Title of Invention	A PROCESS FOR PRODUCING FLAWLESS OPTICAL FIBER PREFORM AND A FIBER PRODUCED THEREFROM
Abstract	The present invention relates to a process for preparing the mandrel suitable for producing flawless optical fiber preform, wherein the mandrel can be easily removed from the soot porous body without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, wherein said process is characterized by heating the mandrel before the start of soot deposition to form a soot porous body having core and clad.

Title of Invention

A PROCESS FOR PRODUCING FLAWLESS OPTICAL FIBER PREFORM AND A FIBER PRODUCED THEREFROM

Abstract

The present invention relates to a process for preparing the mandrel suitable for producing flawless optical fiber preform, wherein the mandrel can be easily removed from the soot porous body without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, wherein said process is characterized by heating the mandrel before the start of soot deposition to form a soot porous body having core and clad.

Full Text	FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 PROVISIONAL SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention:- A process for preparing a mandrel for producing flawless optical fiber preform and a preform produced therefrom 2. Applicants :- (a) Name: STERLITE OPTICAL TECHNOLOGIES LTD. (b) Nationality: An Indian Company (c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA 3. Preamble to the Description:- Provisional Specification: The following specification particularly describes the nature of the Invention. STER/PA/028 Field of the Invention; The present invention relates to a process for preparing a mandrel 5 for producing flawless optical fiber preform and a fiber preform produced therefrom. Particularly, the present invention relates to a process for eliminating defects, such as voids, bubbles, impurities from centerline of optical fiber preform and a fiber preform produced therefrom. More particularly, the present invention relates to a process for preparing a 10 mandrel suitable for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced. Background of the Invention: 15 Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. Optical fiber comprises a core, to which essentially the entire signal is confined, and a clad surrounding the core. The optical fiber is manufactured in a way to 20 have core with higher refractive index in order to achieve light transmission inside the core region. The optical power also spreads in the cladding region near the core region. The optical fibers for telecommunication are required to operate 25 with the lowest possible attenuation loss. As the requirement for optical performance of optical fibers is stringent, the source of attenuation loss in optical fiber needs to be eliminated. However, certain physical constraints in the process for producing optical fiber preform, from which optical fiber is produced, can result in increase in attenuation loss of the 30 fiber and an important one of these physical constraints is formation of defects, such as voids, bubbles, impurities in the centerline of the preform when mandrel is removed to create a centerline in the preform. STERNA/028 The defects, such as voids, bubbles, impurities in the centerline of the preform have also been observed to propagate cracks and breaks in 5 the optical fiber produced from such preform. Therefore, if defects, such as voids, bubbles, impurities are formed in the centerline of the preform on removal of mandrel it results in increase in attenuation loss of the fiber and may also result in formation 10 of cracks and breaks in the fiber produced from such preform. Therefore, there is a need to have a process for preparing a mandrel which is suitable for producing a preform wherein the mandrel can be removed without causing any defects, such as voids, bubbles, 15 impurities in the centerline of the preform thus produced which is then suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein. The prior art teaches a process for producing a preform having 20 reduced formation of defects, such as voids, bubbles, impurities in the centerline of the preform thus produced by carefully removing the mandrel by grinding with the help of a diamond reamer, or by drilling the centerline while removing the mandrel followed by etching with the help of hydrofluoric acid. The main drawback of this method is that the 25 grinding or drilling leaves rough surface in the centerline which requires further processing by laser milling, mechanical polishing and/or fire polishing of the centerline and/or washing the centerline with hydrofluoric acid before further processing to collapsing step. All these process steps are not only time consuming, but also make the overall 30 process highly uneconomical for commercial application. Further, the centerline of the preform produced is still observed to have certain 3 STER/PA/028 defects, such as voids, bubbles, impurities thereby producing a fiber having high attenuation loss, and cracks and breaks therein. 5 Another prior art methods teach that the mandrel can be easily removed if it is made from a refractory material or a fused carbon, but it is observed that even the mandrel made from refractory material or fused carbon does leave behind certain defects, such as voids, bubbles, impurities in the centerline of the preform produced which produces a 10 fiber having high attenuation loss, and cracks and breaks therein. Accordingly, even these processes cannot produce preform having no defects, such as voids, bubbles, impurities in the centerline to produce a fiber having low attenuation loss, and no cracks and breaks therein. 15 Still another prior art method teaches that the mandrel can be easily removed if it is coated with carbon before depositing the soot particles. In accordance with this prior art, the mandrel is coated with carbon by directly exposing the mandrel to acetylene flame or by directly dipping the mandrel in carbon slurry or by directly dipping in wax and 20 then charring the mandrel. The main drawback of this method is that it still requires etching with hydrofluoric acid, which is not only time consuming, but also makes the overall process uneconomical for commercial application. The another drawback of this method is that if coating is done by dipping in carbon slurry or by dipping in wax, then it 25 requires additional step of drying in air for several hours which unduly makes the process highly time consuming. Further, it has been observed that even the mandrel coated with carbon by any one of the above three alternative methods does leave 30 behind certain defects, such as voids, bubbles, impurities in the centerline of the preform produced which produces a fiber having relatively high attenuation loss, and certain cracks and breaks therein. 