WO2008038298A2

WO2008038298A2 - A process for preparing a mandrel for producing flawless optical fiber preform and a preform produced therefrom

Info

Publication number: WO2008038298A2
Application number: PCT/IN2007/000306
Authority: WO
Inventors: Sanket Shah; Jegan Miras; Parag Kulkarni
Original assignee: Sterlite Optical Technologies Ltd.
Priority date: 2006-08-25
Filing date: 2007-07-24
Publication date: 2008-04-03
Also published as: WO2008038298A3

Abstract

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.

Description

Title of the Invention:

A process for preparing a mandrel for producing flawless optical fiber preform and a preform produced therefrom Field of the Invention:

The present invention relates to a process for preparing a mandrel 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 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:

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 have core with higher refractive index than clad 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 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 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.

The defects, such as voids, bubbles, impurities in the centerline of the preform have also been observed to propagate cracks and breaks in 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 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, impurities, uncollapsed portion in the centerline region 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 [US Patent No. 3,823,995] teaches a process for producing a preform having 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 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 process highly uneconomical for commercial application. Further, the centerline of the preform produced is still observed to have certain defects, such as voids, bubbles, impurities, uncollapsed portion thereby producing a fiber having high attenuation loss, and cracks and breaks therein.

Another prior art [US Patent No. 3,933,453] 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, uncollapsed portion in the centerline region of the preform produced which produces a 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.

Still another prior art [US Patent No. 4,204,850] 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 then charring the mandrel. The main drawback of this method is that it still requires etching of capillary at sintered glass preform stage 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 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 does leave behind certain defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform produced which produces a fiber having relatively high attenuation loss, and certain cracks and breaks therein. Accordingly, even the mandrel only coated with carbon does not produce preform having no defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region to produce a 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 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, uncollapsed portion in the centerline region of the preform thus produced which should be capable of producing a fiber having very low attenuation loss and almost no cracks and breaks therein. Objects of the Invention: The main object of the present invention is to have a process for preparing a mandrel for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region 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.

Another object of the present invention is to have an optical fiber preform having no defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region thereof.

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.

Yet another object of the present invention is to have a process for producing a preform wherein the mandrel can be easily removed without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region 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. 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 producing a preform wherein the mandrel can be easily removed without requiring step of etching of capillary of sintered glass preform with the help of an acid - hydrofluoric 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 requiring steps of laser milling, mechanical polishing and/or fire polishing of the centerline and/or washing the centerline of sintered glass preform before further processing to steps of collapsing.

This is yet an object of the present invention to have a process for producing a preform wherein the mandrel can be easily removed with comfort and without wasting undue time, that is which is highly time saving, and also highly economical for commercial applications.

This is still another object of the present invention to have a process for producing a preform wherein the mandrel made from a refractory material or from a fused carbon or any other material can be easily removed, and still does not leave behind any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, and hence, is suitable for use in production of optical fiber having low attenuation loss, and almost no cracks and 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 behind defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, and hence, is suitable for use in production of optical fiber having low attenuation loss, and almost no cracks and breaks therein. _N Other objects and advantages of the present will be apparent from the following description when read in conjunction with the following examples and accompanying drawings which are incorporated for illustration of present invention and are not intended to limit scope thereof. Brief Description of the Accompanying Figures:

Figure IA illustrates mandrel suspension means in accordance with one preferred embodiment of the present invention. Figure IB illustrates suspension tube of mandrel suspension means of present invention as illustrated in Figure IA.

Figure 1C illustrates mandrel of mandrel suspension means of present invention as illustrated in Figure IA.

Figure ID illustrates holding means of mandrel suspension means of present invention as illustrated in Figure IA.

Figure IE illustrates closing means of mandrel suspension means of present invention as illustrated in Figure IA.

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 body having centerline therethrough after removal of mandrel, prepared in accordance with present invention, from the soot porous body.

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 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. 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 sintered [solid] glass preform. Description 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 methods known in the prior art suffers from various drawbacks, disadvantages and limitations as described herein. The prior art methods as described herein have been observed to produce a preform wherein the mandrel cannot be easily removed from soot porous body without causing certain defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region 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.

It has been observed that defects in the centerline region 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 main reason for causing various defects in the centerline region 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 overcome this problem by only providing a carbon coating on the mandrel. However, it has been observed that only a carbon coating does not overcome above described problems and drawbacks completely. As described herein above, the mandrel only with carbon coating still leaves certain defects in the centerline region of the preform thus produced from the carbon coated mandrel.

