Note: Descriptions are shown in the official language in which they were submitted.
il7:~703
This application is a divisional of our copending
patent application Serial No. 381,845 filed on July 16, 1981.
This invention relates to the manufacture of glass fibres for
optical communication. In particular it relates to the vapour-
phase deposition of core and cladding material on a substrateand to the subsequent stage in the procedure in which a solid
preform of a fibre is produced.
Vapour phase techniques involve the oxidation of a
gaseous silicon compound such as SiC14 to deposit a layer of
silicon dioxide on a substrate which, in this invention,
comprises the inner surface of a tube. Various dopant
materials in a vapour phase are added in a controlled manner
to deposit layers which differ in refractive index and in
particular a core of a material is formed that has a greater
refractive index than the cladding. The substrate is usually
pure silica and forms the outer part of the cladding when a
fibre is subsequently drawn from the solid preform which
comprises the collapsed substrate tube.
TiO2, GeO2, P2O5, and A12O3 are all dopants which have
been used in the deposited core layers to increase the refrac-
tive index of the glass in that region. By varying the
concentration of the dopants in the vapour a gradual refractive
index change from the centre to the periphery of the preform
can be obtained or alternatively a discrete step in refractive
index between the core and cladding regions can be introduced.
Both types of profiles are used for multimode fibres. In
monomode fibres a step index profile is required in which the
core radius is small and in which the difference between the
refractive indices of the core and cladding is small compared
,
' .. ... , .
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with a multimode fibre.
Intrinsic losses in SiO2 - GeO2 fibres show a marked
variation with respect to the wavelength of the transmitted
radiation, and a window in the region 1.1 to 1.7~m is generally
recognised as the optimum wavelength range. Thus, in a fibre
with a GeO2 doped core a pure SiO2 cladding would seem to be
the best choice for a ultra-low-loss (~ldB/km) fibre, since it
does not introduce any additional absorbtion mechanisms. This
is particularly true in the case of a monomode fibre where 30%
of the power in a fibre having a normalised frequency of 2
travels in the cladding.
However, a very high temperature is required for the
chemical vapour deposition and sintering of pure silica.
Furthermore, heating to this temperature tends to produce
distortion in the silica substrate tube. A small amount of
P2O5 added to the deposited silica cladding layers considerably
reduces the deposition and sintering temperature, but leads to
an increase in the refractive index of the deposited part of
the cladding. This higher refractive index region forms part
of an undesirable second wave guide with the silica substrate
tube acting alone as the cladding. It has previously been
proposed, for example, in Electronics Letters 1979 15 pp 411-
413, to add a small amount of a refractive index-reducing
dopant to the deposited cladding layers to compensate for the
effect of the P2O5 doping. For the additional dopant, fluorine
has been proposed. The present invention in a first aspect
relates to those fibres having such a compensated or nearly
compensated deposited cladding layer based on silicon and
including both refractive-index-increasing and refractive-
index-decreasing dopants.
Such fibres, whilst alleviating the problems of high
deposition temperature and of the formation of a secondary
waveguide, suffer from the disadvantage of further absorbtion
losses in the transmission spectrum, generally resulting from
the vibrational absorbtion of bonds between the dopant material
` il71703
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and water. In the case of P2O5 a loss at ~1.6~m occurs which
is the first overtone of the P-OH vibration at 3.05~m; and
possibly also there is a loss at wavelengths greater than 1.5~m
due to the tail of the fundamental P-O vibration at 8.1~m.
The present invention in a first aspect is based on a
realisation by the inventors that the compensated dopant
concentrations can be decreased either gradually or merely as
a single step-wise change so that in the extreme case a layer
or layers of substantially undoped silica are deposited
adjacent the core.
According to the present invention there is provided
an optical fibre having a core and a cladding region, wherein
the core comprises silica doped with a refractive-index-
increasing substance, and the cladding has an outer region
comprising substantially undoped silica, with the remainder
of the cladding including regions of silica doped with both
refractive-index-increasing and refractive-index-decreasing
substances, and wherein the concentration of said dopants in
said remainder of the cladding being less towards the core of
the fibre so that there exists at least an intermediate region
of the cladding where the concentration of said dopants is
greater than the remaining inner region of the cladding.
The present invention in a second aspect also relates
to the avoidance of absorbtion mechanisms affecting radiation
in the transmission spectra as an optical fibre. In particular
it relates to the reduction of the water content of the fibre,
which is important since the -OH bond has a particular strong
absorbtion peak in the 1.4~m region. This aspect of the
invention is not limited to the manufacture of fibres having
compensated dopants in the cladding layer, but relates to all
techniques where layers are deposited in a hollow substrate
which is subsequently collapsed to form a preform from which a
fibre is drawn.
It is known to use chlorine as a drying agent in
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optical fibre production, but the inventors have found that
surprisingly its use during the collapse of the substrate tube
has a significant effect on the resultant -OH concentration in
the central region of the preform.
According to a second aspect of the invention there is
provided a method of manufacturing an optical fibre havinga core
and a cladding region, which includes the steps of depositing
successive layers of silica on the inner surface of a tubular
silica substrate by thermal oxidation of a vapour including a
silicon compound, the deposited layers containing said dopant
materials in accordance with the controlled composition of said
vapour, and the composition of the vapour being controlled such
that layers of cladding material are deposited in succession
including layers which include both refractive-index-increasing
and refractive-index-decreasing dopant material, the concentra-
tion of said dopants being less towards the core of the fibre so
that there exists at least a region of the cladding adjacent the
substrate where the concentration of said dopants is greater
than the remaining inner region of the cladding.
Embodiments of the invention will now be described by
way of example only, with reference to the accompanying drawings.
