U.S. patent application number 12/503578 was filed with the patent office on 2009-12-31 for method and burner for manufacturing a glass optical fibre preform by vapour deposition.
This patent application is currently assigned to Prysmian Cavi E Sistemi Energia S.r.L.. Invention is credited to Massimo Nutini, Giacomo Stefano Roba, Franco Veronelli.
Application Number | 20090325111 12/503578 |
Document ID | / |
Family ID | 33522239 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090325111 |
Kind Code |
A1 |
Roba; Giacomo Stefano ; et
al. |
December 31, 2009 |
METHOD AND BURNER FOR MANUFACTURING A GLASS OPTICAL FIBRE PREFORM
BY VAPOUR DEPOSITION
Abstract
A method is disclosed for feeding a flow of gas to a burner for
manufacturing an optical fibre preform, said burner comprising: a
plurality of coaxial tubes, each two adjacent coaxial tubes
defining an annular channel between themselves; an annular gas
distribution chamber at one extremity of said annular channels and
in fluid communication therewith, said annular distribution chamber
being delimited in the radial direction by an inner and an outer
surface; said method comprising introducing the flow of gas into
the distribution chamber so that the direction of its radially
outermost portion is tangential to the radially outer surface of
the distribution chamber. The method allows obtaining a better gas
velocity distribution in said annular channels. A burner for
performing said method is also disclosed.
Inventors: |
Roba; Giacomo Stefano;
(Monza (MI), IT) ; Veronelli; Franco; (Lainate
(MI), IT) ; Nutini; Massimo; (Milano (MI),
IT) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
Prysmian Cavi E Sistemi Energia
S.r.L.
|
Family ID: |
33522239 |
Appl. No.: |
12/503578 |
Filed: |
July 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10478835 |
Aug 31, 2004 |
7587914 |
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PCT/EP02/05790 |
May 27, 2002 |
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12503578 |
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60295591 |
Jun 5, 2001 |
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Current U.S.
Class: |
431/12 |
Current CPC
Class: |
F23D 14/32 20130101;
F23D 14/24 20130101; C03B 2207/42 20130101; F23L 7/00 20130101;
C03B 19/1423 20130101; C03B 2207/87 20130101; C03B 2207/20
20130101; C03B 37/0142 20130101; C03B 2207/06 20130101; F23L
2900/07002 20130101 |
Class at
Publication: |
431/12 |
International
Class: |
F23D 14/32 20060101
F23D014/32 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2001 |
EP |
01113225.5 |
Claims
1. A method for feeding a flow of gas to a burner for manufacturing
an optical fibre preform, said burner comprising: a plurality of
coaxial tubes, each two adjacent coaxial tubes defining an annular
channel between themselves; an annular gas distribution chamber at
one extremity of at least one of said annular channels and in fluid
communication therewith, said annular distribution chamber being
delimited to the radial direction by an inner and an outer surface;
and a feeding duct to feed a flow of gas into said distribution
chamber, said method comprising the step of introducing the gas
into said distribution chamber in a direction not intersecting the
axis of said coaxial tubes and lying on a plane transversal
thereto.
2. The method according to claim 1, wherein the direction of the
gas being introduced into said distribution chamber lies on a plane
perpendicular to the axis of said coaxial tubes.
3. The method according to claim 2, wherein the direction of the
gas being introduced into said distribution chamber does not
intersect the inner surface of said distribution chamber.
4. The method according to claim 3, wherein the direction of the
radially outermost portion of said flow of gas, being introduced
into said distribution chamber, is substantially tangential to the
outer surface of said distribution chamber.
5. The method according to any one of the preceding claims, further
comprising conferring to said flow of gas a direction substantially
parallel to the axis of said coaxial tubes along said annular
channel.
6. The method according to claim 5, wherein said direction is
conferred by flowing said gas through a layer of porous
material.
7. The method according to claim 6, wherein the layer of porous
material is placed between said distribution chamber and said
annular channel.
8-12. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for feeding a flow
of gas to a burner for manufacturing an optical fibre preform used
to make optical glass fibres and to a burner for manufacturing said
optical fibre preform.
[0002] In particular, the present invention relates to a method for
feeding a flow of gas to a burner for manufacturing an optical
fibre preform, said burner comprising a plurality of coaxial
channels, in a manner that allows to achieve a better gas velocity
distribution in said channels and to a burner for performing said
method.
BACKGROUND ART
[0003] Glass fibres for optical communication are made from high
purity, silica-based glass fibres drawn from glass preforms, which
preforms are produced according to various glass deposition
techniques.
