to remove the
10 contaminants therefrom which is provided with a handle rod 4 and fitted on a lathe 5 to form soot porous body 1.
During the step of deposition, the mandrel 3 is rotated in a direction as illustrated by an arrow 6 and also moved along its length with reference to
15 burner 7 to deposit the soot particles 2 on the mandrel 3 for producing soot porous body 1. During the deposition process, the dopant chemicals for example GeCl4 may also be deposited to form the core of the preform and later the dopant chemicals may be terminated to form clad of the preform. The amount of deposition of the clad region 11 and core region 10 is achieved to
20 have any desired ratio diameter of clad region 11 to the diameter of core region 10.
After completion of deposition, the soot porous body 1 is removed from lathe 5 along with mandrel 3 and handle rod 4, and the mandrel 3 is removed/
25 detached, during the mandrel removal step, from the soot porous body 1 thereby resulting in formation of a hollow cylindrical soot porous body 8 (herein after referred to as hollow soot porous body) having a centerline 9 therethrough [Figure 2].
30 The hollow soot porous body 8 thus formed comprises a core region 10
having a centerline hole 9 and a clad region 11 of the optical fiber preform [Figure 3], and said core region 10 has refractive index greater than that of the clad region 11.
3

STER/PA/040
After removal/detachment of mandrel 3 a centerline 9 is created inside the soot porous body 1.
5 Now referring to accompanying Figure 4, the prepared hollow soot porous
body 101 is transferred to the sintering furnace 100 in order to achieve dehydration, and sintering of the hollow soot porous body 101 to form dehydrated and sintered hollow glass body.
10 The dehydrated and sintered hollow glass body is subjected to step of
collapsing of the centerline 102 to form a solid glass preform 103 [Figure 5] with or without requiring any step of drilling or grinding or etching of the centerline 9/102 before steps of consolidation and collapsing.
15 Thus, the prepared hollow soot porous body 101 is dehydrated, sintered
and collapsed to convert it into solid glass preform 103.
In a typical embodiment of ACVD method, the hollow soot porous body 101, one end of which is provided with a plug 116 is inserted inside the furnace
20 100 with the help of the handle rod 106. The driving mechanism (not shown) facilitates lowering of the hollow soot porous body 101 into the furnace 100. The furnace 100 comprises a glass muffle tube 110 having a diameter sufficient to accommodate the hollow soot porous body 101 and to adequately provide the environment necessary for dehydration, sintering and collapsing. The muffle
25 tube 110 is suitable for heating to temperatures necessary for dehydration and simultaneous sintering and collapsing process steps with the heating means (not shown) which are suitably fitted to the sintering furnace 100.

The heating means selected may be suitable to create three heat zones
30 inside the muffle tube 110 over a length. A thermocouple (not shown) provided in the furnace 100 measures the temperature of the hot zones inside the furnace created by the heating means, and the data measurement is fed to the temperature controller (not shown) that controls the temperature inside the muffle tube 110.
35
4

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The furnace 100 is provided with an inlet port 115 located suitably on the furnace, preferably near the bottom of the muffle tube 110 for supplying desired gases in the furnace. The top end of the muffle tube 110 is suitably
5 closed with the lid 113 to achieve the preferred temperature profile inside the muffle tube 110 and to maintain the same during the dehydration, and simultaneous sintering and collapsing process steps, and to avoid leakage of gases from the muffle tube 110 to the outside environment. A suction port 114 is suitably provided near the top of muffle tube 110 to facilitate evacuation of 10 the gases from the muffle tube 110 as and when required or on completion of the process.
In accordance with the known art methods, the mother preform produced is subjected, in a conventional manner, to a step of reducing the
15 diameter to form a core rod having reduced diameter, which is then subjected, in a conventional manner, to a step of overcladding to form a soot preform comprising soot porous body having core rod, also referred as soot preform, which is then subjected, in a conventional manner, to a sintering step to form a daughter preform, which is then subjected to a step of fiber draw to draw the
20 fiber.
In accordance with methods known in the art, the opposite ends of the core rod having reduced diameter are first attached to glass rods by heating means before performing overcladding step thereon to form soot preform. This
25 process step is typically known as core rod assembly preparation step.
Accordingly, a typical process for preparation of optical fiber from daughter preform comprises the steps of:-
30 a) preparing the mandrel;
b) placing the mandrel over a lathe;
c) depositing soot particles on the mandrel to prepare a soot porous body;
d) removing the mandrel to form hollow soot porous body having capillary therethrough;
5

