Title of Invention	OPTICAL FIBER HAVING REDUCED POLARIZATION MODE DISPERSION (PMD) AND METHOD FOR PRODUCING THE SAME
Abstract	N/A

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STER/P A/036 FORM 2 THE PATENTS ACT, 1970 (39 of 1970) THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention: - Optical fiber having reduced polarization mode dispersion [PMD] and method for producing the same. 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. 1 STER/P A/036 Field of the Invention: The present invention relates to optical fiber having reduced polarization mode dispersion [PMD]. Particularly, it relates to method for producing an optical fiber having reduced PMD. More particularly, it relates to a method for 5 producing optical fiber preform having symmetrically and completely collapsed capillary, which is suitable for producing optical fiber having reduced PMD. Even more particularly, it relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary, wherein the preform produced has diameter more than about 90 mm or weight more than 10 about 9Kg, and is suitable for producing optical fiber having reduced PMD. Background of Invention: Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. The primary object of telecommunication industry is to transmit larger amounts of information over 15 longer distances in shorter period of time. In the recent years, certain improvements have taken place in the light carrying ability of the optical fibers. The light carrying ability of the optical fibers for communication is primarily determined by attenuation loss and PMD. The optical losses, that is, attenuation loss in the fiber are caused by many factors including absorption 20 loss, scattering of light, and PMD in fiber are caused by imperfect geometry of the fiber, stress and structural defects in the fiber. It has been observed that imperfect geometry of the fiber, and stress and structural defects in the fiber are caused due to unsymmetric and incompletely collapsed centerline [capillary] in the optical fiber preform from which the fiber 25 is drawn. The problems of imperfect geometry of the fiber, and stress and structural defects in the fiber is further enhanced if the preform from which the fiber is drawn has unsymmetric and incompletely collapsed centerline [capillary] particularly at middle portion thereof. Therefore, the need of the time is to have an optical fiber preform which 30 should have symmetrically and completely collapsed centerline [capillary] so that an optical fiber having reduced PMD can be produced. The co-pending Indian patent application no. 494/MUM/2004 [IPA494] defines a process to achieve complete collapsing of the capillary of the preform, 2 STER/PA/036 by simultaneously carrying out sintering and collapsing process steps on the dehydrated hollow soot porous body. In accordance with this method, preform having completely collapsed capillary is obtained, but the capillary in the top portion of the preform is still not completely collapsed. 5 The another co-pending Indian patent application no. 1530/MUM/2005 [IPA1530] defines an improved process to achieve complete collapsing of the capillary of the preform, wherein the simultaneous sintering and collapsing process steps are carried out on the dehydrated hollow soot porous body under specifically controlled heating. In accordance with this method, preform having 10 completely collapsed capillary is obtained, particularly having collapsed capillary in the top portion of the preform. It has been observed that both the methods taught in IPA494 and IPA1530 are suitable for achieving complete collapsing of capillary, if the preform prepared has diameter of less than about 90 mm or weight of less than 15 about 9Kg. However, if the preform prepared should have diameter more than about 90 mm or weight more than about 9Kg, then both the methods taught in IPA494 and IPA1530 result in incomplete collapsing of the capillary, particularly in the middle portion. The methods taught in IPA494 and IPA1530 do not address the problem 20 of unsymmetric collapsing of the capillary and uncollapsed capillary at the middle portion particularly when the preform prepared is of diameter more than about 90 mm or weight more than about 9 Kg. Need of the Invention: Therefore, there is a need to have a method for preparing an optical fiber 25 preform having symmetrically and completely collapsed centerline [capillary] so that an optical fiber having reduced PMD can be produced therefrom, and to have an optical fiber having reduced PMD. Further, there is a need to have a method for preparing an optical fiber preform wherein the method is suitable even if the preform should have diameter more than about 90 mm or weight 30 more than about 9 Kg. Objects of the Invention: One of the main objects of the present invention is to provide a method for preparing an optical fiber preform having symmetrically and completely 3 STER/PA/036 collapsed centerline [capillary] so that an optical fiber having reduced PMD can be produced therefrom, and to provide an optical fiber having reduced PMD. The another main object of the present invention is to provide a method for preparing an optical fiber preform wherein the method is suitable even for 5 producing preform of diameter more than about 90 mm or weight more than about 9 Kg. Another object of the present invention is to provide a method for preparing an optical fiber preform having symmetrically and completely collapsed capillary, so that an optical fiber having perfect geometry and, 10 reduced stress and structural defects can be produced therefrom, and to provide an optical fiber having perfect geometry, and reduced stress and structural defects. Still another object of the present invention is to provide a method for preparing an optical fiber preform having symmetrically and completely 15 collapsed capillary, particularly at and around middle portion thereof, so that an optical fiber having perfect geometry, and reduced stress and structural defects can be produced therefrom, and to provide an optical fiber having perfect geometry, and reduced stress and structural defects over its entire length. 20 Other objects and advantages will be apparent from the following description when read in conjunction with the accompanying figures. Brief Description of the Invention: It is apparent from the foregoing description that the known methods for producing preform are not suitable for producing a preform having 25 symmetrically and completely collapsed capillary, particularly at the middle portion thereof, and having diameter more than about 90 mm or weight more than about 9Kg, wherein the preform produced is suitable for producing optical fiber having reduced PMD. The present inventors have observed that the collapsing of the capillary 30 in a hollow soot porous body or sintered glass body is carried out while applying a constant vacuum along entire length of hollow soot porous body or sintered glass body. It has been observed that the collapsing of capillary under constant 4 y STER/PA/036 vacuum results in unsymmetrical and incomplete collapsing of the capillary, particularly at and around its middle portion. As the mandrel employed in preparation of a preform has tapered diameter for achieving its easy removal from the soot porous body, the prepared 5 hollow soot porous body and the sintered glass body prepared therefrom are observed to have reducing capillary diameter from one end [referred as top end] to another end [referred as bottom end] of the soot porous body or sintered glass body. The present inventors have observed that the constant vacuum applied 10 in hollow soot porous body or sintered glass body having capillary of reducing [varying] diameter from one end to another end during the collapsing process step surprisingly causes unsymmetric and incomplete collapsing of capillary, particularly at and around its middle portion while producing the optical fiber preform [mother preform]. Such optical fiber preform having unsymmetric and 15 incompletely collapsed capillary has been observed to have imperfect geometry, non-uniform stress and structural defects at its centerline region. It has also been observed that the preform having unsymmetric and incompletely collapsed capillary meaning thereby having imperfect geometry, non-uniform stress and structural defects at its centerline region produces an optical fiber having 20 imperfect geometry, and stress and structural defects. It has been further observed that during the collapsing step, the hollow soot porous body or sintered glass body gets stretched, particularly at and around its middle portion primarily due to its larger weight and gravitational force. The un-symmetric stretch of the soot porous body or sintered glass body 25 also causes unsymmetric and incomplete collapsing of capillary, particularly at and around its middle portion while producing the optical fiber preform [mother preform], which is observed to have imperfect geometry, non-uniform stress and structural defects at its centerline region. The preform having unsymmetric and incompletely collapsed capillary produces an optical fiber having imperfect 30 geometry, and stress and structural defects. The present inventors have surprisingly observed that if the collapsing of capillary in a hollow soot porous body or sintered glass body is carried out under a variable vacuum from one end to another end then the capillary 5 STER/P A/036 surprisingly collapses symmetrically and completely over its entire length, including at and around its middle portion meaning thereby it produces the optical fiber preform [mother preform] having perfect geometry, and reduced stress and structural defects at its centerline region, which has been found 5 suitable to produce an optical fiber having imperfect geometry, and stress and structural defects. Accordingly, in one embodiment, the present invention relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary which is suitable for producing optical fiber having reduced 10 PMD, comprising collapsing the capillary in the hollow soot porous body or sintered glass body under variable vacuum to form optical fiber preform having symmetrically and completely collapsed capillary. In one embodiment, the present invention also relates to a fiber produced from the preform produced by employing method of the present invention, 15 wherein the fiber produced has reduced PMD. Other advantages and preferred embodiments of the present invention will be apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit scope of the present invention. 20 Description of the Accompanying Figures: Figure 1 illustrates a schematic representation of deposition process over a mandrel to produce a soot porous body. Figure 2 illustrates a schematic representation of hollow soot porous body having centerline therethrough after removal of mandrel from the soot 25 porous body. 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. Figure 4 illustrates a schematic representation of hollow soot porous 30 body inside the sintering furnace after removal of mandrel from the soot porous body. Figure 5 illustrates a hollow soot porous body having centerline therethrough after removal of mandrel from the soot porous body which is 6 STER/P A/036 subjected to steps of dehydration, and sintering and collapsing to produce a solid glass preform. Figure 6 illustrates vacuum profile, in accordance with one of the preferred embodiments of the present invention, for simultaneous sintering and 5 collapsing, or collapsing process step to have symmetric and complete collapsing of the capillary to produce a solid glass preform. Figure 7 illustrates vacuum profile, in accordance with another preferred embodiment of the present invention, for simultaneous sintering and collapsing, or collapsing process step to have symmetric and complete 10 collapsing of the capillary to produce a solid glass preform. Figure 8 illustrates vacuum profile, in accordance with still another preferred embodiment of the present invention, for simultaneous sintering and collapsing, or collapsing process step to have symmetric and complete collapsing of the capillary to produce a solid glass preform. 15 Figure 9 illustrates PMD of the fiber produced in accordance with conventional method. Figure 10 illustrates PMD of the fiber produced in accordance with one of the preferred embodiments of the present invention. Figure 11 illustrates PMD distribution of the fiber produced in 20 accordance with conventional method. Figure 12 illustrates PMD distribution of the fiber produced in accordance with one of the preferred embodiments of the present invention. Detailed Description and Preferred embodiments of the Invention; Typically, a preform can be manufactured by a conventional method, for 25 example by ACVD method, which is described herein for reference. In accordance with a typical ACVD process to manufacture a soot porous body, as illustrated in accompanying Figure 1, the preparation of soot porous body 1 comprises the following steps. The glass-forming precursor compounds are oxidized and hydrolyzed to form porous silica based materials 2. The porous 30 silica based materials 2 are deposited on a mandrel 3, which can be any commercially available mandrel with or without any specific preparation, preferably with specific preparation to remove the contaminants therefrom 7 STER/P A/036 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 5 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 10 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/ detached, during the mandrel removal step, from the soot porous body 1 15 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]. 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 20 [Figure 3], and said core region 10 has refractive index greater than that of the clad region 11. After removal/ detachment of mandrel 3 a centerline 9 is created inside the soot porous body 1. Now referring to accompanying Figure 4, the prepared hollow soot porous 25 body 101 is transferred to the sintering furnace 100 in order to achieve dehydration of the hollow soot porous body 101 to form dehydrated hollow soot porous body. The dehydrated hollow soot porous body is subjected to step of sintering and collapsing [simultaneously or one after the other] of the centerline 102 to 30 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. 8 STER/P A/036 Thus, the prepared hollow soot porous body 101 is dehydrated, sintered and collapsed to convert it into solid glass preform 103 [the mother preform]. 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 5 100 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, wherein the 10 sintering and collapsing may either be performed simultaneously or collapsing step may be performed after sintering step. The muffle tube 110 is suitable for heating to temperatures necessary for dehydration, sintering and collapsing process steps with the heating means (not shown) which are suitably fitted to the sintering furnace 100. 15 The heating means selected may be suitable to create three heat zones 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 20 muffle tube 110. 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 closed with the lid 113 to achieve the preferred temperature profile inside the 25 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 the gases from the muffle tube 110 as and when required or on completion of 30 the process. The prepared solid glass preform may optionally be reduced to form a core rod having reduced diameter, which may be overcladded by depositing soot particles to form a soot preform comprising core rod having overclad. The soot 9 STER/PA/036 preform may be sintered in a sintering furnace to form daughter preform. The fiber may also be drawn from the daughter preform. As described herein above, in accordance with known methods, the process step of collapsing of capillary of a hollow soot porous body, whether 5 carried out simultaneously with sintering step or after performing the sintering step, is carried under a constant vacuum, which has been observed to result in unsymmetrical and incomplete collapsing of the capillary, particularly at its middle portion. The problem of unsymmetrical and incomplete collapsing of the capillary enhances further when the preform having diameter more than about 10 90 mm or weight more than about 9Kg is required to be produced. The preform, whether mother preform or daughter preform, having unsymmetrical and incomplete collapsed capillary [centerline] has been observed to produce a fiber having increased PMD. Therefore, the present invention aims to overcome above problems and 15 limitations of the prior art. Accordingly, in one embodiment, the present invention relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary which is suitable for producing optical fiber having reduced PMD, comprising collapsing the capillary in the hollow soot porous body or 20 sintered glass body under variable vacuum to form optical fiber preform having symmetrically and completely collapsed capillary. Accordingly, in another embodiment, the present invention relates to a method for producing optical fiber having reduced PMD from optical fiber preform having symmetrically and completely collapsed capillary which is 25 prepared by collapsing the capillary in the hollow soot porous body or sintered glass body under variable vacuum. In accordance with present invention, the fiber having reduced PMD may either be drawn at mother preform stage or at daughter preform stage. It has been observed by the present inventors that if a mother preform is 30 produced in accordance with present invention by collapsing the capillary in the hollow soot porous body or sintered glass body under variable vacuum, it has symmetrically and completely collapsed capillary, which has been found suitable to produce optical fiber having reduced PMD, and has also been found 10 STER/P A/036 suitable to produce a daughter preform which in-turn has been found suitable to produce optical fiber having reduced PMD. In accordance with present invention, the mother preform having symmetrically and completely collapsed capillary produced in accordance with 5 present invention, preferably has diameter more than about 90 mm or weight more than about 9Kg. Accordingly, in first preferred embodiment, the present invention relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary, which is suitable for producing optical fiber 10 having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; 15 iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; 20 v) simultaneously sintering and collapsing the dehydrated soot porous body to collapse the capillary thereby to form mother preform; characterized in that the simultaneous sintering and collapsing steps on dehydrated soot porous body to collapse the capillary and to form mother preform is carried out under variable vacuum. 25 Accordingly, in second preferred embodiment, which is extension of first embodiment, the present invention relates to a method for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; 30 ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; 11 STER/PA/036 iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) simultaneously sintering and collapsing the dehydrated soot porous body 5 to collapse the capillary thereby to form mother preform; vi) drawing the fiber from the mother preform; characterized in that the simultaneous sintering and collapsing steps on dehydrated soot porous body to collapse the capillary and to form mother preform is carried out under variable vacuum. 10 Accordingly, in third preferred embodiment, which is extension of first embodiment, the present invention relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary, which is suitable for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body 15 having core and clad; ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; 20 iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) simultaneously sintering and collapsing the dehydrated soot porous body to collapse the capillary thereby to form mother preform; 25 vi) reducing the diameter of the mother preform to form a core rod having reduced diameter; vii) overcladding the core rod having reduced diameter to form soot preform comprising soot porous body having core rod; viii) sintering the soot preform to form a daughter preform; 30 characterized in that the simultaneous sintering and collapsing steps on dehydrated soot porous body to collapse the capillary and to form mother preform is carried out under variable vacuum. 12 STER/PA/036 Accordingly, in fourth preferred embodiment, which is extension of third embodiment, the present invention relates to a method for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body 5 having core and clad; ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; 10 iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) simultaneously sintering and collapsing the dehydrated soot porous body to collapse the capillary thereby to form mother preform; 15 vi) reducing the diameter of the mother preform to form a core rod having reduced diameter; vii) overcladding the core rod having reduced diameter to form soot preform comprising soot porous body having core rod; viii) sintering the soot preform to form a daughter preform; 20 ix) drawing the fiber from the daughter preform; characterized in that the simultaneous sintering and collapsing steps on dehydrated soot porous body to collapse the capillary and to form mother preform is carried out under variable vacuum. Accordingly, in fifth preferred embodiment, the present invention relates 25 to a method for producing optical fiber preform having symmetrically and completely collapsed capillary, which is suitable for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; 30 ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; 13 STER/P A/036 iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) sintering the dehydrated soot porous body to form sintered glass body; 5 vi) performing collapsing step on sintered glass body to collapse the capillary thereby to form mother preform; characterized in that the collapsing step on sintered glass body to collapse the capillary and to form mother preform is carried out under variable vacuum. Accordingly, in sixth preferred embodiment, which is extension of fifth 10 embodiment, the present invention relates to a method for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; ii) removing the mandrel from the soot porous body to form hollow soot 15 porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot 20 porous body; v) sintering the dehydrated soot porous body to form sintered glass body; vi) performing collapsing step on sintered glass body to collapse the capillary thereby to form mother preform; vii) drawing the fiber from mother preform; 25 characterized in that the collapsing step on sintered glass body to collapse the capillary and to form mother preform is carried out under variable vacuum. Accordingly, in seventh preferred embodiment, which is extension of fifth embodiment, the present invention relates to a method for producing optical fiber preform having symmetrically and completely collapsed capillary, which is 30 suitable for producing optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; 14 STER/PA/036 ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; 5 iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) sintering the dehydrated soot porous body to form sintered glass body; vi) performing collapsing step on sintered glass body to collapse the 10 capillary thereby to form mother preform; vii) reducing the diameter of the mother preform to form a core rod having reduced diameter; viii) overcladding the core rod having reduced diameter to form soot preform comprising soot porous body having core rod; 15 ix) sintering the soot preform to form a daughter preform; characterized in that the collapsing step on sintered glass body to collapse the capillary and to form mother preform is carried out under variable vacuum. Accordingly, in eighth preferred embodiment, which is extension of seventh embodiment, the present invention relates to a method for producing 20 optical fiber having reduced PMD, comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; 25 iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; iv) dehydrating the hollow soot porous body in a suitable environment to completely remove the moisture in core thereby to form dehydrated soot porous body; 30 v) sintering the dehydrated soot porous body to form sintered glass body; vi) performing collapsing step on sintered glass body to collapse the capillary thereby to form mother preform; 15 STER/P A/036 vii) reducing the diameter of the mother preform to form a core rod having reduced diameter; viii) overcladding the core rod having reduced diameter to form soot preform comprising soot porous body having core rod; 5 ix) sintering the soot preform to form a daughter preform; x) drawing the fiber from daughter preform; characterized in that the collapsing step on sintered glass body to collapse the capillary and to form mother preform is carried out under variable vacuum. As stated herein above, in accordance with present invention, the mother 10 preform having symmetrically and completely collapsed capillary produced in accordance with present invention, preferably has diameter more than about 90 mm or weight more than about 9Kg. In accordance with present invention, the variable vacuum is varied from bottom end to top end of the soot porous body [referred as body] based on part 15 of the soot porous body or sintered glass body being collapsed. In accordance with present invention, the vacuum for X length of soot porous body or sintered glass body is different in its bottom part, ramp down part, middle part, ramp up part and top part. In accordance with preferred embodiment of the present invention, the 20 vacuum inside the body is varied in the range varying from about 130 to about 160 torr while collapsing the bottom portion of the body. The vacuum inside the body is reduced from a range of about 130 to about 160 torr in the bottom portion to a range varying from about 170 to about 190 torr while collapsing ramp down portion of the body. 25 The vacuum inside the body is varied in the range varying from about 170 to about 190 torr while collapsing middle portion of the body. The vacuum inside the body is increased from a range of about 170 to about 190 torr in the middle portion to a range varying from about 15 to about 5 torr while collapsing ramp up portion of the body. 30 The vacuum inside the body is varied in the range varying from about 15 to about 5 torr while collapsing top portion of the body. 16 STER/P A/036 In accordance with preferred embodiment of the present invention for X length of soot porous body or sintered glass body the length of bottom part of the body varies from about OX to about 0.36 X. For X length of soot porous body or sintered glass body the length of 5 ramp down part of the body varies from about 0.36 X to about 0.42 X. For X length of soot porous body or sintered glass body the length of middle part of the body varies from about 0.42 X to about 0.79 X. For X length of soot porous body or sintered glass body the length of ramp up part of the body varies from about 0.79 X to about 0.88 X. 10 For X length of soot porous body or sintered glass body the length of top part of the body varies from about 0.88 X to IX. In accordance with particular embodiment of the present invention, for X length of the body, the vacuum inside the hollow soot porous body or sintered glass body is varied in different portions based on the portion [part] of the soot 15 porous body or sintered glass body being collapsed in following manner:- The vacuum inside the hollow soot porous body is varied in the range varying from about 130 to about 160 torr while collapsing the bottom portion of the soot porous body or sintered glass body; The vacuum inside the hollow soot porous body is reduced from a range 20 of about 130 to about 160 torr in the bottom portion to a range varying from about 170 to about 190 torr while collapsing ramp down portion of the soot porous body or sintered glass body; The vacuum inside the hollow soot porous body is varied in the range varying from about 170 to about 190 torr while collapsing middle portion 25 of the soot porous body or sintered glass body; The vacuum inside the hollow soot porous body is increased from a range of about 170 to about 190 torr in the middle portion to a range varying from about 15 to about 5 torr while collapsing ramp up portion of the soot porous body or sintered glass body; 30 - The vacuum inside the hollow soot porous body is varied in the range varying from about 15 to about 5 torr while collapsing top portion of the soot porous body or sintered glass body; 17 STER/P A/036 wherein the length of bottom portion (part) of the soot porous body or sintered glass body varies from about 0.32 X to about 0.36 X, the length of ramp down portion (part) of the soot porous body varies from about 0.36 X to about 0.42 X, the length of middle portion (part) of the soot porous body varies from about 5 0.42 X to about 0.79 X, the length of ramp up portion (part) of the soot porous body varies from about 0.79 X to about 0.88 X, and the length of top portion (part) of the soot porous body varies from about 0.88 X to IX. In accordance with one of the preferred embodiments of the present invention, the vacuum profile for process step of collapsing, whether 10 simultaneously or preceded by sintering step, is shown in accompanying Figure 6. In accordance with another preferred embodiment of the present invention, the vacuum profile for process step of collapsing, whether simultaneously or preceded by sintering step, is shown in accompanying Figure 15 7. In accordance with still another preferred embodiment of the present invention, the vacuum profile for process step of collapsing, whether simultaneously or preceded by sintering step, is shown in accompanying Figure 8. 20 In one embodiment, the present invention relates to an optical fiber preform having symmetrically and completely collapsed capillary when produced in accordance method of the present invention. In another embodiment, the present invention relates to a method for producing optical fiber having reduced PMD from optical fiber preform which is 25 produced in accordance method of the present invention. In still another embodiment, the present invention relates to an optical fiber having reduced PMD when produced in accordance with present invention. It has been observed that optical fiber preform having symmetrically and completely collapsed capillary prepared in accordance with present invention 30 does not show any seeds or bubbles, and hence, no breakage of fiber has been observed during the fiber draw process step. It has also been observed that optical fiber preform having symmetrically and completely collapsed capillary prepared in accordance with present 18 STER/PA/036 invention shows reduced core ovality in the optical fiber produced from such preform. It is apparent from the foregoing description that the presently disclosed method has overcome disadvantages, limitations and drawbacks of the prior 5 art. It may be noted that various terms, for example mandrel, soot 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 10 preform, sintered core rod, bottom portion, ramp down portion, middle portion, ramp up portion etc. as employed herein are merely intended to illustrate the present invention and are not intended to 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 15 the intended scope of the present invention. It may also be noted that the presently disclosed method has been 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 19 STER/P A/036 We Claim : 1. A method for producing optical fiber preform having symmetrically and completely collapsed capillary which is suitable for producing optical fiber having reduced PMD, comprising collapsing the capillary in the 5 hollow soot porous body or sintered glass body under variable vacuum. 2. A method for producing optical fiber preform having symmetrically and completely collapsed capillary comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; 10 ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; iv) dehydrating the hollow soot porous body in a suitable 15 environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) simultaneously sintering and collapsing the dehydrated soot porous body to collapse the capillary thereby to form mother preform; 20 characterized in that the simultaneous sintering and collapsing steps on dehydrated soot porous body to collapse the capillary and to form mother preform are carried out under variable vacuum. 3. A method as claimed in claim 2, further comprising drawing the fiber from the mother preform produced in process step-v). 25 4. A method as claimed in claim 2, further comprising steps of :- vi) reducing the diameter of the mother preform produced in process step-v) to form a core rod having reduced diameter; vii) overcladding the core rod having reduced diameter to form soot preform comprising soot porous body having core rod; 30 viii) sintering the soot preform to form a daughter preform. 5. A method as claimed in claim 4, further comprising drawing the fiber from the daughter preform produced in process step-viii). 20 STER/PA/036 6. A method for producing optical fiber preform having symmetrically and completely collapsed capillary comprising steps of: i) depositing soot particles on the mandrel to prepare a soot porous body having core and clad; 5 ii) removing the mandrel from the soot porous body to form hollow soot porous body having capillary therethrough; iii) inserting the plug in the bottom end of the capillary of the hollow soot porous body to close one end thereof; iv) dehydrating the hollow soot porous body in a suitable 10 environment to completely remove the moisture in core thereby to form dehydrated soot porous body; v) sintering the dehydrated soot porous body to form sintered glass body; vi) performing collapsing step on sintered glass body to collapse the 15 capillary thereby to form mother preform; characterized in that the collapsing step on sintered glass body to collapse the capillary and to form mother preform is carried out under variable vacuum. 7. A method as claimed in claim 6, further comprising drawing the fiber 20 from the mother preform produced in process step-vi). 8. A method as claimed in claim 6, further comprising steps of :- vii) reducing the diameter of the mother preform produced in process step-vi) to form a core rod having reduced diameter; viii) overcladding the core rod having reduced diameter to form soot 25 preform comprising soot porous body having core rod; ix) sintering the soot preform to form a daughter preform. 9. A method as claimed in claim 8, further comprising drawing the fiber from the daughter preform produced in process step-ix). 10. A method as claimed in any one of the preceding claims, wherein said 30 mother preform has diameter more than about 90 mm or weight more than about 9Kg. 11. A method as claimed in any one of the preceding claims, wherein vacuum is varied from bottom end to top end of the soot porous body 21 STER/PA/036 based on part of the soot porous body or sintered glass body being collapsed. 12. A method as claimed in claim 11, wherein vacuum for X length of soot porous body or sintered glass body is different in its bottom part, ramp 5 down part, middle part, ramp up part and top part. 13. A method as claimed in claim 11 or claim 12, wherein vacuum inside the body is varied in the range varying from about 130 to about 160 torr while collapsing the bottom portion of the body. 14. A method as claimed in claim 11 or claim 12, wherein vacuum inside the 10 body is reduced from a range of about 130 to about 160 torr in the bottom portion to a range varying from about 170 to about 190 torr while collapsing ramp down portion of the body. 15. A method as claimed in claim 11 or claim 12, wherein vacuum inside the body is varied in the range varying from about 170 to about 190 torr 15 while collapsing middle portion of the body. 16. A method as claimed in claim 11 or claim 12, wherein vacuum inside the body is increased from a range of about 170 to about 190 torr in the middle portion to a range varying from about 15 to about 5 torr while collapsing ramp up portion of the body. 20 17. A method as claimed in claim 11 or claim 12, wherein vacuum inside the body is varied in the range varying from about 15 to about 5 torr while collapsing top portion of the body. 18. A method as claimed in claim 12, wherein the length of bottom part of the body varies from about OX to about 0.36 X. 25 19. A method as claimed in claim 12, wherein the length of ramp down part of the body varies from about 0.36 X to about 0.42 X. 20. A method as claimed in claim 12, wherein the length of middle part of the body varies from about 0.42 X to about 0.79 X. 21. A method as claimed in claim 12, wherein the length of ramp up part of 30 the body varies from about 0.79 X to about 0.88 X. 22. A method as claimed in claim 12, wherein the length of top part of the body varies from about 0.88 X to IX. 22 STER/P A/036 23. An optical fiber preform having symmetrically and completely collapsed capillary when produced in accordance with any one of claims 1 to 22. 24. A method for producing optical fiber having reduced PMD from optical fiber preform produced in accordance with any one of the preceding claims 1 to 22. 25. An optical fiber having reduced PMD when produced in accordance with claim 24. 26. A method for producing optical fiber preform substantially as herein described with reference to and as illustrated in the accompanying drawings. 27. A method for producing optical fiber substantially as herein described with reference to and as illustrated in the accompanying drawings. [Dr. Ramesh Kr. MEHTA] Patent Attorney for the Applicants Of Mehta & Mehta Associates Dated this 13th day of October, 2006 23

Documents:

1712-mum-2006-abstract(1-8-2008).doc

1712-MUM-2006-ABSTRACT(1-8-2008).pdf

1712-mum-2006-abstract-1.jpg

1712-mum-2006-assignment(15-5-2008).pdf

1712-MUM-2006-CLAIMS(1-8-2008).pdf

1712-mum-2006-claims(granted)-(1-8-2008).doc

1712-mum-2006-claims(granted)-(1-8-2008).pdf

1712-mum-2006-claims.pdf

1712-mum-2006-correspondance-other.pdf

1712-MUM-2006-CORRESPONDENCE(1-8-2008).pdf

1712-mum-2006-correspondence(ipo)-(22-9-2008).pdf

1712-mum-2006-correspondence1(20-6-2008).pdf

1712-mum-2006-correspondence2(1-2-2007).pdf

1712-mum-2006-description (complete).pdf

1712-MUM-2006-DESCRIPTION(COMPLETE)-(1-8-2008).pdf

1712-MUM-2006-DRAWING(1-8-2008).pdf

1712-mum-2006-drawings0030.jpg

1712-mum-2006-drawings0031.jpg

1712-mum-2006-drawings0032.jpg

1712-mum-2006-drawings0033.jpg

1712-mum-2006-drawings0034.jpg

1712-mum-2006-drawings0035.jpg

1712-mum-2006-drawings0036.jpg

1712-MUM-2006-FORM 1(1-8-2008).pdf

1712-mum-2006-form 1(16-10-2006).pdf

1712-mum-2006-form 1(20-6-2008).pdf

1712-mum-2006-form 13(15-5-2008).pdf

1712-mum-2006-form 18(6-2-2007).pdf

1712-mum-2006-form 2(1-8-2008).pdf

1712-mum-2006-form 2(granted)-(1-8-2008).doc

1712-mum-2006-form 2(granted)-(1-8-2008).pdf

1712-MUM-2006-FORM 2(TITLE PAGE)-(1-8-2008).pdf

1712-mum-2006-form 26(13-5-2008).pdf

1712-mum-2006-form 3(15-5-2008).pdf

1712-mum-2006-form 3(20-6-2008).pdf

1712-mum-2006-form 5(15-5-2008).pdf

1712-mum-2006-form 5(20-6-2008).pdf

1712-mum-2006-form 9(15-12-2006).pdf

1712-mum-2006-form-2.doc

1712-mum-2006-form1.pdf

1712-mum-2006-form2.pdf

1712-mum-2006-form3.pdf

1712-mum-2006-form5.pdf

1712-mum-2006-other documents(14-5-2008).pdf

abstract1.jpg