[0004] Some of these deposition techniques, including vapour axial
deposition (VAD) and outside vapour deposition (OVD), are based on
flame combustion wherein reactants (i.e. silica precursors, such as
SiCl.sub.4, optionally together with dopants materials, such as
GeCl.sub.4, for suitably modifying the refractive index of the
glass) are fed together with combusting gases through a deposition
burner which directs a high temperature flow of forming fine glass
particles onto a rotating growing target preform.
[0005] According to the VAD deposition technique, the growth of the
preform takes place in an axial direction. Thus, the deposition
burner(s) is typically maintained in a substantially fixed
position, while the rotating preform is slowly moved upwardly (or
downwardly) with respect to the burner, in order to cause the axial
growth of the preform. Alternatively, the rotating preform can be
maintained in a substantially fixed position, while the deposition
burner is slowly moved downwardly (or upwardly) with respect to the
preform.
[0006] Differently from the VAD technique, in the OVD technique the
growth of the preform takes place in a radial direction. In this
case, a rotating target (e.g. a quartz glass rod) is generally
positioned in a fixed horizontal or vertical position and the
deposition burner is repeatedly passed along the surface of the
growing preform for causing the radial growth of the same.
[0007] Independently from the applied deposition technique, a
porous glass preform is thus fabricated, which is then consolidated
to form a solid glass preform apt for being subsequently drawn into
an optical fibre.
[0008] Typically, an optical fibre preform comprises a central
portion (core) and an outer portion (cladding), the core and the
cladding differing in their respective chemical composition and
having thus different refractive indexes. As in the optical fibres,
the cladding portion forms the majority of the preform. The preform
is typically manufactured by producing and consolidating a first
preform comprising the core and a first portion of the cladding. An
overcladding layer is then deposited onto said first preform, thus
obtaining a porous preform, which is then consolidated into the
final preform.
[0009] In general, conventional burners for manufacturing optical
fibre preforms are made up of a plurality of coaxial tubes through
which the glass precursor materials (i.e. silica precursors, such
as SiCl.sub.4, optionally together with dopants materials, such as
GeCl.sub.4), the combusting gases (e.g. oxygen and hydrogen or
methane) and, optionally, some inert gas (e.g. argon or helium) are
fed. Typically, the glass precursor material is fed through the
central tube of the burner, while other gases are fed through the
annular channels defined by the coaxially disposed tubes.
[0010] Generally, the gases are introduced at one extremity of each
annular channel.
[0011] U.S. Pat. No. 4,417,692 describes a burner for manufacturing
an optical fibre preform comprising a plurality of coaxial tubes
defining a plurality of annular channels between each pair of
adjacent tubes, having an annular chamber at the extremity of each
annular channel. The chambers are radially delimited by two
cylindrical concentric surfaces. A feeding duct is connected to
each chamber to feed a gas into it. The feeding ducts are disposed
perpendicularly to the axis of the coaxial tubes; their direction
thus intersects the inner surface delimiting the annular
chambers.
[0012] U.S. Pat. No. 4,661,140 describes a burner for manufacturing
an optical fibre preform comprising a plurality of coaxial tubes
defining a plurality of annular channels between each pair of
adjacent tubes. The gas is fed into the annular channels directly
by means of pipes disposed perpendicularly to the axis of the
coaxial tubes. Also in this case, the direction of said pipes
intersects the inner surface delimiting said annular channels.
[0013] The Applicant has however observed that the disposition, of
said feeding ducts or pipes may not allow a completely satisfactory
uniform distribution of the gas flowing through the annular
channels of the burners. For possibly optimising the gas
distribution along the circumference of said channels, the
Applicant has now found a new method for feeding the gases into
distribution chambers that are connected to said annular channels
and a burner for implementing the said method. It has in fact been
found that by imparting to the flow of gas fed to the distribution
chambers a direction not incident to the axis of the coaxial tubes,
in particular a direction substantially tangential to the inner
surface of the distribution chamber the problem can be
overcome.
[0014] Advantageously, an optimised distribution of gases according
to the present invention may allow using deposition burners having
shorter lengths. As a matter of fact, in the burners of the prior
art, a substantial length of the tubes forming the annular channels
is necessary, in order to allow the flow of gas to reach a
substantial uniformity before exiting from said annular channels.
According to the present invention, a substantially uniform flow of
gas is instead obtained within a relatively short distance from the
entrance of said gas into the annular channels. Consequently, the
length of the tubes forming the annular channels can be
advantageously reduced, if desired.