STER/P A/040
e) dehydrating the hollow soot porous body to form dehydrated soot porous
body;
f) performing sintering step on dehydrated soot porous body to form
5 sintered glass body;
g) performing collapsing step to collapse the capillary of the sintered glass
body to form solid glass preform;
h) performing the step of reducing the diameter of the solid glass preform to
form a core rod having reduced diameter;
10 i) performing core rod assembly preparation step to form a core rod
assembly for the overcladding step;
j) performing overcladding step on the core rod assembly to form soot
preform comprising soot porous body having core rod;
k) performing sintering step on the soot preform to form a daughter
15 preform; and
1) performing fiber draw step on the daughter preform having peform cone
of desired shape and dimensions including diameter to draw the fiber
therefrom.
20 It may be noted that for the ease of understanding the soot preform
comprising soot porous body having core rod is referred as "soot preform" which may not be confused with "mother preform" or "daughter preform".
As described herein, in accordance with the known art, the process of
25 core rod assembly preparation suitable for overcladding [above process step i)
involves attaching or fixing or welding opposite ends of the core rod to glass
rods by heating to a temperature for attaching or fixing or welding the gloss
rods to the opposite ends of the core rod.
30 It has been observed that the known methods of core rod assembly
preparation involve process step of heat treatment of opposite ends of the core rod to attach or fix or weld glass rods thereto. The known heat treatment process steps suffer from following drawbacks, disadvantages and limitations.
6

STER/PA/040
The main drawback of the known methods of core rod assembly preparation involving conventional process step of heat treatment of opposite ends of the core rod to attach or fix or weld [herein after for convenience
5 referred as to fix or fixing] glass rods thereto is that it causes stress formation at and around the joints formed between the glass rod and core rod. The stress formation in the core rod assembly has been observed to cause breakage of the core rod assembly during its handling and deposition of soot thereon to form the soot preform.
10
The another main drawback of the known methods of core rod assembly preparation involving conventional process step of heat treatment of opposite ends of the core rod to fix glass rods thereto is that it causes physical defects, for example cracks and breaks at and around the joints formed between the
15 glass rod and core rod, and such physical defects in the core rod assembly have been observed to cause bubbles and voids in the soot preform produced from such core rod assembly which in-turn results in breakage of the optical fiber during fiber drawing step, and hence, a loss of the productivity of the process.
20 The another problem of stress formation and formation of physical
defects in the core rod assembly during conventional heat treatment process is that the breakage of the core rod assembly may take place at any time during the soot deposition which not only results in total loss of soot deposition, but also causes damage to the burner, because there is every possibility that the
25 core rod assembly will fall on the burner meaning thereby the conventional heat treatment process suffer from time loss, production loss and financial loss.
It has been further observed that the stress formation and formation of physical defects in the core rod assembly also lead to transmission loss in the
30 resulting optical fiber or distortion of other optical parameters, for example, polarization mode dispersion, cutoff wavelength etc.
If conventional heat treatment is performed by graphite resistance
method it has been observed that due to relatively higher process time it causes
35 graphite oxidation which in-turn results in formation of oxidation products, for
7

