Note: Descriptions are shown in the official language in which they were submitted.
~lUVU70
~ProcesS for the manufacture of a rod-shaped preform~
The invention relates to a process for the
manufacture of a rod-shaped preform for optical fibers
having a core and having at least one sheath encasing the
core and exhibiting a lower refractive index than the
core, by collapsin~ a ~ubular starting body in a collapse
region, the starting body exhibiting, seen in the radial
direction, a refractive index change over its wall
thickness in at least one interface region and, during
the collapse, an internal pressure which is reduced as
compared with the external pressure acting from outside
on the starting body, being maintained by continued
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evacuation in the tubular part of the starting body, and ;~ ~
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the pre~orm being continuously drawn off from the heating `
zone.
Such processes for the manufacture of preforms,
especially for optical fibers, are generally known. The
manufacture of the tubular starting body can take place
by deposition of particles on a mandrel, which is com-
posed, for example, of glass, graphite or aluminum oxide.
Depending upon the desired radial refractive index
profile of the preform, dopants are either admixed with
the deposited particles or diffused in, for example, also
into the open-pore "body~ which is formed by the
deposition of particles.
~`~i'3 The removal of the mandrel, for example by
drawing out, drilling out or etching out, causes distur-
bances to the internal surface of the tubular starting
body which is produced. With the objective of reducing
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surface defects and material inhomogeneitLes, the inter-
nal isurfaces of the tubular starting bodies are in
general given a subsequent treatment in various cleaning-
smoothing and/or drying processes.
For the purpose of the collapsing, the tubular
starting body is, in the known processes, softened
starting from one end, progressively over its length in
a heating zone, so that it collapses together to form a
solid body, the preform.
EP-A2 0,163,C71 discloses a process for the
manufacture of a preform for the drawing-of optical
fibers, in which process a qjuartz glass tube, which, seen
.; .
in the radial direction, exhibits a nonlinear refractive
index progression over its wall thickness on account of
differing germanium doping, is passed vertically to a
hea~ing zone and, during the collapsing in a heating
zone, remains connected to a vacuum pump; in this case,
the reduced pressure within the glass tube is to be
selected so that as far as possible no impoverishment of
doping materials takes place in the center of the
preform.
In a process disclosed in EP-A 1 0,100,174, for
the manufacture of an optical glass iber, the glass
., ~ .
fiber is drawn out o~ a tubular glass body, which
exhibits layers having differing refractive indexes; in
this case, tha glass tube is sealed at both ends and, to
facilitate the collapsing process, the interior of the
: .
glass tube is evacuated during the drawing of the fiber.
It is common to the known processes that the
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~--entire internal surface of the tubular starting body is
imaged onto the center of the preform in the course o
the collapsing. Quite apart from problems due to bubble ~-
formation on account of gas inclusions or of dopant
impoverishment due to doping material being vaporized off
during the collapsing process, disturbances to the
internal surface of the tubular body, eg. on account of
LmpuritieS~ moisture or other surface defects, also lead
to inhomo~eneities in the center of the preform. However,
a~ precisely that location ~hey are in general particu-
larly troublesome. Frequently, in the case of the pre-
forms which have ~een collapsed ~rith the participation of
reduced pressure, it is even possible to observe oval or ~;;
sheath-like deformations of the core regions of the ~-
preform.
The ob~ect of the present invention is to permit
the manufacture of low disturbance, rod-shaped preforms
for optical elements.
According to the invention, this ob~ect is
achieved in that the starting body is continuously passed
to a heating zone with rotation and horizontally,
external dimensions and internal dimensions of the
starting body, the spacing of the interface region from
the internal surface of the starting body, the viscosity ;;~
of the material of the starting body in the collapse
region, the pressure difference between the internal
pressure and the external pressure, the internal pressure ~ -
head as well as the speed of drawing off of the preform
and the speed of passing forward of the starting body
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~lùu~70
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being selected so that a stalk composed of core material -
is formed from the collapse region, against the direction
of drawing off o~ ~he preform, in the axis of the start-
ing body.
As a result of the fact that the geometry of the
.,,
starting body and the process parameters in the course of
the collapse are set so that from the collapse region a
stalk is formed in the axis of the starting body against
the direction of drawing off of tha collapsed preform, a
premature closing together of mutually opposite internal
wall surfaces of the starting body is prevented. Accor-
dingly, the stalk which is formed stabilizes the symmetry
of the softening starting body directly in front of the
collapse region, and facilitates its transfer to the
preform. At the same time, the material of the internal
surface layer o~ the starting body is turned back and
removed, in the stalk which i~ formed, from the collapse
region. The center of the collapsed preform, which center
thus contain~ no material which had actually formed a ~-~
surface pre~iously, can thus be produced in such a manner
a~ to be substantially free from disturbances, including
those which may ~e caused on account of material vapori- ;
zations in the heating ~one. The nonlinear refractive
index progression over the wall thickness of the starting
body is caused on account of differing concentrations of ~ ;~
dopant~ in the material layers of the starting body. In i -
this case, the term "interface region" is understood as ;
referring to the region from which, seen in a radial
direction, a stepped or a continuous change in the
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refractive index occurs and in which, in the preform, the
interface between core and sheath extends. Depending upon
the dopant concentration, the viscosity of the material
of the starting body also exhibits differing values, so
that in the collapse region the formation of the stalk is
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facilitated on account of the viscosity progression which
is nonlinear when seen radially over the wall thickness
of the starting body.
~ For the purposes of achieving these advantages,
it is immaterial whether the starting body is advanced to
a stationary heating zone or, with reversed kinematics,
the heating zone is guided away past a stationary start-
ing body.
~ he process according to the invention is par-
ticularly suitable for the collapsing of hollow
cylinders. In this case, it has proved to be beneficial
to rotate the hollow cylindex and the preform, during the
: :
collapsing, at a speed between 5 revolutions/min and 50
revolutions/min about the longitudinal axis. As a result
of the rotation, random asymmetries within the heating ~;
zone are compensated and the stalk which i9 formed is
stabilized in the center o~ the hollow cylinder. In`this
case, it has been shown that a starting body geometry
which is advantageou~ is one in which the internal
diameter i~ between 10 mm and 120 mm, wherein the ratio
of the e~ternal to the internal diameter is in the range
.
from l.S to 3 and the spacing of the at least one inter- ~i
face region from the internal surface of the starting
body is at least 3 percent ~f its wall thickness. Since
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the collapse process should proceed as- rapidly as
possible, the temperature in the heating zone i5 prefer-
ably set so high that in the collapse region a viscosity
of the material in the range from 103 dPas to 107 dPas is
achieved. Values up to a maximum of 1008 mbar have proved
to be suitable for the internal pressure in the tubular
part of the starting body, which is a codetermining
factor for the speed at which the stalk i5 formed and for
the mass of mat~rial which is applied for the con-
struction of the stalk; in this case, the pressure
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difference between the internal pressure and the external
pressure acting on the starting body is selected, at
values between S mbar and 813 mbar, expediently so that
the starting body is not deformed in an uncontrolled
manner in the region of the heating zone. In order, in
spite of a sufficient heating through of the material and
an adequate thermal stability in the collapse region,
nevertheless to achieve an economic mass throughput, a
speed of drawing off of the preform from the collapse
region of between 10 mm/min and 80 mmtmin and a speed of
::.; .: . .
advance of the ~tarting body to the heating zone of ;
between 8 mm~min and 35 mm/min have proved to be
beneficial.
It has proved to be advantageous to rotate the `
preform and the starting body at the same speed and in
the same direction of rotation. ~ixtures of layers of the
starting body and of the preform-with differing refrac-
tive indexes are thereby avoided.
The process according to the invention has proved ;
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~l~vl)70
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to be par~icularly beneficial ~or the collapsing of
starting bodies in which the ViSCosLty at a given tem-
perature decreases from the inside towards the ou~side.
Thus, by way of example, by appropriate dopant concen~
trations of the innermost layer of the starting body it
is possible to set high viscosity differences between
this layer and an adjacent, but further outward layer, so
that the inner layer in the collapse region can easily be
peeled off from the adjacent layer and turned back in the ~
form of a stalk which is being foxmed. ~ -
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The process adopts a particularly simple form in
the case of the application of starting bodies which
exhibit a stepped index progression of the refractive
index over their wall thickness. The risk of a mixing of ~ -
regions of differing refractive indexes as a result of a
possible a~ymmetric formation of the stalk lS substan-
tially reduced as a result of this. The process has
proved to be advantageous for the collapsing of starting
bodies which are predominantly composed of SiO2,
especially those which, seen in the radial direction,
exhibit at least one layer of germanium-doped SiO2 / as
well as those which, seen in the radial direction,
exhibit at least one Iayer of undoped SiO2 and, adjacent
thereto and further outward, at least one sheath glass
layer of fluorine-doped SiO2.
~ he process according to the invention is des-
cribed by way of example hereinbelow with reference to a
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diagrammatic representation.
The reference numeral 12 designates an ~lectrical
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re-sista~ce heating arrangement, which encases the heating
zone 1, which surrounds a portion of a quartz glass tube
2 to be collapsed and an already collapsed preform 3.
Nithin the heating zone 1, the internal walls 4 of the
quartz glass tube 2 close together in a collapse region
~, ..
5; in this case, a stalk 6 is formed and is drawn off
from the collapse region 5 against the direction of
drawing off of the preform 3. The direction of drawing
off of the preform 3 i identified by the directional
arrow 7, and that of the stalk is identified by the
directional arrow 8. The quartz glass tube 2 is sealed
with à plug 10 at its end face remote from the collapse
region 5. Within the quartz glass tube 2, during the
collapse, an internal pressure of 900 mbar is maintained
by means o~ a vacuum pump 9, which is connected via a
vacuum-tight passage through the plug lO to the quartz
glass tube 2. The quartz glass tube 2, which has an
external diameter of 120 mm and an internal diameter of
., ~ `1 .:
60 mm, exhibits as Lnnermost layer with a layer thickness
, of approximately 6 mm, a core glass layer 13 of pure,
synthetic quaxtz glass and, ad~acent thereto and further
i~ outward, a sheath gla~s layer 14 of synthetic quartz
glass doped with 0.5 weight ~i fluorine tfor the sake of
clarity, the layer thicknesses in ~he figure are not
shown to scale). The quartz glass tube 2 is oriented
horizontally and is advanced with continuous rotation of
20 revolutions/min at a speed of advance of 23 mm/min to
the heating zone 1 and in that zone is heated to a
temperature around 2100C. In the collapse region 5, the
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quartz glass has in this case a mean v-rs-cosity of.
.
105 dPas. During the collapse, the vacuum pump 9 main-
tains an internal pressurs of 900 mbar in the quartz
glass tube 2, so tha~, as against the atmospheric pres-
sure acting externally on the quar~z glass tube surface,
i
a pressure difference of 113 mbar is maintained. The
speed of drawing off of the preform 3 is 23.5 mm/min, and
is thus slightly greater than the speed of advance of the
quartz glass tube 2. As a result of this, the quartz
glass tube 2 and the quartz glass preform 3 are steadily
maintained under tension. - ;
On account of the experimental parameters listed
above, a stalk 6 is formed in the axis of the quartz
glass tube 2, which stalk is composed of material from
the near-surface regions 11 tshown in the figure by dark~ :
shading) of the inner walls 4 of the quartz glass tube 2,
which have in the collapse region 5 such a low viscosity
that they are deformed by the compressive or vacuum ~ :
forces acting in the direction of the longitudinal axis,
against the direction 7 of drawing off of the preform 3 .
and are turned back in the direction 8 of the growth of .
the stalk. ~ccordingly, the contaminations and distur-
bances of the near-surface regions 11 are removed,
together with the stalk 6, from thr collapsQ region 5.
Moreover, as a result of the formation of the stalk 6 a
closing together of opposite inner walls 4 of the quartz ~.
-glass tube 2 is prevented, and thus the symmetry of the
quartz glass tube.2 directly ahead of the collapse region
5 is stabilized and transferred to the preform 3. The
l) r~ o ~.
_ . _
diameter of the preform 3 produced in this manner is
approximately 96 mm, and of this approximately 8 mm are
attributable to the core glass region. The stalk 6
exhibits a dlaxeter of approxlmately 40 =m.
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