SUMMARY OF THE INVENTION
[0015] The Applicant has now developed a method for feeding a flow
of gas into a burner for manufacturing an optical fibre preform,
said burner comprising:
[0016] a plurality of coaxial tubes, each two adjacent coaxial
tubes defining an annular channel therebetween;
[0017] an annular gas distribution chamber at one extremity of at
least one of said annular channels and in fluid communication
therewith, said annular distribution chamber being delimited in the
radial direction by an inner and an outer surface; and
[0018] a feeding duct to feed a flow of gas into said distribution
chamber,
[0019] said method comprising the step of introducing the gas into
said distribution chamber with a direction not intersecting the
axis of said coaxial tubes and lying on a plane intersecting said
axis.
[0020] According to a preferred embodiment, said gas has a
direction lying on a plane perpendicular to the axis of said
coaxial tubes.
[0021] It is preferred that said flow of gas entering the
distribution chamber has a direction not intersecting the inner
surface of the distribution chamber. More preferably, the direction
of the radially outermost portion of the flow of gas entering said
distribution chamber is substantially tangential to the radially
outer surface thereof. The term "radially" is always referred to
the radial distances as measured from the axis of the coaxial
tubes.
[0022] After having been fed to the distribution chamber, the gas
leaves said distribution chamber and is introduced into the annular
channel with which the distribution chamber is in fluid
communication. It is preferred that a direction substantially
parallel to the axis of the coaxial tubes is conferred to the gas
flowing along said annular channel. According to a preferred
embodiment, this can be achieved by passing the gas through a layer
of porous material. Said layer may be advantageously placed
transversally to the gas flow in the said channel, or, preferably,
at the entrance of the channel. More preferably, the layer is
placed between the distribution chamber and the annular
channel.
[0023] According to another aspect, the present invention relates
to a burner for manufacturing an optical fibre preform said burner
comprising: [0024] a plurality of coaxial tubes, each two adjacent
coaxial tubes defining an annular channel between themselves [0025]
a gas distribution chamber at one extremity of at least one of said
annular channels and [0026] a device for feeding a gas into said
distribution chamber,
[0027] wherein said device imparts to the flow of said gas entering
the distribution chamber a direction not intersecting the axis of
said coaxial tubes and lying on a plane transversal thereto.
[0028] According to a preferred embodiment, the device comprises a
feeding duct connected to said distribution chamber, said feeding
duct being disposed in a direction substantially tangential with
respect to the inner surface of the distribution chamber said inner
surface being as above defined.
[0029] A layer of porous material is advantageously placed at the
inlet section of the annular channel.
[0030] Preferably, the coaxial tubes have a circular cross-section.
However, they may also have cross-sections of other shape, such as
an elliptical cross-section. The section of the distribution
chamber on a plane perpendicular to the axis of the coaxial tubes
has preferably the same shape of the cross-section of said annular
channels.
[0031] According to an embodiment of the invention, the lower
extremities of two adjacent coaxial tubes defining an annular
channel lie on different planes in particular, the innermost of the
two tubes has a portion extending below the outermost one; a
portion of the outer surface of the inner tube outside the annular
channel may thus constitute the inner surface of the distribution
chamber.
[0032] A further aspect of the present invention relates to a
method for manufacturing an optical fibre preform by directing a
flow of fine glass particles from a deposition burner comprising a
plurality of channels onto a rotating elongated target preform by
using a deposition burner as above described.
[0033] The coaxial tubes can be made of any suitable material, such
as metallic materials or quartz glass. Preferred are metallic
materials, stainless steel being more preferred.
[0034] The porous material layer may be made of a porous metallic
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 shows a schematic longitudinal sectional view of a
part of a burner according to the present invention;
[0036] FIGS. 2 and 3 respectively show a lateral sectional view and
a top view of a cylindrical member constituting part of the burner,
as will be later described, bearing a distribution chamber.
[0037] FIG. 4 shows a bottom view of a part of a burner according
to the present invention;
[0038] FIG. 5 shows a schematic transversal cross-sectional view of
an embodiment of a burner according to the present invention;
[0039] FIG. 6 schematically shows an overcladding deposition step
of a method for manufacturing an optical fibre preform according to
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] FIG. 1 shows schematically a longitudinal section of a
burner according to one embodiment of the invention.
[0041] The burner comprises a metallic body 200 containing the
lower extremities of eight coaxial tubes, indicated with references
101 to 108, and the distribution chambers 204 (only one of said
chambers being numerically identified). The tubes 101 to 108
constitute the terminal part of the burner itself and end in eight
circular orifices placed at their extremities opposite to those
contained in the body 200. From those extremities, the process
gases are discharged and are then made to react in a flame.
[0042] Eight pipes, of which only two (402, 403) are shown in the
figure, are used to feed the process gases to the burner.