4 STER/PA/028 Accordingly, even the mandrel coated with carbon by any one of the above three alternative methods cannot produce preform having no defects, such as voids, bubbles, impurities in the centerline to produce a 5 fiber having low attenuation loss, and no cracks and breaks therein. Need of the Invention: Therefore, the need of time, as described herein above, is to have a 10 process for preparing a mandrel which is suitable for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which should be capable of producing a fiber having very low attenuation loss and almost no cracks and breaks 15 therein. Objects of the Invention: The main object of the present invention is to have a process for 20 preparing a mandrel for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which is then capable of producing a fiber having very low attenuation loss and almost no cracks and breaks therein. 25 Another object of the present invention is to have an optical fiber preform having no defects, such as voids, bubbles, impurities in the centerline thereof. 30 Still another object of the present invention is to have an optical fiber having very low attenuation loss and almost no cracks and breaks therein. 5 STER/PA/028 Yet another object of the present invention is to have a process for producing a preform wherein the mandrel can be easily removed without 5 causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which is then capable of producing a fiber having very low attenuation loss and almost no cracks and breaks therein. 10 This is an object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed without step of grinding and/or drilling. This is another object of the present invention to have a process for 15 producing a preform wherein the mandrel can be easily removed without a step of etching with the help of an acid. This is still an object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed without 20 requiring steps of laser milling, mechanical polishing and/or fire polishing of the centerline and/or washing the centerline with an acid before further processing to steps of dehydration, consolidation and collapsing. 25 This is yet an object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed without wasting undue time, that is which is highly time saving, and also highly economical for commercial applications. 30 This is still another object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed even if it is made from a refractory material or from a fused 6 STER/PA/028 carbon, and still does not leave behind any defects, such as voids, bubbles, impurities in the centerline of the preform produced, and hence can produce a fiber having low attenuation loss, and no cracks and 5 breaks therein. This is yet another object of the present invention to provide a process for producing a preform wherein the mandrel can be easily removed even if it is not coated with carbon, and still does not leave 10 behind defects, such as voids, bubbles, impurities in the centerline of the preform produced, and hence can produce a fiber having low attenuation loss, and no cracks and breaks therein. The other objects and advantages of the present will be apparent 15 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. 20 Brief Description of the Accompanying Figures; Figure 1 shows mandrel hanging mechanism in accordance with present invention. 25 Figure 2 shows a schematic representation of deposition process over a mandrel prepared in accordance with present invention to produce a soot porous body. Figure 3 shows a schematic representation of hollow soot porous 30 body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous body. 7 STER/PA/028 Figure 4 shows a schematic cross-sectional view of hollow soot porous body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous 5 body. Figure 5 shows a schematic representation of hollow soot porous body in side the sintering furnace after removal of mandrel prepared in accordance with present invention from the soot porous body. 10 Figure 6 shows a hollow soot porous body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous body which is subjected to steps of dehydration, sintering and collapsing to produce a solid glass preform. 15 Description [nature] and preferred embodiments of the Invention: It is apparently clear from the forgoing description that the optical fiber preform produced by the mandrel prepared in accordance with 20 methods known in the prior art suffers from various drawbacks, disadvantages and limitations as described herein. The prior art methods as described herein above have been observed to produce a preform wherein the mandrel cannot be easily 25 removed without causing certain defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, and hence, the preform thus produced cannot produce a fiber having very low attenuation loss and almost no cracks and breaks therein. 30 It is known that defects in the centerline of the preform occur during removal of the mandrel from the soot porous body to form a hollow soot porous body. The present inventors have observed that the 8 STER/PA/028 main reason for causing various defects in the centerline of the preform while removing the mandrel from the soot porous body is due to rough surface of the mandrel. The prior art methods have made an attempt to 5 overcome this problem by providing a coating on the mandrel. However, it has been observed that even a coating does not overcome the problem completely. As described herein above, the mandrel with a coating still leaves certain defects in the centerhne of the preform produced from the coated mandrel. 10 It has been surprising observed by the inventors of the present invention that if mandrel for producing a preform is preheated before soot deposition or is preheated before depositing carbon coating, then the mandrel can be easily removed from the preform without causing any 15 defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein meaning thereby the above-described problems of the prior art methods can be overcome to a greater extent. 20 Accordingly, in first embodiment, the present invention relates to a process for preparing the mandrel suitable for producing flawless optical fiber preform, wherein the mandrel can be easily removed from the preform without causing any defects, such as voids, bubbles, impurities 25 in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process is characterized by heating the mandrel before soot deposition to form a soot porous body having core and clad. 30 Accordingly, in second embodiment, the present invention relates to a process for preparing the mandrel suitable for producing flawless 9 STER/PA/028 optical fiber preform, wherein the mandrel can be easily removed from the preform without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has 5 been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process is characterized by heating the mandrel before depositing carbon coating which is then subjected to soot deposition to form a soot porous body having core and clad. 10 In one embodiment, the mandrel prepared in accordance with present invention is employed for producing a flawless optical fiber preform. 