STER/PA/040
example ash, graphite particles etc. which adheres to the core rod surface, and such contamination of the core rod with unwanted particles results in production of a daughter preform which will produce a fiber having increased
5 transmission loss and poor strength.
Similarly, if conventional heat treatment is performed by lasers, such as carbon dioxide lasers, which are clean heat source to use, power consumption has been observed to be very high rendering the overall process very expensive.
10
Further, it has also been observed that large amount of thermal induced stress in the core rod at and around the joint of the core rod and glass rod area adversely effects joint strength which upon further processing may shatter the core rod.
15
Accordingly, the known methods of core rod assembly preparation are observed to be uneconomical for commercial applications.
Need of the Invention:
20
Therefore, there is a need to have a method for preparation of core rod
assembly for overcladding, particularly a method for preparation of core rod
assembly which is suitable for overcladding so as to form daughter preform
which is suitable for fiber draw process.
25
Objects of the Invention:
Accordingly, the main object of the present invention is to provide a
method for preparation of core rod assembly which is suitable for overcladding
30 so as to form daughter preform which in-turn is suitable for fiber draw process.
The another main object of the present invention is to provide a method for preparation of core rod assembly wherein no stress is formed at and around the joints formed between the glass rod and core rod meaning thereby the core
8

STER/PA/040
rod assembly produced will not break, due to stress, during its handling and deposition of soot thereon to form the soot preform.
5 Still another main object of the present invention is to provide a method
for preparation of core rod assembly wherein formation of physical defects, for example cracks and breaks at and around the joints formed between the glass rod and core rod is reduced, and hence, possibility of formation of bubbles and voids in the soot preform produced from such core rod assembly is reduced
10 meaning thereby possibility of breakage of the optical fiber, due to physical defects during fiber drawing step is reduced, and accordingly, possibility of loss of productivity of the process is reduced.
Yet another main object of the present invention is to provide a method
15 for preparation of core rod assembly wherein possibility of stress formation and formation of physical defects in the core rod assembly is eliminated or greatly reduced, and hence, the possibility of breakage of the core rod assembly is eliminated or greatly reduced meaning thereby possibility of loss of soot deposition and damage of the burners is eliminated or greatly reduced.
20
Further object of the present invention is to provide a method for preparation of core rod assembly for overcladding wherein the core rod assembly is suitable for overcladding and to form suitable daughter preform.
25 One particular object of the present invention is to provide a method for
preparation of core rod assembly for overcladding which can overcome drawbacks, disadvantages and limitations of the prior art.
Another particular object of the present invention is to provide a method
30 for preparation of core rod assembly for overcladding wherein no or reduced physical defects and stress is caused in the core rod, and hence the possibility of transmission loss in the resulting optical fiber or distortion of other optical parameters, for example, polarization mode dispersion, cutoff wavelength etc. is greatly reduced.
35
9

STER/P A/040
Still another particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding which even by performing heating by graphite resistance method does not cause graphite
5 oxidation, and hence, overcomes associated problems of formation of oxidation products, for example ash, graphite particles etc. which are known to adhere to the core rod surface. Accordingly, the present invention aims to provide a method for core rod assembly preparation wherein possibility of contamination of core rod with unwanted particles is greatly reduced, and hence, the daughter
10 preform produced from such core rod assembly will produce a fiber having decreased transmission loss and increased strength.
Still further particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding which even if
15 heating is performed by lasers, such as carbon dioxide lasers, does not require higher power consumption meaning thereby which is relatively less expensive.
Yet further particular object of the present invention is to provide a method for preparation of core rod assembly for overcladding wherein
20 possibility of thermal induced stress in the core rod at and around the joint of the core rod and glass rod area is greatly reduced meaning thereby the joint strength is not adversely effected, and hence, the further processing of the produced core rod assembly will be safer and convenient.
25 It is also an object of the present invention to provide a core rod
assembly which is suitable for overcladding to form a suitable daughter preform which will be capable of fiber draw process to produce suitable optical fiber.
The other objects and advantages of the present invention will be
30 apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for the purpose of illustration of present invention and not to limit scope thereof.
Brief Description of the Invention:
35
10