[0043] According to a further embodiment, the body 200 of the
burner comprises nine superimposed metallic cylindrical members
201-209.
[0044] Each of the tubes 101 to 108 is welded or otherwise firmly
connected to one of the cylindrical members 201-209 in a
corresponding seat. An annular channel is defined between each pair
of said tubes; the annular channels are indicated with ref.
102a-108a.
[0045] In FIGS. 2 and 3 an enlarged longitudinal section and a top
view of a cylindrical member, indicated in FIG. 1 with ref. 204, is
given. The cylindrical member 204, specifically the fourth from the
bottom end of the body 200 of the burner, is shown when dismantled
from the burner and the extremity of the tube 103 connected with
said member 204 is also shown.
[0046] The member 204 presents a central bore 306, perforating it
from part to part, coaxial with member 204. Coaxial with the member
is also a seat 304 made to receive the extremity of the tube 103
connected with member 204. The bore 306 and the seat 304 may be
obtained for example by boring.
[0047] In the cylindrical member, an annular distribution chamber
301 is also obtained. It can be appreciated from FIG. 2 that the
chamber 301 is delimited radially outwardly and downwardly by
surfaces obtained in the cylindrical member 204, for example by
boring. Radially inwardly, the chamber 301 is delimited by the
outer wall of the tube 103 when the latter is inserted into its
seat 304. Upwardly it is delimited by a layer of porous material
302, inserted in a seat expressly obtained in the cylindrical
member 204. The porous material layer can suitably have the form of
a disk bearing a central bore to externally fit the tube 103.
[0048] Suitable porous materials include multi-layered sintered
metal fibres, manufactured of stainless steel, and are commercially
available with a porosity ranging from 68% to 83%. An example of
porous material layer, which can be advantageously used, is porous
filters FIBERMET AO Series, by MEMTEC, composed by metal fibres and
having a porosity of 78.4%.
[0049] A seat 303 is also obtained in the cylindrical member 204
for receiving a sealing means, for example an O-ring or a
gasket.
[0050] A duct 401 is provided to connect a gas-feeding pipe 402 to
the distribution chamber 301. The end part of said duct enters the
chamber 301 tangentially with respect to the inner surface of the
chamber, as can be appreciated from FIG. 3. An extremity of pipe
402 enters a seat obtained in the cylindrical member 204, with
which it is welded or otherwise gas-tightly connected.
[0051] Referring again to FIG. 1, cylindrical members 202, 203 and
201 are perforated so to provide an opening through the body 200 of
the burner through which the pipe 402 passes.
[0052] Alternatively, the superposed openings through body 200 may
themselves define a passage for the gas, in connection with the
duct 401; sealing means should then be provided between each two
adjacent cylindrical members passed through by said passage, to
make it gas-tight.
[0053] The distribution chamber 301 is in connection through the
porous material layer 302 with the annular channel 104a, defined
between the tubes 103 and 104.
[0054] All the cylindrical members 201-209 have a structure similar
to that above-described of member 204, except the uppermost and the
lowermost members 201 and 209. They all present distribution
chambers, ducts (analogue to the duct 401 of member 204) for
connecting the distribution chambers with a gas feeding pipe
(analogue to pipe 402); those structure are not shown in FIG. 1 in
order to preserve the readability of the drawing.
[0055] FIG. 4 presents a bottom view of the body 200 of a burner
according to the present invention, where the coaxial tubes, the
porous material layers and the seats for containing the annular
gaskets do not appear. More precisely, in this view every
unnecessary particular has been omitted, in order to have a clearer
vision. From that view, a suitable disposition of the said ducts
(collectively indicated with ref. 400) for connecting the
distribution chambers with the said gas-feeding pipes (collectively
indicated with ref. 500) can be evinced, as well as the inlet of
said ducts 400 into the distribution chambers (only the radially
outer surface 300 of said chambers is shown in this figure). It can
be seen how the outer portion 405 of the ducts 400 at their
entrance into the distribution chambers, is, according to a
preferred embodiment, tangential to the outer surface 305 of said
chambers. Any other suitable disposition of ducts and pipes can be
applied.
[0056] The members 201-209 are assembled together as shown in FIG.
1 for example by means of screws (not shown in FIGS. 1 to 3), which
pass through the whole body 200 through suitably obtained holes
(not shown in FIGS. 1 to 3). Pins or other suitable means (not
shown in FIGS. 1 to 3) are advantageously employed, according to
the knowledge of the skilled in the art, to assure the precise
relative positioning of said members 201-209, when they are
assembled.