15 Accordingly, in third embodiment, the present invention relates to a process for producing flawless optical fiber preform, particularly to a process for eliminating defects, such as voids, bubbles, impurities from centerline of optical fiber preform, more particularly to a process for producing a preform wherein the mandrel can be easily removed without 20 causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process comprises :- 25 i) providing a cylindrical and tapered mandrel; ii) depositing soot over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to the form hollow soot porous body; 30 iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture in core; 10 STER/PA/028 v) simultaneously sintering and collapsing the dehydrated hollow soot porous body inside the sintering furnace to form solid glass optical fiber preform having collapsed capillary; 5 characterized in that said mandrel is heated prior to step of deposition of soot particles thereon. In another embodiment, the mandrel prepared in accordance with present invention is provided with a coating and then employed for 10 producing a flawless optical fiber preform. Accordingly, in fourth embodiment, the present invention relates to a process for producing flawless optical fiber preform, particularly to a process for eliminating defects, such as voids, bubbles, impurities from 15 centerline of optical fiber preform, more particularly to a process for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks 20 and breaks therein wherein the process comprises:- i) providing a cylindrical and tapered mandrel; ii) depositing a carbon coating on said cylindrical mandrel; iii) depositing soot over said mandrel coated with carbon to form 25 soot porous body having core and clad; iv) detaching said mandrel from the soot porous body to the form hollow soot porous body; v) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture in core; 30 vi) simultaneously sintering and collapsing the dehydrated hollow soot porous body inside the sintering furnace to form solid glass optical fiber preform having collapsed capillary; 11 STER/PA/028 characterized in that said mandrel is heated prior to step of deposition of carbon coating thereon. 5 Now referring to accompanying Figure 1, in accordance with present invention, the mandrel 201 is suspended in the heating furnace [not shown] after covering with suspension means 202. The suspension means 202 comprises a glass tube 202 provided with a holder 204 which may also be referred as an anchor, which is suitable for suspending the 10 mandrel 201 in the heating furnace [not shown in Figure 1, refer to accompanying Figure 5]. In accordance with present invention, the mandrel can be selected from the conventionally available mandrels. However, the mandrel made 15 from a refractory material, preferably alumina or fused carbon material has shown better results. In accordance with one of the preferred embodiments of the present invention, the mandrel is cleaned with an acid for removal of 20 sticky material therefrom prior to a step of heating thereby producing a cleaned mandrel suitable for heat treatment in accordance with present invention. The acid selected is preferably hydrochloric acid or hydrofluoric acid. 25 Now referring to accompanying Figure 2, the soot porous body 1 can be prepared by any method known in the art. For example, by atmospheric chemical vapour deposition [ACVD] method. The preparation of soot porous body 1 comprises the following steps. The glass-forming precursor compounds are oxidized and hydrolyzed to form 30 porous silica based materials 2. The porous silica based materials 2 are deposited on a tapered cylindrical member referred as mandrel 3 which 12 STER/PA/028 is prepared in accordance with presently disclosed process and provided with a handle rod 4 and fitted on a lathe 5 to form soot porous body 1. As described herein, in accordance with one embodiment of the 5 present invention, the process for preparing a mandrel suitable for producing flawless preform is characterized by heating the mandrel made from a refractory material or from a fused carbon material before subjecting it to a step of deposition of carbon coating thereon. 10 Further, as described herein, in accordance with another embodiment of the present invention, the process for preparing a mandrel suitable for producing flawless preform is characterized by heating the mandrel made from a refractory material or from a fused carbon material before subjecting it to a step of deposition of soot 15 particles. In accordance with present invention, the preheated mandrel with or without carbon coating is subjected to step of deposition of soot particles. 20 In accordance with present invention, the preparation of mandrel is carried out by heating the mandrel at a temperature varying from about 400°C to about 800°C, preferably at a temperature varying from about 500°C to about 700°C. 25 In accordance with present invention, the mandrel is heating in a suitable heating furnace for a period varying from about 90 mins to about 210 mins, preferably for a period varying from about 120 mins to about 180 mins. 30 The cleaning of the mandrel as defined herein may be carried out by any known method, preferably it is carried out with an acid having 13 STER/PA/028 concentration varying from about 20% by weight to about 30% by weight, preferably of about 25% by weight. 5 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 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 deposited to form the core 10 of the preform and later the dopant chemicals may be terminated to form clad of the preform. After completion of deposition, the soot porous body 1 is removed from lathe 5 along with mandrel 3 and handle rod 4, and thereafter the 15 mandrel 3 which was prepared in accordance with process of present invention is detached 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 3]. 20 It has been observed upon examination of the centerline 9 of the hollow soot porous body 8 that it is free from defects, such as voids, bubbles, impurities. 25 The hollow soot porous body 8 thus formed comprises a core region 10 having a centerline hole 9, which is observed to be free from defects, such as voids, bubbles, impurities and clad region 11 of the optical fiber preform, and said core region 10 has refractive index greater than that of the clad region 11 [Figure 4]. 30 After detachment of mandrel 3 a centerline 9 free from defects, such as voids, bubbles, impurities is created inside the soot porous body 14 STER/PA/028 1. The amount of deposition of the clad region 11 and core region 10 is achieved to have any desired ratio diameter of clad region 11 to the diameter of core region 10. 