It has been surprising observed by the inventors of the present invention that if mandrel employed for producing a preform is preheated before start of soot deposition to prepare soot porous body, then 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 which has been found to be suitable for producing a fiber having very low attenuation loss, and almost no cracks and breaks therein meaning thereby the above -de scribed problems of the prior art methods can be overcome to a greater extent. It has also been surprising observed by the inventors of the present invention that if mandrel employed for producing a preform is preheated before depositing one or more coatings from a group consisting of carbon and soft soot before start of soot deposition to prepare soot porous body, then 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 which has been found to be 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. 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 soot porous body without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, which has been found to be 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 the start of soot deposition to form a soot porous body having core and clad.

In accordance with one of the preferred embodiments of this invention, the carbon coating is deposited on the heated mandrel before the start of soot deposition to form a soot porous body having core and clad. Accordingly, in second 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 soot porous body without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, which has been found to be suitable for producing a fiber having very low attenuation loss and, almost no cracks and breaks therein _f wherein the process is characterized by heating the mandrel and depositing carbon coating thereon before the start of soot deposition to form a soot porous body having core and clad. In accordance with another preferred embodiment of this invention, the soft soot coating is deposited on the heated mandrel_^ which is subsequently removed to have smooth and clean surface before the start of soot deposition to form a soot porous body having core and clad.

Accordingly, in third 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 soot porous body without causing any defects, such as voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, which has been found to be 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 and depositing coating of soft soot thereon which is subsequently removed to have smooth and clean surface of mandrel before the start of soot deposition to form a soot porous body having core and clad. v In accordance with still another preferred embodiment of this invention, the carbon coating and soft soot coating are deposited on the heated mandrel which are subsequently removed to have smooth and clean surface of mandrel before the start of soot deposition to form a soot porous body having core and clad.

Accordingly, in fourth 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 soot porous body without causing any defects, such as > voids, bubbles, impurities, uncollapsed portion in the centerline region of the preform thus produced, which has been found to be 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 and depositing carbon coating followed by coating of soft soot thereon which are subsequently removed to have smooth and clean surface of the mandrel before the start of soot deposition to form a soot porous body having core and clad. In accordance with present invention, the mandrel prepared in accordance with any one of the above embodiments is employed for producing soot porous body which is dehydrated, sintered and collapsed to produce flawless optical fiber preform [also referred to as mother preform].

Accordingly, in accordance with first embodiment of this invention, the process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) • depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated before the start of step of deposition of soot particles thereon.

Accordingly, in accordance with second embodiment of this invention, the process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated and carbon coating is deposited thereon before the start of step of deposition of soot particles thereon.

Accordingly, in accordance with third embodiment of this invention, the process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated and soft soot coating is deposited thereon, which is subsequently removed to have smooth and clean surface of mandrel before the start of step of deposition of soot particles thereon.

Accordingly, in accordance with fourth embodiment of this invention, the process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated, and carbon coating and soft soot coating are deposited thereon, which are subsequently removed to have smooth and clean surface of mandrel before the start of step of deposition of soot particles thereon.

The term soft soot as employed herein means depositing soot layer having low density varying between about 0.3 to about 0.45 gm/cm³.

In accordance with one of the preferred embodiments of this invention, the mandrel is heated in a furnace after suspending it in a suspension means to avoid contamination of its surface.

Accordingly, in fifth embodiment, the present invention relates to a mandrel suspension means suitable for suspending the mandrel in a furnace [not shown] comprising a suspension tube including neck portion and mouth wherein mandrel is suspended in suspension tube after fixing a holding means on one end thereof and suspension tube is closed with a closing means before placing in the furnace.

Now referring to accompanying Figure 1, in accordance with fifth embodiment of this invention a mandrel suspension means 200 suitable for suspending the mandrel 201 in a furnace [not shown] comprises a suspension tube 202 having a cylindrical body 203 including neck portion 204 and a mouth portion 205 wherein mandrel 201 is suspended in suspension tube 202 after fixing a holding means 206 on one end thereof and suspension tube is closed with a closing means 207 provided with an opening 208 for release of expanded air on heating. In accordance with this invention, the mandrel 201 fitted with holding means 206 is made to rest above neck portion 204.

In accordance with present invention the suspension tube, holding means and closing means can be made of any suitable material capable of withstanding a temperature of about 900⁰C. In accordance with one of the preferred embodiments of the present invention, the suspension tube, holding means and closing means are made of quartz glass capable of withstanding a temperature of about 900⁰C.

In accordance with one of the preferred embodiments of the present invention, the mandrel is cleaned for removal of sticky material therefrom prior to a step of heating thereby producing a cleaned mandrel suitable for heat treatment in accordance with present invention. The cleaning of the mandrel may be carried out by any known method, preferably it is carried out with an acid. The acid selected is preferably hydrochloric acid having concentration varying from about 20% by weight to about 30% by weight, preferably of about 25% by weight.

In accordance with present invention, the mandrel is heated at a temperature varying from about 400⁰C to about 800⁰C, preferably at a temperature varying from about 500⁰C to about 700⁰C.

It has been observed that if mandrel is heated above a temperature of 800⁰C, the mandrel becomes reddish and brittle due to excess heat which has been found to reduce its life, and if it is heated below a temperature of 400⁰C, the heating has been found to be ineffective to create a desired smooth and clean surface.

In accordance with present invention, the mandrel is heated 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.

It has been observed that if mandrel is heated above a temperature of 800⁰C for a duration less than 90 mins, the heating has been found to be ineffective to create a desired smooth and clean surface, and if mandrel is heated above a temperature of 800⁰C for a duration more than 210 mins, the mandrel becomes reddish and brittle due to excess heat which has been found to recluce its life. It has also been observed that if mandrel is heated at a temperature lower than 400⁰C for a duration less than 90 mins, the heating has been found to be ineffective to create a desired smooth and clean surface, and if mandrel is heated at a temperature lower than 400⁰C for a duration more than 210 mins, the heating has been found to be ineffective to create a desired smooth and clean surface.

In accordance with present invention, the mandrel can be selected from the conventionally available mandrels. However, the mandrel made from a refractory material, preferably alumina or fused carbon material. In accordance with present invention, carbon coating on the heat treated mandrel can be done by any known method. However, in accordance with preferred embodiment of the present invention, the carbon coating is done by employing acetylene flame preferably with flow rate varying from about 3 lpm [liters per minute] to about 5 lpm for thickness varying from about 1 mm to about 3 mm to achieve a carbon coating making its removal comfortable. It has been observed that if thickness of carbon coating is less than 1 mm, the removal of mandrel becomes difficult and if thickness of carbon coating is more than 3 mm, the carbon coating will not disintegrate and residuals of carbon will remain inside the centerline of soot porous body thereby contaminate the same.

In accordance with present invention, soft soot coating on the heat treated mandrel can be done by any known method. However, in accordance with preferred embodiment of the present invention, the soft soot coating is done by employing hydrogen-oxygen flame having silica flow at central line.

In accordance with preferred embodiment of this invention, the flow rate of hydrogen varies from about 35 lpm to about 45 lpm and flow rate of oxygen varies from about 10 lpm to about 20 lpm. It has been observed that if flow rate of hydrogen is less than 35 lpm, the soft soot does not get deposited on the mandrel and if it is greater than 45 lpm, the density of soot deposited increases meaning thereby the soot deposited will not be soft in nature. In accordance with this invention, the soft soot coating is deposited for at least a thickness of about 8 mm. It has been observed that if thickness of soft soot coating is less than 8 mm, the mandrel prepared does not have desired level of smoothness, and hence its removal becomes difficult.

If soft soot coating, and carbon coating followed by soft soot coating is prepared in accordance with present invention, it has been found that peeling of these coatings is easier to achieve desired smooth and clean surface of mandrel.

It has been found that the mandrel prepared employing method of this invention can be employed in any conventional method for preparing soot porous body. In one embodiment, the mandrel prepared is employed in atmospheric chemical vapour deposition [ACVD] method as described herein.

Now referring to accompanying Figure 2, the soot porous body 1 can be prepared by ACVD method. 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 silica based materials 2 are deposited on a tapered cylindrical member referred as mandrel 3 which has been prepared in accordance with any one of the embodiments described herein. The mandrel is provided with a handle rod 4 and fitted on a lathe 5 to form soot porous body 1.

As described herein, in accordance with first 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 by suspending it in a mandrel suspension means before subjecting it to a step of deposition of soot particles thereon to produce soot porous body. As described herein, in accordance with second 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 by suspending it in a mandrel suspension means and depositing carbon coating on the heat treated mandrel before subjecting the mandrel to a step of deposition of soot particles to produce ,soot porous body.

As described herein, in accordance with third 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 by suspending it in a mandrel suspension means and depositing soft soot coating on the heat treated mandrel followed by removal of soft soot coating from the surface of mandrel to achieve smooth and clean surface of the mandrel before subjecting the mandrel to a step of deposition of soot particles to produce soot porous body.

As described herein, in accordance with fourth 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 by suspending it in a mandrel suspension means, and depositing carbon coating followed by depositing soft soot coating on the heat treated mandrel followed by removal of carbon coating and soft soot coating from the surface of mandrel to achieve smooth and clean surface thereof before subjecting the mandrel to a step of deposition of soot particles to produce soot porous body. During the step of deposition, the mandrel 3 prepared in accordance with present invention 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 said mandrel 3 for producing soot porous body 1. During the deposition process, the dopant chemicals for example Geθ2 may also deposited to form the core 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 said mandrel 3 and handle rod 4, and thereafter said 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 may also be referred to as hollow soot porous body) having a centerline 9 therethrough [Figure 3], which upon visual inspection did not reveal any physical defects confirming that it is free from physical defects.

The hollow soot porous body 8 thus formed comprises a core region

10 having a centerline hole [capillary] 9, which upon preliminary visual inspection has been observed to be free from physical defects 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].

After detachment of mandrel 3 a centerline 9 free from physical defects is created inside the soot porous body 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.

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 to form a solid glass preform 103 [Figure 6]. It is observed that desired solid glass preform is prepared when employing mandrel prepared in accordance with present invention without requiring any step of drilling or grinding or etching of the centerline 9/ 102 of sintered glass preform. As 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 acid, the present process has been found to be highly time saving as well as highly economical for commercial applications.

Figure 5 shows exemplary embodiment to illustrate a general arrangement of the apparatus used in the present invention for 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 comprises a 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, sintering and collapsing process steps with the heating means (not shown) that are fitted to the sintering furnace 100.

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 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 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, 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 the process.

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 assembly rod 111. The inside of the assembly rod 111 consists of smaller dimension 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.

The sintered glass preform produced after dehydration and sintering of the hollow soot porous body is examined for physical defects and surprisingly no physical defects were observed in the capillary region of the sintered glass preform.

The sintered glass preform was subjected to step of collapsing of the centerline 9/ 102 therein to produce solid glass preform 103. The prepared solid glass preform 103 [Figure 6] was also examined for physical defects and surprisingly no physical defects were observed in centerline region of the solid glass preform. The removal of mandrel 3 prepared in accordance with present invention from the soot porous body 1 has been found to be very easy and convenient, and hollow soot porous body thus produced was found suitable for further processing to prepare dehydrated and sintered glass preform, which was examined to evaluate physical defects in capillary region therein, and surprisingly no physical defects were observed. Therefore, the sintered glass preform was subjected to step of collapsing to produce solid glass preform [may also be referred to as mother preform], which was also examined for physical defects and surprisingly no physical defects were found in the capillary region thereof. Accordingly, the solid glass preform was processed to produce core rods of reduced diameter, which were also examined for physical defects and surprisingly no physical defects were found in the capillary region thereof. Accordingly, the fiber was drawn from the core rod, which was also examined for physical defects and other characteristics, and surprisingly no physical defects were observed and fiber produced had other characteristics including attenuation loss, mode field diameter and cutoff wavelength within desired ranges.

The present process has also been found to be suitable for effecting the removal of mandrel 3 without causing shearing of the core layer 10 and without forming flaky pitted centerline 9/ 102, and hence, has been observed to produce a preform without formation of defects, such as voids, bubbles, impurities, uncollapsed capillary in its centerline region.

The present invention is now described with the help of examples which are incorporated to illustrate the manner of performing the invention and are not intended to limit scope thereof.

Examples:

Example 1 [First Embodiment of Present Invention] :- A mandrel made of alumina [refractory material] was cleaned with a cleaning solution comprising 20% diluted hydrochloric acid solution for 20 min to remove foreign particles sticked on the mandrel. The cleaned mandrel was taken in the mandrel suspension means of present invention. The mandrel suspension means with mandrel was inserted inside the refractory furnace for heating it at a temperature of 550⁰C for 160 min. After heating, the mandrel suspension means was removed from refractory furnace and allowed to cool down to room temperature. After cooling, the heated treated mandrel was removed from the mandrel suspension means and fixed on the deposition lathe after fixing a handle tube on one end thereof. The soot comprising SiU2 and dopant Geθ2 was deposited on a rotating mandrel by employing conventional ACVD method described hereinabove. After deposition of desired layers of core to form 40 mm diameter, the dopant was discontinued and clad layers were deposited to form soot porous body of 160 mm diameter. After formation of soot porous body of 160 mm diameter, it was removed from the deposition lathe and allowed to cool down to room temperature followed by removal of mandrel therefrom to form a hollow soot porous body having a centerline therein. The removal of mandrel was observed to be very easy and convenient without causing any shearing and formation of flaky pitted centerline. The hollow soot porous body was physically examined by visual inspection and no physical defects were observed therein. The hollow soot porous body was transferred to sintering furnace after fixing a glass plug in open end of capillary therein and was subjected to a step of dehydration at a temperature of 1250 degree C to form dehydrated hollow soot porous body followed by sintering at a temperature of 1500 degree C to form sintered glass preform. The sintered glass preform was removed out of furnace and allowed to cool down in a closed and filled with nitrogen clean room environment. After cooling, the sintered glass preform was also examined by CCD [Charged Couple Device] Camera for physical defects in its capillary region thereof. Surprisingly, no physical defects were observed in the capillary region of the sintered glass preform. The sintered glass preform was transferred back to the sintering furnace for collapsing of capillary therein at a temperature of 1530 degree C to form solid glass preform [also known as mother preform]. The solid glass preform was removed from the sintering furnace and allowed to cool down to room temperature. After cooling, the solid glass preform was also examined by CCD Camera for physical defects in its capillary region. Surprisingly, no physical defects were observed in the capillary region of the solid glass preform. The examined solid glass preform was transferred to rod draw furnace for drawing 9 nos. of core rods at a temperature of 1800 degree C, each having average diameter of about 20 mm. The drawn core rods were removed from rod draw furnace and allowed to cool down to room temperature. The cooled core rods were also examined by CCD Camera for physical defects in its capillary region. Surprisingly, physical defects were observed in the capillary region of 7% of the core rods drawn from a solid glass preform prepared by employing mandrel prepared in accordance with present invention. One of the examined core rods was transferred to deposition lathe to overclad by depositing clad layers to form soot porous body with core rod of 180 mm diameter, which was dehydrated and sintered by employing sintering furnace to form sintered glass preform [also known as daughter preform]. The daughter preform was transferred to fiber draw furnace to draw optical fiber therefrom. The drawn optical fiber was examined for attenuation loss at 1310 nm and 1550 nm, and its cutoff wavelength and mode field diameter. The optical fiber drawn exhibited attenuation loss of 0.325 dB/km at 1310 nm and attenuation loss of 0.19 dB/km at 1550 nm, and cutoff wavelength of 1230 nm with mode field diameter of 9.13 μm.

Accordingly, it is clear from this example that the method of present invention is suitable for preparation of mandrel for producing soot porous body, sintered glass preform, solid glass preform, core rod, daughter preform and optical fiber having no physical defects. Example 2 [Second Embodiment of Present Invention]:-

A mandrel made of alumina [refractory material] was cleaned with a cleaning solution comprising 20% diluted hydrochloric acid solution for 20 min to remove foreign particles sticked on the mandrel. The cleaned mandrel was taken in the mandrel suspension means of present invention. The mandrel suspension means with mandrel was inserted inside the refractory furnace for heating it at a temperature of 550⁰C for 160 min. After heating, the mandrel suspension means was removed from refractory furnace and allowed to cool down to room temperature. After cooling, the heated treated mandrel was removed from the mandrel suspension means and fixed on the deposition lathe after fixing a handle tube on one end thereof for carbon coating 1.5 mm thickness using acetylene flame. The soot comprising Siθ2 and dopant Geθ2 was deposited on a rotating mandrel having carbon coating deposited thereon by employing conventional ACVD method described hereinabove. After deposition of desired layers of core to form 41 mm diameter, the dopant was discontinued and clad layers were deposited to form soot porous body of 162 mm diameter. After formation of soot porous body of 162 mm diameter, it was removed from the deposition lathe and allowed to cool down to room temperature followed by removal of mandrel therefrom to form a hollow soot porous body having a centerline therein. The removal of mandrel was observed to be very easy without causing any shearing and formation of flaky pitted centerline. The hollow soot porous body was physically examined by visual inspection and no physical defects were observed therein. The hollow soot porous body was transferred to sintering furnace after fixing a glass plug in open end of capillary therein and was subjected to a step of dehydration at a temperature of 1250 degree C to form dehydrated hollow soot porous body followed by sintering at a temperature of 1500 degree C to form sintered glass preform. The sintered glass preform was removed out of furnace and allowed to cool down in a closed and filled with nitrogen clean room environment. After cooling, the sintered glass preform was also examined by CCD Camera for physical defects in its capillary region thereof. Surprisingly, no physical defects were observed in the capillary region of the sintered glass preform. The sintered glass preform was transferred back to the sintering furnace for collapsing of capillary therein at a temperature of 1530 degree C to form solid glass preform [also known as mother preform]. The solid glass preform was removed from the sintering furnace and allowed to cool down to room temperature. After cooling, the solid glass preform was also examined by CCD Camera for physical defects in its capillary region. Surprisingly, no physical defects were observed in the capillary region of the solid glass preform. The examined solid glass preform was transferred to rod draw furnace for drawing 9 nos. of core rods at a temperature of 1800 degree C, each having average diameter of about 20 mm. The drawn core rods were removed from rod draw furnace and allowed to cool down to room temperature. The cooled core rods were also examined by CCD Camera for physical defects in its capillary region. Surprisingly, physical defects were observed in the capillary region of 3% of the core rods drawn from a solid glass preform prepared by employing mandrel prepared in accordance with present invention. One of the examined core rods was transferred to deposition lathe to overclad by depositing clad layers to form soot porous body with core rod of 182 mm diameter, which was dehydrated and sintered by employing sintering furnace' to form sintered glass preform [also known as daughter preform]. The daughter preform was transferred to fiber draw furnace to draw optical fiber therefrom. The drawn optical fiber was examined for attenuation loss at 1310 nm and 1550 nm, and its cutoff wavelength and mode field diameter. The optical fiber drawn exhibited attenuation loss of 0.327 dB/km at 1310 nm and attenuation loss of 0.188 dB/km at 1550 nm, and cutoff wavelength of 1242 nm with mode field diameter of 9.18 μm. Aςcordingly, it is also clear from this example that this method of present invention is also suitable for preparation of mandrel for producing soot porous body, sintered glass preform^ solid glass preform, core rod, daughter preform and optical fiber having no physical defects. Example 3 [Third Embodiment of Present Invention] :-

A mandrel made of alumina [refractory material] was cleaned with a cleaning solution comprising 20% diluted hydrochloric acid solution for 20 min to remove foreign particles sticked on the mandrel. The cleaned mandrel was taken in the mandrel suspension means of present invention. The mandrel suspension means with mandrel was inserted inside the refractory furnace for heating it at a temperature of 550⁰C for 160 min. After heating, the mandrel suspension means was removed from refractory furnace and allowed to cool down to room temperature. After cooling, the heated treated mandrel was removed from the mandrel suspension means and fixed on the deposition lathe after fixing a handle tube on one end thereof for depositing coating of 150 layers of soft soot by employing ACVD method. Before start of soot deposition, the soft soot coating was removed to achieve smooth and clean surface of the heat treated mandrel. Thereafter, the soot comprising Siθ2 and dopant Geθ2 was deposited on a rotating mandrel having soft soot deposited thereon by employing conventional ACVD method described hereinabove. After deposition of desired layers of core to form 42 mm diameter, the dopant was discontinued and clad layers were deposited to form soot porous body of 163 mm diameter. After formation of soot porous body of 163 mm diameter, it was removed from the deposition lathe and allowed to cool down to room temperature followed by removal of mandrel therefrom to form a hollow soot porous body having a centerline therein. The removal of mandrel was observed to be very easy without causing any shearing and formation of flaky pitted centerline. The hollow soot porous body was physically examined by visual inspection and no physical defects were observed therein. The hollow soot porous body was transferred to sintering furnace after fixing a glass plug in open end of capillary therein and was subjected to a step of dehydration at a temperature of 1250 degree C to form dehydrated hollow soot porous body followed by sintering at a temperature of 1500 degree C to form sintered glass preform. The sintered glass preform was removed out of furnace and allowed to cool down in a closed and filled with nitrogen clean room environment. After cooling, the sintered glass preform was also examined by CCD Camera for physical defects in its capillary region thereof. Surprisingly, no physical defects were observed in the capillary region of the sintered glass preform. The sintered glass preform was transferred back to the sintering furnace for collapsing of capillary therein at a temperature of 1530 degree C to form solid glass preform [also known as mother preform]. The solid glass preform was removed from the sintering furnace and allowed to cool down to room temperature. After cooling, the solid glass preform was also examined by CCD Camera for physical defects in its capillary region. Surprisingly, no physical defects were observed in the capillary region of the solid glass preform. The examined solid glass preform was transferred to rod draw furnace for drawing 9 nos. of core rods at a temperature of 1800 degree C, each having average diameter of about 20 mm. The drawn core rods were removed from rod draw furnace and allowed to cool down to room temperature. The cooled core rods were also examined by CCD Camera for physical defects in its capillary region. Surprisingly, physical defects were observed in the capillary region of 7% of the core rods drawn from a solid glass preform prepared by employing mandrel prepared in accordance with present invention. One of the examined core rods was transferred to deposition lathe to overclad by depositing clad layers to form soot porous body with core rod of 181 mm diameter, which was dehydrated and sintered by employing sintering furnace to form sintered glass preform [also known as daughter preform]. The daughter preform was transferred to fiber draw

' furnace to draw optical fiber therefrom. The drawn optical fiber was examined for attenuation loss at 1310 nm and 1550 nm, and its cutoff wavelength and mode field diameter. The optical fiber drawn exhibited attenuation loss of 0.329 dB/km at 1310 nm and attenuation loss of 0.189 dB/km at 1550 nm, and cutoff wavelength of 1229 nm with mode field diameter of 9.12 μm. Accordingly, it is also clear from this example that this method of present invention is also suitable for preparation of mandrel for producing soot porous body, sintered glass preform, solid glass preform, core rod, daughter preform and optical fiber having no physical defects. Example 4 [Fourth Embodiment of Present Invention] :- A mandrel made of alumina [refractory material] was cleaned with a cleaning solution comprising 20% diluted hydrochloric acid solution for 20 min to remove foreign particles sticked on the mandrel. The cleaned mandrel was taken in the mandrel suspension means of present invention. The mandrel suspension means with mandrel was inserted inside the refractory furnace for heating it at a temperature of 550⁰C for 160 min. After heating, the mandrel suspension means was removed from refractory furnace and allowed to cool down to room temperature. After cooling, the heated treated mandrel was removed from the mandrel suspension means and fixed on the deposition lathe after fixing a handle tube on one end thereof for carbon coating of 1.5 mm followed by depositing coating of 150 layers of soft soot by employing ACVD method. Before start of soot deposition, the carbon coating and soft soot coating were removed to achieve smooth and clean surface of the heat treated mandrel. Thereafter, the soot comprising S-O2 and dopant Geθ2 was deposited on a rotating mandrel having carbon coating of 1.5 mm and 150 layers of soft soot deposited thereon by employing conventional ACVD method described hereinabove. After deposition of desired layers of core to form 43 mm diameter, the dopant was discontinued and clad layers were deposited to form soot porous body of 161 mm diameter. After formation of soot porous body of 161 mm diameter, it was removed from the deposition lathe and allowed to cool down to room temperature followed by removal of mandrel therefrom to form a hollow soot porous body having a centerline therein. The removal of mandrel was observed to be very easy without causing any shearing and formation of flaky pitted centerline. The hollow soot porous body was physically examined by visual inspection and no physical defects were observed therein. The hollow soot porous body was transferred to sintering furnace after fixing a glass plug in open end of capillary therein and was subjected to a step of dehydration at a temperature of 1250 degree C to form dehydrated hollow soot porous body followed by sintering at a temperature of 1500 degree C to form sintered glass preform. The sintered glass preform was removed out of furnace and allowed to cool down in a closed and filled with nitrogen clean room environment. After cooling, the sintered glass preform was also examined by CCD Camera for physical defects in its capillary region thereof. Surprisingly, no physical defects were observed in the capillary region of the sintered glass preform. The sintered glass preform was transferred back to the sintering furnace for collapsing of capillary therein at a temperature of 1530 degree C to form solid glass preform [also known as mother preform]. The solid glass preform was removed from the sintering furnace and allowed to cool down to room temperature. After cooling, the solid glass preform was also examined by CCD Camera for physical defects in its capillary region. Surprisingly, no physical defects were observed in the capillary region of the solid glass preform. The examined solid glass preform was transferred to rod draw furnace for drawing 9 nos. of core rods at a temperature of 1800 degree C, each having average diameter of about 20 mm. The drawn core rods were removed from rod draw furnace and allowed to cool down to room temperature. The cooled core rods were also examined by CCD Camera for physical defects in its capillary region. Surprisingly, no physical defects were observed in the capillary region of any of the core rod drawn from a solid glass preform prepared by employing mandrel prepared in accordance with present invention. One of the examined core rods was transferred to deposition lathe to overclad by depositing clad layers to form soot porous body with core rod of 183 mm diameter, which was dehydrated and sintered by employing sintering furnace to form sintered glass preform [also known as daughter preform]. The daughter preform was transferred to fiber draw furnace to draw optical fiber therefrom. The drawn optical fiber was examined for attenuation loss at 1310 nm and 1550 nm, and its cutoff wavelength and mode field diameter. The optical fiber drawn exhibited attenuation loss of 0.326 dB/km at 1310 nm and attenuation loss of 0.187 dB/km at 1550 nm, and cutoff wavelength of 1240 nm with mode field diameter of 9.11 μm.

Accordingly, it is also clear from this example that this method of present invention is also suitable for preparation of mandrel for producing soot porous body, sintered glass preform, solid glass preform, core rod, daughter preform and optical fiber having no physical defects. Example 5 [Prior Art]: -

A mandrel made of alumina [refractory material] was cleaned with a cleaning solution comprising 20% diluted hydrochloric acid solution for 20 min to remove foreign particles sticked on the mandrel. The cleaned mandrel was fixed on the deposition lathe after fixing a handle tube on one end thereof. The soot comprising Siθ2 and dopant Geθ2 was deposited on a rotating mandrel by employing conventional ACVD method described - hereinabove. After deposition of desired layers of core to form 40 mm diameter, the dopant was discontinued and clad layers were deposited to form soot porous body of 160 mm diameter. After formation of soot porous body of 160 mm diameter, it was removed from the deposition lathe and allowed to cool down to room temperature followed by removal of mandrel therefrom to form a hollow soot porous body having a centerline therein. The removal of mandrel was observed to be very difficult and it appeared to have caused shearing and formed flaky pitted centerline. The hollow soot porous body was physically examined by visual inspection and few physical defects were observed therein. The hollow soot porous body was transferred to sintering furnace after fixing a glass plug in open end of capillary therein and was subjected to a step of dehydration at a temperature of 1250 degree C to form dehydrated hollow soot porous body followed by sintering at a temperature of 1500 degree C to form sintered glass preform. The sintered glass preform was removed out of furnace and allowed to cool down in a closed and filled with nitrogen clean room environment. After cooling, the sintered glass preform was also examined by CCD Camera for physical defects in its capillary region thereof. Several physical defects were observed in the capillary region of the sintered glass preform. Therefore, the further processing was discarded.

The above experimental studies confirm that if a cleaned mandrel is heated before start of further processing, it has surprising advantages of easy and convenient removal and of avoiding formation of physical defects in the capillary region of the hollow soot porous body. The above findings also confirm that the mandrel prepared in accordance with present invention is suitable for producing a flawless preform without requiring any step of drilling or grinding or etching with acid or mechanical milling or fire polishing etc. of capillary region of sintered glass preform, and hence, the overall process for producing a flawless preform not only becomes highly time saving, but also highly economical for commercial applications.

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 from soot porous body without causing any defects therein.

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 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. Therefore, it is 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 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. The present method has been described for ACVD method wherein sintering and collapsing steps have been performed one after the another. However, the mandrel prepared in accordance with present invention has been found suitable for ACVD wherein sintering and collapsing steps can be performed simultaneously. Further, the mandrel prepared in accordance with present invention has been found suitable for all other known methods for manufacturing optical performs.

Claims

1. 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.

2. A process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated before start of step of deposition of soot particles thereon.

3. 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 and depositing carbon coating thereon before the start of soot deposition to form a soot porous body having core and clad.

4. A process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated and carbon coating is deposited thereon before start of step of deposition of soot particles thereon.

5. 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 and depositing coating of soft soot thereon which is subsequently removed to have smooth and clean surface of heat treated mandrel before start of soot deposition to form a soot porous body having core and clad.

6. A process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated and soft soot coating is deposited thereon, which is subsequently removed to have smooth and clean surface of heat treated mandrel before start of step of deposition of soot particles thereon.

7. 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 and depositing carbon coating followed by coating of soft soot thereon which are subsequently removed to have smooth and clean surface of heat treated mandrel before start of soot deposition to form a soot porous body having core and clad.

8. A process for producing flawless optical fiber preform comprises :- i) providing a cylindrical and tapered mandrel; ii) depositing soot particles over said mandrel to form soot porous body having core and clad; _/ iii) detaching said mandrel from the soot porous body to form hollow soot porous body; iv) dehydrating the hollow soot porous body in an environment suitable to completely remove the moisture therefrom; v) sintering and collapsing dehydrated hollow soot porous body inside the sintering furnace to form solid glass preform having collapsed capillary therein; characterized in that said mandrel is heated, and carbon coating and soft soot coating are deposited thereon, which are subsequently removed to have smooth and clean surface of heat treated mandrel before start of step of deposition of soot particles thereon.

9. A process as claimed in any one of the preceding claims, wherein said mandrel is heated in a furnace after suspending it in a suspension means to avoid contamination of its surface.

10. A process as claimed in any one of the preceding claims, wherein said mandrel is heated at a temperature varying from about 400⁰C to about 800⁰C, preferably at a temperature varying from about 500⁰C to about 700°C.

11. A process as claimed in any one of the preceding claims, wherein said mandrel is heated 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.

12. A process as claimed in any one of the preceding claims, wherein said mandrel is made from a refractory material, preferably alumina or fused carbon material.

13. A process as claimed in any one of preceding claims 3, 4, 7, 8, wherein said carbon coating is done by employing acetylene flame preferably with flow rate varying from about 3 lpm to about 5 lpm.

14. A process as claimed in claim 13, wherein thickness of said carbon coating varies from about 1 mm to about 3 mm.

15. A process as claimed in any one of preceding claims 5 to 8, wherein said soft soot has density varying between about 0.3 to about 0.45 gm/Cm³.

16. A process as claimed in claim 15, wherein said soft soot coating is done by employing hydrogen-oxygen flame having silica flow at central line.

17. A process as claimed in claim 16, wherein flow rate of hydrogen varies from about 35 lpm to about 45 lpm.

18. A process as claimed in claim 16, wherein flow rate of oxygen varies from about 10 lpm to about 20 lpm.

19. A mandrel suspension means suitable for suspending mandrel in a furnace comprising a suspension tube including neck portion and mouth, wherein mandrel is suspended in suspension tube after fixing a holding means on one end thereof and suspension tube is closed with a closing means.

20. A mandrel suspension means as claimed in claim 19, wherein said suspension tube comprises a cylindrical body including neck portion and a mouth portion.

21. A mandrel suspension means as claimed in claim 19 or 20, wherein said closing means is provided with an opening for release of expanded air on heating.

22. A mandrel suspension means as claimed in any of claims 19 to 21, wherein said mandrel fitted with said holding means is made to rest above said neck portion.

23. A process for preparing the mandrel suitable for producing flawless optical fiber preform substantially as herein described with the help of forgoing examples and as illustrated in accompanying figures.

24. A process for producing flawless optical fiber preform substantially as herein described with the help of forgoing examples and as illustrated in accompanying figures.

25. A mandrel suspension means suitable for suspending mandrel in a furnace substantially as herein described with the help of forgoing examples and as illustrated in accompanying figures.