In the drawings:-
Figure 1 is a diagrammatic axial section through a
substrate tube;
Figure 2 is a graph of integrated power against core
radius for fibres having different values of normalised
frequency;
Figure 3(a) is a graph of refractive index against core
radius for a fibre made in accordance with a first aspect of the
in~ention;
Figure 3(b) is a diagram associated with Figure 3(a)
giving figures for the deposited layers at various radial
11717~3
-- 5 --
distances; and
Figure 4 is a graph of loss per unit length against
transmitted wavelength for a number of fibres.
In the process known as the MCVD (modified chemical
vapour deposition) process, low loss optical fibres are pro-
duced from a solid preform. The preform itself is formed by
collapsing a tubular substrate on the inside of which layers
of material are deposited by thermal oxidation of chemical
vapours. The deposited material is in the form of the solid
products of the vapour phase reaction which becomes sintered
to a glass layer during the traverse of the heat source.
Referring to Figure 1, a substrate tube 1 is formed
from pure silica and is rotated about its axis over a source
of heat, typically an oxy-hydrogen burner. Combinations of
vapours are introduced at one end of the tube and the heater
is moved slowly in the direction of vapour flow. The vapour
is primarily SiC14 but includes chlorides of the dopant
substances such as GeC14 and POC13. Oxidation of the vapour
causes solid products 2 to be deposited inside the tube in a
region adjacent the source of heat. The actual deposition
mechanism lies between the deposition of soot formed in the
vapour and the mechanism of the unmodified process where the
solid products are deposited where the hot vapour contacts
the less hot region of the substrate tube adjacent the heat
source. As the heater passes these regions the deposited
material is sintered to form a glass layer 3.
As was discussed above the use of pure silica as the
cladding material whilst desirable in some respects causes
problems as a result of the high temperature required in
subsequent stages. Thus a refractive-index-compensated
combination of P2O5 and F dopants is desirable in the deposited
layers of cladding to reduce the temperature required.
In a single-mode fibre which is required to have a
1 1'71703
low loss at both 1.3~m and 1.6~m, the LPl1 mode of propogation
preferably has a cut off at a wavelength between 1.1 to 1.2~m.
To achieve this the normalised frequency V, given by the
expression:-
2 a 2 2 1/2
where nl, n2 are the refractive indices of the core andcladding, A is the operating wavelength, and a is the core
radius;
has a value of ~2.2 at 1.2~m and ~1.8 at 1.6~m.
Figure 2 shows the integrated power profiles of a step-index
single-mode fibre as a function of fibre radius for different
values of V, from which it can be seen that for V = 1.8 only
~67~ of the power is contained in the core region, whereas ~88%
of the power is contained within a radius of 1.5a and ~96%
within a radius of 2a.
Therefore, if the cladding dopants can be diminished
or eliminated out to a radius 1.5a or 2a then the effect of
the P-OH and P-O bonds can be greatly reduced.
Accordingly, in this method the composition of the
vapour at the beginning of the deposition includes F and POC13
together with the major component, SiC14, until 49 layers of
cladding are deposited between radii of 5a and 1.5a. This is
illustrated in Figures 3(a) and (b). The supply of doping
vapours is then extinguished so that 7 layers of pure SiO2 are
deposited in the region between radii a to 1.5a. Finally
GeC14 is added to the vapour to deposit the 4 layers comprising
the core. A deposition temperature for the pure SiO2 inner
cladding that is 200C greater than that for the doped cladding
has been found to be satisfactory, and a deposition rate for
the doped cladding of 4 times that for the pure SiO2 region was
also found to be suitable.
In this example the radial thickness of the pure
silica layer is limited to 0.5a to minimise the heating
required and hence to minimise the possibility of serious
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distortion of the substrate tube. Further advantageous
extension outwards of the pure silica layer may be possible
but is likely to require pressurisation to reduce distortion
of the substrate. Alternatively the concentration of the
compensated dopants may be gradated in the region adjacent the -
pure silica layer, so that a compromise between the reduction
of P2O5 and the increase in deposition temperature obtains.
In a further alternative the concentration of the compensated
dopants in the cladding may be gradually diminished in a
direction toward the core without being eliminated, and so
without a pure silica layer being formed.
Once the deposition is complete the temperature is
increased to l " 00-1,900C to cause the collapse of the tube
to a solid preform. In accordance with the second aspect of
lS the invention chlorine gas is introduced upon the termination
of the deposition stage and then continued during the collapse
stage. This chlorine vapour is of similar high purity to the
other materials used and has been found by the inventors to
have a significant drying effect at this stage in the process.
During the collapse stage -OH concentration tends to increase
in the tube material, particularly at the exposed, most
recently deposited, core layers. This may be due to absorb-
tion from any remaining hydrogen containing impurities in the
carrier gas or by back diffusion. It will be appreciated that
this contamination is particularly important in monomode fibres
where the transmitted power is concentrated at the centre of
the fibre.
Figure 4 illustrates the loss spectra of three lengths
of fibres produced by collapsing the substrate tube and drawing
the fibre in one operation, and in which procedure, during the
collapse of the preform tube, chlorine was used as a drying
agent. The -OH content estimated from the peaks at 1.25~m in
long fibre lengths and from measurements at 1.39~m made on lkm
lengths of fibres was 30 to 100 parts in 109. The intensity of
the -OH overtone at 1.39~m has been reduced to ~3dB/km, and
further reduction should be possible.
703
Although in the method described, the chlorine used
as a drying agent is introduced as chlorine gas, it could also
be obtained from the decomposition of a vapour, such as thionyl
chloride which decomposes without causing further deposition.
Vapours, such as SiC14, may also be used and supply may then
be continued from the deposition stage, but in such a case the
extra deposition of vitreous layers must be accommodated.