STER/P A/040
The inventors have surprisingly observed that if core rod is heated in a controlled manner to have its controlled heating after fixing the glass rods at its opposite ends and before performing step of overcladding, the problems
5 associated with known methods of core rod assembly preparation as described herein above can be overcome and a core rod assembly suitable for overcladding which in-turn is suitable for producing desired daughter preform.
Accordingly, the present invention provides a method for preparation of
10 core rod assembly suitable for overcladding comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter;
15 characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step
process to produce core rod assembly suitable for overcladding, wherein
first step is hard fire polish and second step is soft fire polish.
20
The other embodiments and advantages of the present will be apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for illustration of preferred embodiments of the present invention and are not intended to limit scope thereof.
25
Brief Description of the Accompanying Drawings:
Figure 1 illustrates a schematic representation of deposition process over a mandrel to produce a soot porous body.
30
Figure 2 illustrates a schematic representation of hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body.
11

STER/PA/040
Figure 3 illustrates a schematic cross-sectional view of hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body.
5
Figure 4 illustrates a schematic representation of hollow soot porous body inside the sintering furnace after removal of mandrel from the soot porous body.
10 Figure 5 illustrates a hollow soot porous body having centerline
therethrough after removal of mandrel from the soot porous body which is subjected to steps of dehydration, sintering and collapsing to produce a solid glass preform.
15 Figure 6 illustrates a schematic representation of the mother preform
comprising a core and a clad provided with handle rod on each of its opposite ends.
Figure 6a illustrates a schematic representation of the daughter preform
20 comprising a core rod and overclad.
Figure 7 illustrates a schematic representation of process step of fixing a glass rod at one of the two opposite ends of the core rod in accordance with method for preparing a core rod assembly according to one embodiment of the
25 present invention.
Figure 8 illustrates a schematic representation of process step of fixing a glass rod at another opposite end of the core rod in accordance with method for preparing a core rod assembly according to one embodiment of the present
30 invention.
Figure 9 illustrates a schematic representation of core rod assembly ready for controlled heat treatment in accordance with method of the present invention.
35
12

STER/P A/040
Figure 10 illustrates a schematic representation of process step of heat treatment of the core rod assembly in accordance with method for preparing a core rod assembly according to one embodiment of the present invention.
5
Figure 11 illustrates a schematic representation of the temperature profile of the hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention.
10 Figure 12 illustrates a schematic representation of the burner speed
profile of the hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention.

Figure 13 illustrates a schematic representation of temperature profile of
15 the soft fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention.

Figure 14 illustrates a schematic representation of burner speed profile of the soft fire polish heat treatment process step of the core rod assembly in
20 accordance with one embodiment of the present invention.
Detailed Description and preferred embodiments of the Invention:
Accordingly, the present invention relates to a method for preparation of
25 core rod assembly suitable for overcladding comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter;
30 characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step
process to produce core rod assembly suitable for overcladding, wherein
first step is hard fire polish and second step is soft fire polish.
13

STER/P A/040
In accordance with one of the preferred embodiments of the present invention, the overcladding step is performed on the core rod assembly produced in above process step-c) to produce a core rod having overclad
5 referred as soot preform comprising soot porous body having core rod.
In accordance with another preferred embodiment of the present invention, the soot preform comprising soot porous body having core rod produced from core rod having overclad which in-turn is produced produced
10 from core rod assembly produced in above process step-c) is subjected to sintering step to form a daughter preform, which is suitable for storage or immediate processing for fiber draw step.

Accordingly, in one embodiment, the present invention relates to a
15 method for preparation of core rod having overcladding, that is, soot preform comprising soot porous body having core rod comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having
20 reduced diameter;
characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step
process to produce core rod assembly suitable for overcladding, wherein
25 first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod.
30 Accordingly, in another embodiment, the present invention relates to a
method for preparation of daughter preform comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having
35 reduced diameter;
14

STER/P A/040
characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step
5 process to produce core rod assembly suitable for overcladding, wherein
first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod; and
10 e) performing the sintering step on the soot preform produced in above
process step-d) to form a daughter preform.