[0057] All the disks, except top disk 209, have an annular gasket,
placed in a seat analogue to the seat 303 of the member 204, to
create a gas tight connection with the adjacent disk.
[0058] According to a preferred embodiment, all the gas-feeding
pipes, analogue to pipe 402, pass through the members placed below
the member to whose distribution chamber they are connected. The
pipes for feeding gas to the members above member 204, as well as
the holes that permit them to pass trough member 204, are not shown
in FIGS. 1 to 3.
[0059] The lowermost member 201 presents a bore 404 connecting
gas-feeding pipe 403 to channel 101a, which is the opening of the
central tube 101. A disk 302 of porous material can also be placed
across that connection.
[0060] According to an embodiment of the invention, a tube 109,
made from heat resistant material, can be disposed externally to
the outer tube 108, extending for a certain length farther from the
tip of said tube 108, for confining the flame. Preferably, the tube
109 extends for about 150 mm to about 220 mm from the tips of the
outer metal tube 108. A suitably shaped part 110 provides a seat
for said tube 109; said part 110 can be advantageously shaped as a
flanged portion of tube, as shown in FIG. 1.
[0061] The heat resistant material of tube 109 is for instance
quartz glass or ceramic material, such as alumina. Preferably,
quartz, in particular high purity quartz, is employed.
[0062] According to the method to which the present invention
relates, and with reference to FIG. 1, a gas is fed through pipe
402 and duct 401 to the distribution chamber 301 of member 204,
following the path indicted by arrows D and D'. As can be
appreciated from FIG. 3, the gas enters the distribution chamber
301 so that its radially outermost portion has a direction
substantially tangential to the radially outer surface 305
delimiting said chamber 301. The gas then flows through layer 302
to the annular channel 104a, as shown by arrows D'', with a
direction substantially parallel to the longitudinal axis of the
burner. The gas to be flown through the other annular channels is
fed in an analogue way. The gas for the central channel 101a is
preferably fed through an axial pipe 403.
[0063] As observed by the applicant, the above described optimised
distribution of the gas results in a substantially uniform axial
flow of gas, achievable in a relatively short length of the annular
channels, thus allowing to reduce the length of the tubes forming
the burner, when desirable.
[0064] It is preferred that the coaxial tubes are made from a
metallic material, more preferably from an easily machinable and
heat/corrosion resistant stainless steel. An example of a suitable
metal material is AISI (American Institute Steel and Iron) 316L,
which is a stainless steel comprising about 0.03% C about 16-18% of
Cr, about 11.5%-14.5% of Ni, about 2% of Mn and about 2.5%-3% of
Mo.
[0065] Referring to FIG. 5, showing a cross section of the coaxial
tubes in a burner according to an embodiment of the invention,
reference 101 to 108 being the section of said tubes, 101a to 108a
being the channels defined by said tubes and 109 the section of a
tube of heat resistant material, designed to confine the flame as
above described, the inner tube 101 has typically an inner diameter
of from about 6 mm to about 8 mm and a thickness of from about 0.5
mm to about 2 mm.
[0066] The other tubes, having preferably a thickness comprised
from about 0.5 mm to about 2.5 mm, are then arranged concentrically
one to each other to form channels 102a-108a having widths of from
about 1 mm to about 3.5 mm, depending on the relative diameter of
the tube and flow rate of gas through the aperture.
[0067] In particular, the width of each channel is selected
according to the amount and kind of gas that is flown through said
channel and to the relative radial position of said channel. For
instance, in a burner particularly designed for the outer cladding
deposition, channels through which inert gas is flown are
dimensioned so to obtain an exit velocity of the gas of from about
0.1 and about 2 m/s. Said annular channels may thus have a width of
from about 1 mm to about 1.5 mm. On the other side, channels
through which combustion gases are flown are dimensioned so to
obtain an exit velocity of the gas of from about 2 and about 10
m/s. Said annular channels may thus have a width of from about 2 mm
to about 3.5 mm.
[0068] According to the preferred embodiment described above with
reference to FIGS. 1 to 4, the sections perpendicular to the axis
of the coaxial tubes of the distribution chambers have a
circular-corona shape, the inner diameter being the diameter of the
inner tube defining the annular channel with which the distribution
chamber is connected, as described above. The half part of the
difference between the outer diameter and the inner diameter of the
said circular-corona section will be referred to as "width" of the
distribution chamber. The "height" of the distribution chamber is
the distance between the two plane surfaces that axially delimitate
the distribution chamber, one of them being, according to the
embodiment described in FIG. 1 to 3, the surface of the porous
material layer that separates the chamber from the entrance to the
annular channel in communication therewith. The section of the duct
for feeding a gas into the distribution chamber may advantageously
be circular. According to a preferred embodiment of the invention,
the height of the distribution chambers ranges from half to twice
their width; the height and the width of a chamber may suitably
have similar values. Preferably, the diameter of the duct for
feeding a gas into a distribution chamber is not larger than the
height of the chamber; preferably, the width of the chamber is
comprised between once and three times the diameter of said duct.
The width of a distribution chamber is preferably at least the
width of the annular channel in connection therewith chamber;
according to a preferred embodiment it is comprised between once
and ten times the width of said annular channel, preferably between
once and five times. Suitably, all the distribution chambers of a
burner have the same values of height and width; the ducts for
feeding gases into the distribution chambers may suitably have the
same diameters as well.
[0069] Generally, through the central channel 101a, a glass
precursor material is flown.
[0070] The flow of glass precursor material is surrounded by a
flame generated by a combusting gas and a combustion sustaining gas
flowing through further channels of said burner.
[0071] In the present description, the term glass precursor
material is intended to refer to any suitable raw material capable
of reacting in the presence of a flame to form glass (pure silica)
or doped glass particles. Preferably, silicon tetrachloride
(SiCl.sub.4) can be used. Alternatively, other silicon containing
reactants can be used, such as SiHCl.sub.3, SiH.sub.2Cl.sub.2,
SiH.sub.3Cl or SiH.sub.4. In addition, chlorine-free silicon
containing reactants can be used, such as the siloxane compounds
disclosed in U.S. Pat. No. 5,043,002, e.g.
octamethylcyclotetrasiloxane, or the organosilicone compounds
disclosed in EP A 1,016,635, e.g. hexamethyldisilane.
[0072] A preferred glass precursor material capable of forming
doped glass particles under the reaction conditions of a flame
burner according to the invention is Germanium tetrachloride.
Alternative dopant materials are POCl.sub.3 or BBr.sub.3.
[0073] Mixtures of the above glass precursor materials (e.g.
SiCl.sub.4 and GeCl.sub.4) in variable proportion can be used to
suitably modify the refractive index of the manufactured
preform.
[0074] As the above glass precursor materials are generally liquid
at ambient temperature, they are preferably heated in advance into
a vaporizer, so that high temperature vapours of the glass
precursor material are flown through the central tube of the
burner. For instance, silicon tetrachloride, having a boiling point
of about 57.degree. C. (at 101.330 Pa) is heated at about
100.degree. C. in the vaporizer before being fed into the
burner.
[0075] It may be advantageous, in particular for relatively large
dimension burners (e.g. cladding burners), to add a predetermined
amount of a high thermal diffusivity gas to the flow of glass
precursor material, in order to increase the heat transfer from the
flame towards the inner core of said flow.
[0076] The thermal diffusivity of a gas is defined as the ratio of
the thermal conductivity to the heat capacity. It measures the
ability of a material to conduct thermal energy relative to its
ability to store thermal energy. Typical values of thermal
diffusivity of gases can be found on a number of reference books,
such as R. B. Bird, "Transport Phenomena", Wiley & Sons, New
York 1960, or F. P. Incropera, D. P. DeWitt, "Fundamentals of heat
and mass Transfer", Wiley and Sons; 3rd edition, New York,
1996.
[0077] For the purposes of the present description, a high thermal
diffusivity gas is a gas having a thermal diffusivity of at least
3.010.sup.-5 m.sup.2/s or higher, e.g. up to about 2.010.sup.-4
m.sup.2/s (values at 400.degree. K). Examples of suitable high
thermal diffusivity gases are oxygen, nitrogen, argon, helium or
hydrogen, having a thermal diffusivity at 400.degree. K. of
3.610.sup.-5 m.sup.2/s, 3.710.sup.-5 m.sup.2/s, 3.810.sup.-5
m.sup.2/s, 3.010.sup.-4 m.sup.2/s and 2.310.sup.-4 m.sup.2/s,
respectively.
[0078] As the thermal diffusivity of a gas depends, further from
its specific thermal diffusivity coefficient, also from the mass
fraction of the added gas, it is preferable to use gases with a
higher molecular weight, in order to reduce the volume fraction of
added gas (or, alternatively, using the same volume fraction of
gas, increase its mass fraction). Oxygen is thus preferred for its
higher molecular weight and for its relatively high coefficient of
thermal diffusivity.
[0079] Said high thermal diffusivity gas should preferably be added
to the flow of glass precursor material in an amount such that the
overall thermal diffusivity of the so obtained mixture is about 50%
higher than the thermal diffusivity of the glass precursor
material. In particular, when silicon tetrachloride is used, the
thermal diffusivity of the mixture should preferably be higher than
about 4.010.sup.-6 m.sup.2/s at 400.degree. K. Preferably, the
thermal diffusivity of the mixture is comprised between
4.010.sup.-6 m.sup.2/s and 5.510.sup.-6 m.sup.2/s at 400.degree.
K.
[0080] The high thermal diffusivity gas is preferably admixed in a
volume fraction of from about 0.05 to about 0.5 parts with respect
to the total volume of the mixture, preferably of from about 0.1 to
about 0.4 parts, depending also from the thermal diffusivity of the
glass precursor material (e.g. 2.8410.sup.-6 m.sup.2/s at
400.degree. K. for SiCl.sub.4). The addition of a high thermal
diffusivity gas to a glass precursor material to be fed to a burner
for manufacturing an optical fibre preform is described in
copending European Patent Application 00127850.6.
[0081] According to a method for manufacturing an optical fibre
preform, which is a further embodiment of the present invention, a
combustible gas and a combustion sustaining gas are flown through
at least two of the annular channels defined 102a-108a. Examples of
suitable combustible gas are hydrogen or hydrocarbons, such as
methane. Oxygen is typically used as the combustion sustaining
gas.
[0082] If desired, an inert gas may be flown through some of the
annular channels 102a-108a, either alone or admixed with the above
combustible gas or combustion sustaining gas. For instance, an
inert gas may be flown through an annular channel disposed between
a first annular channel dedicated to the inlet of a combustible gas
and a second annular channel dedicated to the inlet of a combustion
sustaining gas. This allows a physical separation of the two flows
of combustible gas and of combustion sustaining gas, thus
displacing the flame away from the tips of the metal tubes and
avoiding possible overheating of the same. Similarly, the flame can
be displaced away from the tips of the metal tubes by suitably
increasing the inlet speed of the combustible gas and of combustion
sustaining gas. Examples of suitable inert gases are argon, helium,
nitrogen.
[0083] FIG. 6 schematically illustrates a typical overcladding
deposition process, according to the VAD technique, for embodying
the method of the present invention. The deposition typically
starts onto a glass rod 701 of about 20 mm diameter, comprising the
core of the preform and a first portion of the cladding layer,
separately manufactured according to conventional techniques. The
target preform is rotated about is longitudinal axis and slowly
upwardly translated. A lower overcladding burner 703 deposits a
first portion of overcladding layer 702a, e.g. up to a diameter of
about 90-100 mm onto the preform. An upper burner 704 then
completes the deposition by depositing a second overcladding layer
702b, e.g. increasing the diameter of the deposited soot at about
180-200 mm. Typically, the upper burner 704 has increased
dimensions with respect to the lower one, in order to allow the
deposition of higher amount of silica particles in the time
unit.
[0084] The so obtained preform is then heated into a furnace and
collapsed to obtain a final preform of about 60-80 mm diameter,
which is then drawn into an optical fibre according to conventional
techniques.
[0085] While a burner according to the present invention can
advantageously be used in the above process for depositing the
overcladding layer of the preform, in particular the outer
overcladding portion (i.e. as burner 704), it will be appreciated
that such a burner, when suitably dimensioned, can also be used for
the deposition of the core and of the inner portion of the
cladding.
[0086] In a VAD deposition process, it may be advantageous to
separate the flame in an inner and an outer flame concentrically
disposed. This can be achieved by interposing a tube of heat
resistant material between the two flames; said tube may be
disposed into the annular housing between two coaxial tubes, said
tube of heat resistant material extending for a certain length
farther from the tips of the pipes of the inner portion of the
burner. The heat resistant material may be suitably chosen among
those that can be employed for the tube 109 of FIG. 1, as discussed
above.
[0087] The use of a tube of heat resistant material is particularly
indicated when the coaxial tubes of the burner do not end on the
same plane at their extremity outside the burner; this allows both
to physically separate the inner flame from the tubes of the outer
section, which tubes protrude outside the burner more than the
tubes of the inner section of the burner and to confine the inner
flame. This protects the tubes of the outer section, which can thus
be made of metal. Preferably, the flame separating tube extends for
a length such to entirely surround the reaction zone where the
glass precursor material reacts to form the glass particles.
Particularly for overcladding burners, the flame separating tube
should preferably extend for at least about 80 mm from the tips of
the pipes of the inner section of the burner. The length of the
tube should however preferably not exceed about 150 mm. Preferably,
said length is from about 90 to about 130 mm. The use of a flame
separating tube as described above in a multi-flame burner for
manufacturing an optical fibre preform is described in copending
European Patent Application 00127851.4.
[0088] It may also be preferred to suitably redistribute the flow
of forming glass particles before said flow impacts onto the target
preform. To this purpose, a multi-flame burner with a flame
separating tube as previously discussed is particularly suitable.
In particular, the outlet of the flame separating tube may be
suitably modified so to increase the deposition rate of the burner.
The modification is such as to confer to the outlet of the quartz
separating tube a cross-section having a major and a minor axis. It
has been observed that the deposition rate can be increased by
increasing the dimensions of the flow of glass particles in a
direction substantially perpendicular with respect to the
longitudinal axis of the target preform. To achieve this the
terminal portion of the flame separating tube should have a section
with a minor and a major axis. An elliptical section is
particularly suited. In a VAD process for manufacturing an optical
fibre preform, the major axis of the section of the terminal
portion of the flame separating tube should lay on a plane
substantially perpendicular to the longitudinal axis of the growing
optical fibre preform. The technique of redistributing flow of
forming glass particles as described above in a multi-flame burner
for manufacturing an optical fibre preform is described in
copending European Patent Application 00127849.8.
Example
[0089] For this experiment, a burner comprising eight co-axial
metal pipes as shown in FIGS. 1 and 5 has been used. The material
used for the metal pipes was AISI316L stainless steel. Tubes
101-108 and channels 101a-103a and 105a to 108a of FIG. 5 will be
referred to in the present example as tubes 1-8 and channels 1a-7a,
respectively. A quartz glass tube has been inserted between the
third and the fourth metal tube into channel 104a of FIG. 5 for
providing a flame confinement. The following table 1 indicates the
relative internal (ID) and outer (OD) diameter of the annular
channels determined by the metal tubes; for the innermost channel
1a, having a circular cross section, only the OD has been reported.
The inner section of the burner is formed by tubes 1 to 3 (and
corresponding channels 1a to 3a), while the outer section of the
burner is formed by tubes 4 to 8 (and corresponding channels 4a to
7a)
TABLE-US-00001 TABLE 1 dimensions of channels Channel no. 1a 2a 3a
4a 5a 6a 7a ID (mm) -- 11 21.34 37.6 44.2 55.8 61.1 OD (mm) 7 17.6
24.4 40.2 50.5 58.3 67.55
[0090] The internal confining quartz glass tube, having a thickness
of about 1.5 mm, an inner diameter of 28.4 mm and an outer diameter
of 31.4, has been inserted into the annular channel 104a of FIG. 5
(ID 27.4 mm, OD 33.6 mm); said channel was consequently not
connected to any feed of gas. The lower portion of the glass tube
has been wrapped with a Teflon.RTM. tape up to the outer diameter
of the clearance, in order to maintain it in a fixed position.
[0091] An outer quartz glass tube (ref. 109 of FIGS. 1 and 5)
having a thickness of about 2 mm has been further disposed around
the outer metal tube 8.
[0092] All the metal tubes protruded outside the body of the burner
for the same length (201 mm). The outer quartz tube protruded for
about 165 mm from the tips of the metal tubes, while the internal
quartz tube protruded for about 133 mm from the tips of the metal
tubes.
[0093] All the distribution chambers had a width of 6 mm and a
height of 7.5 mm.
[0094] The diameter of all the ducts for feeding gases into the
distribution chambers was of 4 mm.
[0095] The reactants employed and their relative flow rate and
inlet speed are reported in the following table 2, where the
innermost opening of the burner is identified with no. 1a. Silica
tetrachloride has been supplied by vaporizing the liquid material
and feeding it at a temperature of about 80.degree. C. through the
central tube, together with oxygen.
TABLE-US-00002 TABLE 2 Reactants and flow rate Channel no. 1a 2a 3a
4a 5a 6a 7a Reactant SiCl.sub.4 + O.sub.2 H.sub.2 O.sub.2 Ar
H.sub.2 Ar O.sub.2 Flow Rate (slm) 12 + 7 27 65 14 160 10 115 Inlet
velocity 8.2 3.4 9.9 1.5 5.7 0.7 2.9 (m/s)
[0096] The target preform was a rotating quartz tube of about 90 mm
diameter and the burner (i.e. the upper end of the outer glass tube
of the burner) has been kept at a distance of about 90 mm from the
perform, with an inclination of about 12.degree. with respect to
the longitudinal axis of the preform.
[0097] The preform was translated upwardly at a speed of 168 mm/h
and rotated at about 60 r.p.m.
[0098] The deposition was stopped when the preform reached a
diameter of about 140-150 mm.
[0099] By following the above-described procedure, a regular soot
deposition was obtained, without formation of cracks or other
defects.
* * * * *