5 Now referring to accompanying Figure 5, 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, and collapsing of the centerline 102 of the hollow soot porous body 101 10 to form a solid glass preform 103 [Figure 6] without requiring any step of drilling or grinding or etching of the centerline 9/102 before steps of consolidation and collapsing. Thus, the prepared hollow soot porous body 101 is dehydrated, sintered and collapsed to convert it into solid glass preform 103 without any step of drilling or grinding or etching with 15 acid thereby making the present process not only highly time saving, but also highly economical for commercial applications. Figure 5 shows exemplary embodiment to illustrate a general arrangement of the apparatus used in the present invention for 20 dehydration, sintering and collapsing of the hollow soot porous body 101. The hollow soot porous body 101, one end of which is provided with a plug 116 is inserted inside the furnace 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 25 comprises a glass muffle tube 110 having a diameter sufficient to accommodate the preform 101 and to adequately provide the environment necessary for dehydration, sintering and collapsing. The muffle tube 110 is heated to temperatures necessary for dehydration and simultaneous sintering and collapsing process steps with the heating 30 means (not shown) that is fitted to the sintering furnace 100. 15 STER/PA/028 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 5 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. The furnace 100 is provided with an inlet port 115 located suitably 10 on the furnace, preferably near the bottom of the muffle tube 110 for inserting desired gases in the furnace. The top end of the muffle tube 110 is 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, 15 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 the process. 20 The bottom end of the soot porous body 101 is closed with glass frustum 116 before inserting inside the sintering furnace 100. The top end of the hollow soot porous body 101 supported with the handle rod 106 is mounted over the coupler 108 with the help of ball 109. The coupler 108 is connected with the glass assembly rod 111. The inside of 25 the glass assembly rod 111 consists of smaller dimension glass tube 112 which is connected to the top end of the hollow soot porous body 101 through ball 109 and its opposite end 116A is connected to the vacuum generator (not shown) provided for achieving the required negative pressure inside the capillary 102 of the hollow soot porous body 101. 30 The solid glass preform 103 [Figure 6] produced after dehydration and sintering of the hollow soot porous body 101, and collapsing of the 16 STER/PA/028 centerline 9/102 of the hollow soot porous body 101 has been observed to be without any defects, etc. thereby confirming that the mandrel prepared in accordance with present invention is suitable for producing a 5 flawless preform without requiring any step of drilling or grinding or etching with acid or mechanical milling or fire polishing etc. and hence, the overall process for producing a flawless preform is not only highly time saving, but also highly economical for commercial applications. 10 The removal of mandrel 3 from the soot porous body 1 prepared in accordance with present process has been observed to be very easy without requiring any step of grinding and/or drilling and/or step of etching with an acid, and without causing any defects, such as voids, bubbles, impurities in the centerline 9/102 of the preform thus 15 produced, which have been confirmed upon examination of the centerline 9/102 created in the preform by presently disclosed process. Further, the removal of mandrel 3 from the soot porous body 1 prepared in accordance with present process has been observed to be 20 very easy without requiring any step of laser milling, mechanical polishing and/or fire polishing of the centerline 9/102 and/or washing the centerline 9/102 with an acid before further processing to steps of dehydration, consolidation and collapsing, and still has been observed to be without causing any defects, such as voids, bubbles, impurities in the 25 centerline of the preform thus produced, which, as stated herein above, have been confirmed upon examination of the centerline 9/102 created in the preform by presently disclosed process. The present process has also been found suitable for effecting the 30 removal of mandrel 3 without causing shearing of the core layer 10 and without forming flaky pitted centerline 9/102, and hence, has been 17 STER/PA/028 observed to produce a preform without formation of defects, such as voids, bubbles, impurities in its centerline 9/102. 5 It is apparently clear that the presently disclosed process for preparing a mandrel is not only highly time saving and economical for commercial applications, but it also makes the mandrel produced suitable for producing a flawless preform effecting removal of mandrel without causing defects, such as bubbles, voids, impurities in the 10 centerline created in the hollow soot porous body. It is also apparently clear that the presently disclosed process for preparing a mandrel suitable for producing a flawless preform makes the process for producing a flawless preform not only highly time saving, but 15 also highly economical by avoiding steps of grinding and/or drilling and/or etching with an acid, and/or laser miling and/or mechanical polishing and/or fire polishing of the centerline created by easy removal of mandrel from the soot porous body. 20 The preform produced by employing mandrel prepared in accordance with present process has also been observed to produce a fiber having very low attenuation loss and almost no cracks and breaks therein. 25 Therefore, it is also apparently clear that the presently disclosed process for preparing a mandrel suitable for producing a flawless preform makes the process for producing optical fiber having very low attenuation loss and almost no cracks and breaks therein not only highly time saving, but also highly economical by avoiding steps of grinding and/or drilling 30 and/or etching with an acid, and/or laser miling and/or mechanical polishing and/or fire polishing of the centerline created by easy removal of mandrel from the soot porous body. 18 STER/PA/028 The present method has been observed to be suitable for a preform even if cladding region is provided with overclad region. 5 The present method has been described for ACVD method. However, it is suitable even for all other known methods for manufacturing optical performs. 10 Dated this 22nd day of August, 2006. [Dr. Ramesh Kr. Mehta] Patent Attorney for the Applicants 15 Of Mehta & Mehta Associates 19