Accordingly, in still another embodiment, the present invention relates to a method for preparation of optical fiber comprising the steps of:-
15
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter;
characterized in that
20 c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
25 above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod;
e) performing the sintering step on the soot preform produced in above
process step-d) to form a daughter preform; and
f) performing fiber draw step on the daughter preform obtained in above
30 process step-e) to draw the fiber therefrom.
Now referring to accompanying Figure 6, the mother preform 120 comprises a core 121 and a clad 122 and is provided with dummy glass handle 123 and 124 on each opposite ends 125 and 126 thereof for the purpose of
15

STER/P A/040
handling and holding the preform. The opposite end 125 [or 126] comprises a cone shape structure Cb [or Ct].
5 The accompanying Figure 6a illustrates the daughter preform 130
comprising a core rod 131 and a overclad 132 wherein the extended parts 133 and 134 on the opposite ends 135 and 136 of the core rod 131 serve the purpose of handle rods on each of the opposite ends of the core rod 131 for handling and holding the preform. The opposite ends 135 and 136 comprise a
10 cone shape structure Cb [or Ct].
Now referring to accompanying Figure 7 illustrating a schematic representation of process step of fixing a glass rod at one of the two opposite ends of the core rod in accordance with method of the present invention, a glass
15 rod 201 is fixed onto one end of the core rod 202 after attaching the core rod over a lathe 203 having rotation means [not shown] to rotate the core rod and glass rod, and heating means 204 for heating the glass rod 201 and core rod 202 to form a joint 205 between the two [Figure 9].
20 Now referring to accompanying Figure 8 illustrating a schematic
representation of process step of fixing a second glass rod at another opposite end of the core rod in accordance with method of the present invention a second piece of glass rod 301 is fixed onto another end of the core rod 202 after attaching the core rod over a lathe 203 having rotation means [not shown] to
25 rotate the core rod and second glass rod, and heating means 204 for heating the glass rod 301 and core rod 202 to form a joint 305 between the two [Figure 9].

The core rod assembly prepared in accordance with present invention is
30 illustrated in accompanying Figure 9 which comprises one glass rod 201 fixed onto one end of the core rod 202 forming a joint 205 and second glass rod 301 fixed onto another end of the core rod 202 forming a joint 305. The points A, B, C and D are marked on the core rod before start of controlled heat treatment in accordance with present invention, wherein point A is marked at suitable end of
35 the one glass rod 201, and point D is marked at a suitable end of another glass
16

STER/P A/040
rod 301 in a manner that the glass rods are suitably held in the lathe for controlled heat treatment in accordance with present invention. The points B
and C are the points of joint 205 formed between one glass rod 201 and core
5 rod 202 and joint 305 formed between second glass rod 301 and core rod 202 respectively.
In accordance with present invention, the said first step of hard fire polish is performed by increasing the temperature from room temperature to a
10 temperature varying from about 1400°C to about 1800°C from point A to point B on the core rod, maintaining this temperature within said range varying from about 1400°C to about 1800°C from point B to point C on the core rod followed by reducing said temperature varying within said range varying from about 1400QC to about 1800°C to a temperature of about 3000C from point C to point
15 D on the core rod.
The above temperature profile of the first hard fire polish heat treatment process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 11.
20
In accordance with present invention, the said second step of soft fire polish is performed by increasing the temperature from a temperature of about 300°C to a temperature varying from about 1100°C to about 12500C from point D to point C on the core rod, maintaining this temperature within said range
25 varying from about 1100°C to about 1250°C from point C to point B on the core rod followed by reducing said temperature varying within said range varying from about 1100°C to about 1250°C to a temperature of about room temperature from point B to point A on the core rod.
30 The above temperature profile of the second soft fire polish heat
treatment process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 13.
In accordance with present invention, the said first step of hard fire 35 polish is performed by changing said temperature from point A to D while
17

STER/PA/040
increasing the burner speed from a speed of zero to a speed varying from about 150 to about 250 mm/min from point A to point B on the core rod, maintaining said burner speed varying within said range varying from about 150 to about
5 250 mm/min from point B to point C on the core rod followed by reducing said burner speed varying within said range varying from about 150 to about 250 mm/min to a speed of about 50 mm/min from point C to point D on the core rod.
10 The above burner speed profile of the first hard fire polish heat treatment
process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 12.

In accordance with present invention, the said second step of soft fire
15 polish is performed by changing said temperature from point D to A while increasing the burner speed from a speed of about 50 mm/min to a speed varying within a range varying from about 300 to about 350 mm/min from point D to point C on the core rod, maintaining said burner speed varying within said range varying from about 300 to about 350 mm/min from point C
20 to point B on the core rod followed by reducing said burner speed from said speed varying within said range varying from about 300 to about 350 mm/min to a zero speed from point C to point D on the core rod.

The above burner speed profile of the first hard fire polish heat treatment
25 process step of the core rod assembly in accordance with one embodiment of the present invention is illustrated in Figure 14.
It may be noted that the present invention is not restricted by performing the first hard fire polish step from point A to D and second soft fire polish step
30 from point D to A. One may also perform these steps in reverse direction, that is first hard fire polish step from point D to A and second soft fire polish step from point A to D. Accordingly, the present invention is restricted by performing second soft fire polish step in direction opposite to the direction of first hard fire polish step.
35
18

STER/P A/040
In accordance with one of the preferred embodiments of the present
invention, the fire polish, that is, the controlled heat treatment of the core rod
assembly is carried out while rotating the core rod assembly to achieve the
5 uniform heat treatment. Preferably, the rotation speed varies from about 50 to
about 70 rpm.
It has been found that on following above described controlled heat treatment method of the present invention no stress formation takes place at
10 and around the joints formed between the glass rod and core rod. Accordingly, no breaks have been observed in the core rod assembly produced by present method during its handling and deposition of soot thereon to form the soot preform.
15 It has also been found that on following above described controlled heat
treatment method of the present invention, the formation of physical defects, for example cracks and breaks at and around the joints formed between the glass rod and core rod is greatly reduced, and hence, the possibility of formation of bubbles and voids in the soot preform produced from such core rod assembly is
20 greatly reduced meaning thereby the possibility of breakage of the optical fiber, due to physical defects during fiber drawing step is greatly reduced, and accordingly, the possibility of loss of productivity of the process is greatly reduced, which make the present method economically viable.
25 As the present method has been found to be free from stress formation
and formation of physical defects in the core rod assembly, the possibility of breakage of the core rod assembly during soot deposition is eliminated or greatly reduced meaning thereby the possibility of loss of soot deposition and damage of the burners is eliminated or greatly reduced.
30
In accordance with present invention, the core rod with reduced diameter can be prepared by any method. As exemplary embodiment, the core rod having reduced diameter may be prepared in following manner:-
35 i) preparing the mandrel;
19

STER/P A/040
ii) placing the mandrel over a lathe;
iii) depositing soot particles on the mandrel to prepare a soot porous body;
iv) removing the mandrel to form hollow soot porous body having capillary
5 therethrough;
v) dehydrating the hollow soot porous body to form dehydrated soot porous
body; vi) performing sintering step on dehydrated soot porous body to form
sintered glass body;
10 vii) performing collapsing step to collapse the capillary of the sintered glass
body to form solid glass preform; and viii) performing the step of reducing the diameter of the solid glass preform to
form a core rod having reduced diameter.
15 It is apparent from the foregoing description that the presently disclosed
method has overcome disadvantages, limitations and drawbacks of the prior
art.

It may be noted that various terms, for example adjustable mandrel, soot
20 porous body, hollow soot porous body, capillary, dehydrated soot porous body, sintered glass body, solid glass preform, core rod having reduced diameter, soot porous body having core rod, core rod, mother preform, soot preform, daughter preform, preform end, preform cone, sintered core rod etc. as employed herein are merely intended to illustrate the present invention and are not intended to
25 restrict scope of the present invention. It is obvious for the persons skilled in the art that alternative terms may also be employed to describe the present method without deviating from the intended scope of the present invention.

It may also be noted that the presently disclosed method has been
30 described with reference to ACVD method. However, the present method is suitable even for other alternative methods known for producing mother preform and daughter preform.
20

STER/P A/040
We claim:-
1. A method for preparation of core rod assembly suitable for overcladding
comprising the steps of:-
5 a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having
reduced diameter;
characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
10 process step-b) is fire polished in a controlled manner and two step
process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish.
2. A method for preparation of soot preform comprising soot porous body
15 having core rod comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter;
characterized in that
20 c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
25 above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod.
3. A method for preparation of daughter preform comprising the steps of:-
a) preparing core rod having reduced diameter;
30 b) fixing glass rods at each of the opposite ends of the core rod having
reduced diameter; characterized in that
c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step
21

STER/PA/040
process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
5 above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod; and
e) performing the sintering step on the soot preform produced in above
process step-d) to form a daughter preform.
10 4. A method for preparation of optical fiber comprising the steps of:-
a) preparing core rod having reduced diameter;
b) fixing glass rods at each of the opposite ends of the core rod having reduced diameter;
characterized in that
15 c) the core rod with fixed glass rods at its opposite ends obtained in above
process step-b) is fire polished in a controlled manner and two step process to produce core rod assembly suitable for overcladding, wherein first step is hard fire polish and second step is soft fire polish;
d) performing the overcladding step on the core rod assembly produced in
20 above process step-c) to produce a core rod having overcladding, that is
soot preform comprising soot porous body having core rod;
e) performing the sintering step on the soot preform produced in above
process step-d) to form a daughter preform; and
f) performing fiber draw step on the daughter preform obtained in above
25 process step-e) to draw the fiber therefrom.
5. A method as claimed in any of the preceding claims, wherein said first step of hard fire polish is performed by increasing the temperature from room temperature to a temperature varying from about 1400°C to about 1800°C from
30 point A to point B on the core rod, maintaining this temperature within said range varying from about 1400°C to about 1800°C from point B to point C on the core rod followed by reducing said temperature varying within said range varying from about 1400°C to about 1800°C to a temperature of about 300°C
from point C to point D on the core rod.
35
22

STER/P A/040
6. A method as claimed in any one of the preceding claims 1 to 4, wherein
said second step of soft fire polish is performed by increasing the temperature
from a temperature of about 300°C to a temperature varying from about 1100°C
5 to about 1250°C from point D to point C on the core rod, maintaining this temperature within said range varying from about 1100°C to about 1250°C from point C to point B on the core rod followed by reducing said temperature varying within said range varying from about 1100°C to about 1250°C to a temperature of about room temperature from point B to point A on the core rod.
10
7. A method as claimed in any one of the preceding claims 1 to 4, wherein
said first step of hard fire polish is performed by changing said temperature
from point A to D while increasing the burner speed from a speed of zero to a
speed varying from about 150 to about 250 mm/min from point A to point B on
15 the core rod, maintaining said burner speed varying within said range varying from about 150 to about 250 mm/min from point B to point C on the core rod followed by reducing said burner speed varying within said range varying from about 150 to about 250 mm/min to a speed of about 50 mm/min from point C to point D on the core rod.
20
8. A method as claimed in any one of the preceding claims 1 to 4, wherein
said second step of soft fire polish is performed by changing said temperature
from point D to A while increasing the burner speed from a speed of about 50
mm/min to a speed varying within a range varying from about 300 to about
25 350 mm/min from point D to point C on the core rod, maintaining said burner speed varying within said range varying from about 300 to about 350 mm/min from point C to point B on the core rod followed by reducing said burner speed from said speed varying within said range varying from about 300 to about 350 mm/min to a zero speed from point C to point D on the core rod.
30
9. A method as claimed in any one of the preceding claims, wherein said
first step of hard fire polish and said second step of soft fire polish are carried
in reverse directions.
23

STER/P A/040

10. A method as claimed in claim 5, wherein the temperature profile of Figure 11 is followed.
5 11. A method as claimed in claim 6, wherein the temperature profile of Figure 13 is followed.
12. A method as claimed in claim 7, wherein the burner speed profile of the
Figure 12 is followed.
10
13. A method as claimed in claim 8, wherein the burner speed profile of the
Figure 14 is followed.
14. A method as claimed in any of the preceding claims, wherein the fire
15 polish of the core rod assembly is carried out while rotating the core rod
assembly.
15. A core rod assembly prepared in accordance with any one of the
preceding claims 1 to 14.
20
16. A soot preform prepared from core rod assembly of claim 15.
17. A daughter preform prepared from soot preform of claim 16.
25 18. A fiber prepared from daughter preform of claim 17.
19. A method for preparation of core rod assembly substantially as herein described with reference to and as illustrated in the accompanying drawings.
30 Dated this 9th day of October, 2006.
[Dr. Ramesh Kr. Mehta]
Patent Attorney for the Applicants
35 Of Mehta & Mehta Associates
24

Documents:

1687-mum-2005-claims.doc

1687-mum-2005-form-2.doc

1687-MUM-2006-ABSTRACT 24-7-2008.pdf

1687-mum-2006-abstract(24-06-2008).doc

1687-mum-2006-abstract(24-06-2008).pdf

1687-mum-2006-abstract-1.jpg

1687-mum-2006-cancelled page(24-06-2008).pdf

1687-mum-2006-claim(granted)-(24-06-2008).doc

1687-mum-2006-claim(granted)-(24-06-2008).pdf

1687-MUM-2006-CLAIMS 24-7-2008.pdf

1687-mum-2006-claims.pdf

1687-mum-2006-correspondance-received-111006.pdf

1687-mum-2006-correspondance-received-131206.pdf

1687-MUM-2006-CORRESPONDENCE 8-7-2008.pdf

1687-MUM-2006-CORRESPONDENCE(IPO) 8-7-2008.pdf

1687-mum-2006-correspondence(ipo)-(02-07-2008).pdf

1687-mum-2006-correspondence1(08-07-2008).pdf

1687-mum-2006-correspondence2(05-02-2007).pdf

1687-mum-2006-description (complete).pdf

1687-MUM-2006-DRAWING 24-7-2008.pdf

1687-mum-2006-drawing(24-06-2008).pdf

1687-mum-2006-drawings.pdf

1687-MUM-2006-FORM 1 24-6-2008.pdf

1687-mum-2006-form 1(01-12-2006).pdf

1687-mum-2006-form 1(24-06-2008).pdf

1687-mum-2006-form 13(15-05-2008).pdf

1687-mum-2006-form 18(06-02-2007).pdf

1687-mum-2006-form 2(granted)-(24-06-2008).doc

1687-mum-2006-form 2(granted)-(24-06-2008).pdf

1687-mum-2006-form 26(08-07-2008).pdf

1687-mum-2006-form 26(13-05-2008).pdf

1687-mum-2006-form 26(15-05-2008).pdf

1687-mum-2006-form 3(12-10-2006).pdf

1687-mum-2006-form 3(24-06-2008).pdf

1687-mum-2006-form 5(12-10-2006).pdf

1687-mum-2006-form 5(24-06-2008).pdf

1687-mum-2006-form 9(15-12-2006).pdf

1687-mum-2006-form-1.pdf

1687-mum-2006-form-2.pdf

1687-mum-2006-form-3.pdf

1687-mum-2006-form-5.pdf

1687-mum-2006-form-9.pdf

1687-MUM-2006-OTHER DOCUMENT 8-7-2008.pdf

1687-mum-2006-power of attorney (13-05-2008).pdf

1687-mum-2006-power of attorney(13-05-2008).pdf

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Patent Number

223433

Indian Patent Application Number

1687/MUM/2006

PG Journal Number

06/2009

Publication Date

06-Feb-2009

Grant Date

11-Sep-2008

Date of Filing

12-Oct-2006

Name of Patentee

STERLITE TECHNOLOGIES LIMITED

Applicant Address

E1/E2/E3 MIDC,WALUJ, AURANGABAD 431136, MAHARASHTRA, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	Dattatray PASARE	STERLITE OPTICAL TECHNOLOGIES LTD., E1/E2/E3.MIDC. Waluj, Aurangabad-431136. Maharashtra, INDIA.
2	PARAG KULKARNI	E1/E2/E3 MIDC,WALUJ, AURANGABAD 431136, MAHARASHTRA, INDIA

PCT International Classification Number

C03B

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA