Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1999-10-01
2003-03-18
Hoffman, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S399000, C065S421000
Reexamination Certificate
active
06532775
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of manufacturing an optical fiber preform, the method comprising forming at least one silica-based external deposition layer by depositing silica on a primary preform consisting in a bar mainly comprising silica and including a silica-based outer peripheral portion.
In known manner, external deposition is performed on a primary preform, e.g. as shown in
FIG. 1
which relates to plasma build-up, in order to enlarge the diameter of the preform, thereby increasing the length of the optical fiber, as shown in
FIG. 2
, which can be made by hot drawing the resulting preform. The primary preform, whether it is made by the modified chemical vapor deposition (MCVD) technique or by the vapor axial deposition (VAD) technique, is a bar mainly comprising silica and including an outer peripheral portion which is based on silica, said outer peripheral portion usually consisting in a silica-based tube forming the side wall of the primary preform, with said bar comprising, inside the outer peripheral portion, optical cladding and a core which have different refractive indices. Silica, generally in the form of a silica grains when the external deposition is performed by plasma build-up, is deposited on the silica-based outer peripheral portion of the primary preform so that the build-up layer has substantially the same refractive index as said peripheral portion.
FIG. 1
shows, in highly diagrammatic manner, plasma build-up apparatus comprising an enclosure
1
having a transparent window
2
, a preform
3
of longitudinal axis X seen end-on, and onto which there are directed a plasma torch
4
and a nozzle
5
for supplying build-up grains. Outside the enclosure
1
, a CCD camera
6
located behind the window
2
is directed towards the preform
3
. It provides a measurement of the diameter of the preform at the location towards which it is pointed and in the form of a value which is transmitted over a link
7
to apparatus
8
for controlling the build-up process. Over a multiple link
9
, the apparatus
8
also receives other indications about build-up process conditions. Under the effect of an internal program for controlling the build-up process, and at constant grain feed rate, the apparatus
8
delivers over an outlet link
10
connected to a control apparatus
11
, a control value for positioning the nozzle
5
relative to the preform
3
so that the nozzle
5
is positioned accordingly by being displaced along an axis parallel to the longitudinal X of the preform
3
. Over a multiple output link
12
, the apparatus
8
also delivers other control values that govern other aspects of the control process.
All of the elements of the apparatus shown in
FIG. 1
are well known to the person skilled in the art. Other elements (not shown) are also well known. This applies to means for supporting the preform
3
while enabling it to be driven in rotation and in translation, a carriage for supporting the plasma torch
4
and the nozzle
5
, and suitable for being driven in translation parallel to the longitudinal axis of the preform
3
, and means for evaluating the angular position of the preform
3
and the longitudinal position of the carriage, e.g. as described in European patent application EP-A1-0 440 130. In conventional manner, these means together enable the preform
3
to be moved away from the torch
4
as the preform
3
becomes larger. Means for directing the camera
6
to successive locations on the preform
3
during a measurement pass likewise form part of the prior art and can be constituted by a second carriage whose displacement is coupled to that of the first carriage.
Plasma build-up takes place in passes, from right to left and then from left to right, during which the plasma torch
4
and the nozzle
5
scan the length of the preform
3
.
The entire control process is optimized so as to obtain high yield concerning the quantity of silica deposited at a given speed of translation and for a given refractive index for the build-up layer.
Simultaneously, the camera
6
performs a measurement pass, providing successive values for the diameter of the preform
3
along its entire length. In order to simplify matters, it is preferable for the camera
6
to perform measurement only on every other pass, e.g. on the right to left pass, while during the following pass the output from the camera
6
is not enabled.
FIG. 2
is a diagrammatic section view through an optical fiber
15
made by hot drawing down of a preform
3
obtained by a silica-based external deposition method applied to a primary preform
24
, e.g. a preform made by the MCVD method. The layers of the optical fiber
15
and of the preform
3
correspond, ignoring scale, so the same numerals
20
to
24
are used both for the preform
3
and for the optical fiber
15
shown in FIG.
2
. The primary preform
24
comprises an optical core
20
, cladding
21
, and an outer peripheral portion
22
. The primary preform
24
is made using the MCVD method by internal deposition of optionally-doped silica-based layers forming the optical core
20
and the optical cladding
21
inside the tube
22
, followed by the tube that has been internally coated in this way being transformed into a bar by being collapsed, which bar constitutes the primary preform
24
, after which the final preform
3
is made by external silica-based deposition of external deposition layers
23
deposited on the primary preform
24
.
The problem which arises is that of obtaining an optical fiber having a refractive index that is constant throughout its thicknesses which is scaled, during the hot drawing operation, from both the silica-based outer deposition layer and the silica-based outer peripheral portion of the primary preform. It has been observed that an undesirable index step is to be found in the optical fiber that results from hot drawing of the preform, said step being located (ignoring scale) at the boundary between the outer peripheral portion and the silica-based outer deposition layer, said outer peripheral portion apparently not being subjected to the same compression stresses during hot drawing as is the outer deposition layer.
SUMMARY OF THE INVENTION
To this end, the invention provides a method of manufacturing an optical fiber preform comprising: forming at least one silica-based outer deposition layer by depositing silica on a primary preform constituted by a bar mainly comprising silica and including a silica-based outer peripheral portion, the method being characterized in that the viscosity of the outer deposition layer is adjusted to be substantially identical to the viscosity of the outer peripheral portion of the primary preform by adding to the silica, over a substantial portion of the outer deposition layer, at least one compound selected from the group formed by the following compounds: CaF
2
, MgF
2
, AlF
3
, B
2
O
3
, and Al
2
O
3
.
Preferably, if the dopant is selected from the group constituted by CaF
2
, MgF
2
, and AlF
3
, the proportion of the dopant lies in the range 5 ppm to 100 ppm, preferably in the range 10 ppm to 40 ppm, by weight of the fluorided element (i.e. the calcium, the magnesium, or the aluminum) relative to silica. It is necessary to have sufficient dopant in order to obtain a significant effect on viscosity. Furthermore, if the proportion of said dopant is excessive, a problem is observed concerning the stability of the diameter of the optical fiber manufactured from the preform, and also variation of the index around the nominal value can be too great compared with the tolerance range set by the manufacturer.
Preferably, if the dopant is selected from the group formed by B
2
O
3
and Al
2
O
3
, the proportion of dopant lies in the range 0.01% to 1%, preferably in the range 0.1% to 0.6%, by weight of the oxidized element (i.e. the boron or the aluminum) relative to the silica. It is necessary to have sufficient dopant to obtain a significant effect on viscosity. Furthermore, if the proportion of said dopant is excessive, there is observed,
Campion Jean-Florent
Chariot Jean-François
Fauche Remi
Jolly Jacques
Jourdier Pierryle
Alcatel
Hoffman John
Sughrue & Mion, PLLC
LandOfFree
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