Full Text

FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULES, 2003
PROVISIONAL SPECIFICATION
(See Section 10 and Rule 13)
1. Title of the Invention:-
A process for preparing a mandrel for producing flawless optical fiber
preform and a preform produced therefrom
2. Applicants :-
(a) Name: STERLITE OPTICAL TECHNOLOGIES LTD.
(b) Nationality: An Indian Company
(c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA
3. Preamble to the Description:-
Provisional Specification:
The following specification particularly describes the nature of the
Invention.

STER/PA/028
Field of the Invention;
The present invention relates to a process for preparing a mandrel
5 for producing flawless optical fiber preform and a fiber preform produced therefrom. Particularly, the present invention relates to a process for eliminating defects, such as voids, bubbles, impurities from centerline of optical fiber preform and a fiber preform produced therefrom. More particularly, the present invention relates to a process for preparing a
10 mandrel suitable for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced.
Background of the Invention:
15
Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. Optical fiber comprises a core, to which essentially the entire signal is confined, and a clad surrounding the core. The optical fiber is manufactured in a way to
20 have core with higher refractive index in order to achieve light transmission inside the core region. The optical power also spreads in the cladding region near the core region.
The optical fibers for telecommunication are required to operate
25 with the lowest possible attenuation loss. As the requirement for optical performance of optical fibers is stringent, the source of attenuation loss in optical fiber needs to be eliminated. However, certain physical constraints in the process for producing optical fiber preform, from which optical fiber is produced, can result in increase in attenuation loss of the
30 fiber and an important one of these physical constraints is formation of defects, such as voids, bubbles, impurities in the centerline of the preform when mandrel is removed to create a centerline in the preform.

STERNA/028
The defects, such as voids, bubbles, impurities in the centerline of the preform have also been observed to propagate cracks and breaks in
5 the optical fiber produced from such preform.
Therefore, if defects, such as voids, bubbles, impurities are formed in the centerline of the preform on removal of mandrel it results in increase in attenuation loss of the fiber and may also result in formation
10 of cracks and breaks in the fiber produced from such preform.
Therefore, there is a need to have a process for preparing a mandrel which is suitable for producing a preform wherein the mandrel can be removed without causing any defects, such as voids, bubbles, 15 impurities in the centerline of the preform thus produced which is then suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein.
The prior art teaches a process for producing a preform having
20 reduced formation of defects, such as voids, bubbles, impurities in the
centerline of the preform thus produced by carefully removing the
mandrel by grinding with the help of a diamond reamer, or by drilling the
centerline while removing the mandrel followed by etching with the help
of hydrofluoric acid. The main drawback of this method is that the
25 grinding or drilling leaves rough surface in the centerline which requires
further processing by laser milling, mechanical polishing and/or fire
polishing of the centerline and/or washing the centerline with
hydrofluoric acid before further processing to collapsing step. All these
process steps are not only time consuming, but also make the overall
30 process highly uneconomical for commercial application. Further, the
centerline of the preform produced is still observed to have certain
3

STER/PA/028
defects, such as voids, bubbles, impurities thereby producing a fiber having high attenuation loss, and cracks and breaks therein.
5 Another prior art methods teach that the mandrel can be easily
removed if it is made from a refractory material or a fused carbon, but it is observed that even the mandrel made from refractory material or fused carbon does leave behind certain defects, such as voids, bubbles, impurities in the centerline of the preform produced which produces a
10 fiber having high attenuation loss, and cracks and breaks therein. Accordingly, even these processes cannot produce preform having no defects, such as voids, bubbles, impurities in the centerline to produce a fiber having low attenuation loss, and no cracks and breaks therein.
15 Still another prior art method teaches that the mandrel can be
easily removed if it is coated with carbon before depositing the soot particles. In accordance with this prior art, the mandrel is coated with carbon by directly exposing the mandrel to acetylene flame or by directly dipping the mandrel in carbon slurry or by directly dipping in wax and
20 then charring the mandrel. The main drawback of this method is that it still requires etching with hydrofluoric acid, which is not only time consuming, but also makes the overall process uneconomical for commercial application. The another drawback of this method is that if coating is done by dipping in carbon slurry or by dipping in wax, then it
25 requires additional step of drying in air for several hours which unduly makes the process highly time consuming.
Further, it has been observed that even the mandrel coated with
carbon by any one of the above three alternative methods does leave
30 behind certain defects, such as voids, bubbles, impurities in the
centerline of the preform produced which produces a fiber having
relatively high attenuation loss, and certain cracks and breaks therein.
4

STER/PA/028
Accordingly, even the mandrel coated with carbon by any one of the above three alternative methods cannot produce preform having no defects, such as voids, bubbles, impurities in the centerline to produce a
5 fiber having low attenuation loss, and no cracks and breaks therein.
Need of the Invention:
Therefore, the need of time, as described herein above, is to have a
10 process for preparing a mandrel which is suitable for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which should be capable of producing a fiber having very low attenuation loss and almost no cracks and breaks
15 therein.
Objects of the Invention:
The main object of the present invention is to have a process for
20 preparing a mandrel for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which is then capable of producing a fiber having very low attenuation loss and almost no cracks and breaks therein.
25
Another object of the present invention is to have an optical fiber preform having no defects, such as voids, bubbles, impurities in the centerline thereof.
30 Still another object of the present invention is to have an optical
fiber having very low attenuation loss and almost no cracks and breaks therein.
5

STER/PA/028
Yet another object of the present invention is to have a process for producing a preform wherein the mandrel can be easily removed without
5 causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which is then capable of producing a fiber having very low attenuation loss and almost no cracks and breaks therein.
10 This is an object of the present invention to have a process for
producing a preform wherein the mandrel can be easily removed without step of grinding and/or drilling.
This is another object of the present invention to have a process for
15 producing a preform wherein the mandrel can be easily removed without a step of etching with the help of an acid.
This is still an object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed without
20 requiring steps of laser milling, mechanical polishing and/or fire polishing of the centerline and/or washing the centerline with an acid before further processing to steps of dehydration, consolidation and collapsing.
25 This is yet an object of the present invention to have a process for
producing a preform wherein the mandrel can be easily removed without wasting undue time, that is which is highly time saving, and also highly economical for commercial applications.
30 This is still another object of the present invention to have a
process for producing a preform wherein the mandrel can be easily removed even if it is made from a refractory material or from a fused
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carbon, and still does not leave behind any defects, such as voids, bubbles, impurities in the centerline of the preform produced, and hence can produce a fiber having low attenuation loss, and no cracks and
5 breaks therein.
This is yet another object of the present invention to provide a process for producing a preform wherein the mandrel can be easily removed even if it is not coated with carbon, and still does not leave
10 behind defects, such as voids, bubbles, impurities in the centerline of the preform produced, and hence can produce a fiber having low attenuation loss, and no cracks and breaks therein.
The other objects and advantages of the present will be apparent
15 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.
20 Brief Description of the Accompanying Figures;
Figure 1 shows mandrel hanging mechanism in accordance with present invention.
25 Figure 2 shows a schematic representation of deposition process
over a mandrel prepared in accordance with present invention to produce a soot porous body.
Figure 3 shows a schematic representation of hollow soot porous
30 body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous body.
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Figure 4 shows a schematic cross-sectional view of hollow soot porous body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous
5 body.
Figure 5 shows a schematic representation of hollow soot porous body in side the sintering furnace after removal of mandrel prepared in accordance with present invention from the soot porous body.
10
Figure 6 shows a hollow soot porous body having centerline therethrough after removal of mandrel prepared in accordance with present invention from the soot porous body which is subjected to steps of dehydration, sintering and collapsing to produce a solid glass preform.
15
Description [nature] and preferred embodiments of the Invention:
It is apparently clear from the forgoing description that the optical fiber preform produced by the mandrel prepared in accordance with
20 methods known in the prior art suffers from various drawbacks, disadvantages and limitations as described herein.
The prior art methods as described herein above have been observed to produce a preform wherein the mandrel cannot be easily
25 removed without causing certain defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, and hence, the preform thus produced cannot produce a fiber having very low attenuation loss and almost no cracks and breaks therein.
30 It is known that defects in the centerline of the preform occur
during removal of the mandrel from the soot porous body to form a hollow soot porous body. The present inventors have observed that the
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main reason for causing various defects in the centerline of the preform while removing the mandrel from the soot porous body is due to rough surface of the mandrel. The prior art methods have made an attempt to
5 overcome this problem by providing a coating on the mandrel. However, it has been observed that even a coating does not overcome the problem completely. As described herein above, the mandrel with a coating still leaves certain defects in the centerhne of the preform produced from the coated mandrel.
10
It has been surprising observed by the inventors of the present invention that if mandrel for producing a preform is preheated before soot deposition or is preheated before depositing carbon coating, then the mandrel can be easily removed from the preform without causing any
15 defects, such as voids, bubbles, impurities in the centerline of the preform thus produced which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein meaning thereby the above-described problems of the prior art methods can be overcome to a greater extent.
20
Accordingly, in first embodiment, the present invention relates to a process for preparing the mandrel suitable for producing flawless optical fiber preform, wherein the mandrel can be easily removed from the preform without causing any defects, such as voids, bubbles, impurities
25 in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process is characterized by heating the mandrel before soot deposition to form a soot porous body having core and clad.
30
Accordingly, in second embodiment, the present invention relates to a process for preparing the mandrel suitable for producing flawless
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optical fiber preform, wherein the mandrel can be easily removed from the preform without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has
5 been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process is characterized by heating the mandrel before depositing carbon coating which is then subjected to soot deposition to form a soot porous body having core and clad.
10
In one embodiment, the mandrel prepared in accordance with present invention is employed for producing a flawless optical fiber preform.
15 Accordingly, in third embodiment, the present invention relates to
a process for producing flawless optical fiber preform, particularly to a process for eliminating defects, such as voids, bubbles, impurities from centerline of optical fiber preform, more particularly to a process for producing a preform wherein the mandrel can be easily removed without
20 causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks and breaks therein, wherein the process comprises :-
25 i) providing a cylindrical and tapered mandrel;
ii) depositing soot over said mandrel to form soot porous body
having core and clad;
iii) detaching said mandrel from the soot porous body to the form
hollow soot porous body;

30 iv) dehydrating the hollow soot porous body in an environment
suitable to completely remove the moisture in core;
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v) simultaneously sintering and collapsing the dehydrated hollow
soot porous body inside the sintering furnace to form solid glass
optical fiber preform having collapsed capillary;
5 characterized in that said mandrel is heated prior to step of deposition
of soot particles thereon.
In another embodiment, the mandrel prepared in accordance with present invention is provided with a coating and then employed for
10 producing a flawless optical fiber preform.
Accordingly, in fourth embodiment, the present invention relates to a process for producing flawless optical fiber preform, particularly to a process for eliminating defects, such as voids, bubbles, impurities from
15 centerline of optical fiber preform, more particularly to a process for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities in the centerline of the preform thus produced, which has been found suitable for producing a fiber having very low attenuation loss and almost no cracks
20 and breaks therein wherein the process comprises:-
i) providing a cylindrical and tapered mandrel;
ii) depositing a carbon coating on said cylindrical mandrel;
iii) depositing soot over said mandrel coated with carbon to form
25 soot porous body having core and clad;
iv) detaching said mandrel from the soot porous body to the form
hollow soot porous body;
v) dehydrating the hollow soot porous body in an environment
suitable to completely remove the moisture in core;
30 vi) simultaneously sintering and collapsing the dehydrated hollow
soot porous body inside the sintering furnace to form solid glass
optical fiber preform having collapsed capillary;
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characterized in that said mandrel is heated prior to step of deposition of carbon coating thereon.
5 Now referring to accompanying Figure 1, in accordance with
present invention, the mandrel 201 is suspended in the heating furnace [not shown] after covering with suspension means 202. The suspension means 202 comprises a glass tube 202 provided with a holder 204 which may also be referred as an anchor, which is suitable for suspending the
10 mandrel 201 in the heating furnace [not shown in Figure 1, refer to accompanying Figure 5].

In accordance with present invention, the mandrel can be selected from the conventionally available mandrels. However, the mandrel made
15 from a refractory material, preferably alumina or fused carbon material has shown better results.
In accordance with one of the preferred embodiments of the present invention, the mandrel is cleaned with an acid for removal of
20 sticky material therefrom prior to a step of heating thereby producing a cleaned mandrel suitable for heat treatment in accordance with present invention. The acid selected is preferably hydrochloric acid or hydrofluoric acid.
25 Now referring to accompanying Figure 2, the soot porous body 1
can be prepared by any method known in the art. For example, by atmospheric chemical vapour deposition [ACVD] method. The preparation of soot porous body 1 comprises the following steps. The glass-forming precursor compounds are oxidized and hydrolyzed to form
30 porous silica based materials 2. The porous silica based materials 2 are deposited on a tapered cylindrical member referred as mandrel 3 which
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is prepared in accordance with presently disclosed process and provided with a handle rod 4 and fitted on a lathe 5 to form soot porous body 1.
As described herein, in accordance with one embodiment of the
5 present invention, the process for preparing a mandrel suitable for producing flawless preform is characterized by heating the mandrel made from a refractory material or from a fused carbon material before subjecting it to a step of deposition of carbon coating thereon.
10 Further, as described herein, in accordance with another
embodiment of the present invention, the process for preparing a mandrel suitable for producing flawless preform is characterized by heating the mandrel made from a refractory material or from a fused carbon material before subjecting it to a step of deposition of soot
15 particles.
In accordance with present invention, the preheated mandrel with or without carbon coating is subjected to step of deposition of soot particles.
20
In accordance with present invention, the preparation of mandrel is carried out by heating the mandrel at a temperature varying from about 400°C to about 800°C, preferably at a temperature varying from about 500°C to about 700°C.
25
In accordance with present invention, the mandrel is heating in a suitable heating furnace for a period varying from about 90 mins to about 210 mins, preferably for a period varying from about 120 mins to about 180 mins.
30
The cleaning of the mandrel as defined herein may be carried out by any known method, preferably it is carried out with an acid having
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concentration varying from about 20% by weight to about 30% by weight, preferably of about 25% by weight.
5 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 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 deposited to form the core
10 of the preform and later the dopant chemicals may be terminated to form clad of the preform.
After completion of deposition, the soot porous body 1 is removed from lathe 5 along with mandrel 3 and handle rod 4, and thereafter the
15 mandrel 3 which was prepared in accordance with process of present invention is detached 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 3].
20
It has been observed upon examination of the centerline 9 of the hollow soot porous body 8 that it is free from defects, such as voids, bubbles, impurities.
25 The hollow soot porous body 8 thus formed comprises a core
region 10 having a centerline hole 9, which is observed to be free from defects, such as voids, bubbles, impurities and clad region 11 of the optical fiber preform, and said core region 10 has refractive index greater than that of the clad region 11
[Figure 4].
30
After detachment of mandrel 3 a centerline 9 free from defects, such as voids, bubbles, impurities is created inside the soot porous body
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1. The amount of deposition of the clad region 11 and core region 10 is achieved to have any desired ratio diameter of clad region 11 to the diameter of core region 10.
5
Now referring to accompanying Figure 5, 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, and collapsing of the centerline 102 of the hollow soot porous body 101
10 to form a solid glass preform 103 [Figure 6] without requiring any step of drilling or grinding or etching of the centerline 9/102 before steps of consolidation and collapsing. Thus, the prepared hollow soot porous body 101 is dehydrated, sintered and collapsed to convert it into solid glass preform 103 without any step of drilling or grinding or etching with
15 acid thereby making the present process not only highly time saving, but also highly economical for commercial applications.
Figure 5 shows exemplary embodiment to illustrate a general arrangement of the apparatus used in the present invention for
20 dehydration, sintering and collapsing of the hollow soot porous body 101. The hollow soot porous body 101, one end of which is provided with a plug 116 is inserted inside the furnace 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
25 comprises a glass muffle tube 110 having a diameter sufficient to accommodate the preform 101 and to adequately provide the environment necessary for dehydration, sintering and collapsing. The muffle tube 110 is heated to temperatures necessary for dehydration and simultaneous sintering and collapsing process steps with the heating
30 means (not shown) that is fitted to the sintering furnace 100.
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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
5 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.
The furnace 100 is provided with an inlet port 115 located suitably
10 on the furnace, preferably near the bottom of the muffle tube 110 for inserting desired gases in the furnace. The top end of the muffle tube 110 is 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,
15 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 the process.
20 The bottom end of the soot porous body 101 is closed with glass
frustum 116 before inserting inside the sintering furnace 100. The top end of the hollow soot porous body 101 supported with the handle rod 106 is mounted over the coupler 108 with the help of ball 109. The coupler 108 is connected with the glass assembly rod 111. The inside of
25 the glass assembly rod 111 consists of smaller dimension glass tube 112 which is connected to the top end of the hollow soot porous body 101 through ball 109 and its opposite end 116A is connected to the vacuum generator (not shown) provided for achieving the required negative pressure inside the capillary 102 of the hollow soot porous body 101.
30
The solid glass preform 103 [Figure 6] produced after dehydration and sintering of the hollow soot porous body 101, and collapsing of the
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centerline 9/102 of the hollow soot porous body 101 has been observed to be without any defects, etc. thereby confirming that the mandrel prepared in accordance with present invention is suitable for producing a
5 flawless preform without requiring any step of drilling or grinding or etching with acid or mechanical milling or fire polishing etc. and hence, the overall process for producing a flawless preform is not only highly time saving, but also highly economical for commercial applications.
10 The removal of mandrel 3 from the soot porous body 1 prepared in
accordance with present process has been observed to be very easy without requiring any step of grinding and/or drilling and/or step of etching with an acid, and without causing any defects, such as voids, bubbles, impurities in the centerline 9/102 of the preform thus
15 produced, which have been confirmed upon examination of the centerline 9/102 created in the preform by presently disclosed process.
Further, the removal of mandrel 3 from the soot porous body 1 prepared in accordance with present process has been observed to be
20 very easy without requiring any step of laser milling, mechanical polishing and/or fire polishing of the centerline 9/102 and/or washing the centerline 9/102 with an acid before further processing to steps of dehydration, consolidation and collapsing, and still has been observed to be without causing any defects, such as voids, bubbles, impurities in the
25 centerline of the preform thus produced, which, as stated herein above, have been confirmed upon examination of the centerline 9/102 created in the preform by presently disclosed process.
The present process has also been found suitable for effecting the
30 removal of mandrel 3 without causing shearing of the core layer 10 and
without forming flaky pitted centerline 9/102, and hence, has been
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observed to produce a preform without formation of defects, such as voids, bubbles, impurities in its centerline 9/102.
5 It is apparently clear that the presently disclosed process for
preparing a mandrel is not only highly time saving and economical for commercial applications, but it also makes the mandrel produced suitable for producing a flawless preform effecting removal of mandrel without causing defects, such as bubbles, voids, impurities in the
10 centerline created in the hollow soot porous body.
It is also apparently clear that the presently disclosed process for preparing a mandrel suitable for producing a flawless preform makes the process for producing a flawless preform not only highly time saving, but
15 also highly economical by avoiding steps of grinding and/or drilling and/or etching with an acid, and/or laser miling and/or mechanical polishing and/or fire polishing of the centerline created by easy removal of mandrel from the soot porous body.
20 The preform produced by employing mandrel prepared in
accordance with present process has also been observed to produce a fiber having very low attenuation loss and almost no cracks and breaks therein.
25 Therefore, it is also apparently clear that the presently disclosed process for preparing a mandrel suitable for producing a flawless preform makes the process for producing optical fiber having very low attenuation loss and almost no cracks and breaks therein not only highly time saving, but also highly economical by avoiding steps of grinding and/or drilling
30 and/or etching with an acid, and/or laser miling and/or mechanical polishing and/or fire polishing of the centerline created by easy removal of mandrel from the soot porous body.
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The present method has been observed to be suitable for a preform even if cladding region is provided with overclad region.
5
The present method has been described for ACVD method. However, it is suitable even for all other known methods for manufacturing optical performs.
10 Dated this 22nd day of August, 2006.
[Dr. Ramesh Kr. Mehta]
Patent Attorney for the Applicants
15 Of Mehta & Mehta Associates
19

A PROCESS FOR PRODUCING FLAWLESS OPTICAL FIBER PREFORM AND A FIBER PRODUCED THEREFROM

Documents:

Inventors:

PCT Conventions: