Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~8L~:~Yy~8 5_L~ _In~ention
This application is a division oE appllcation 172,817~ filed May
31, 1973.
Field of the Invention
mis invention relates to the manufacture of optical fibres and
particularly to an improved m~thod of coatin~ the bore of a glass tube9
United States Patent No. 3,659,915 issued May 2~ 1972 describes a
process for making optical fibres wherein a layer of doped fused silica is
formed on the inside wall of a tube of pure fused silica and the composite
structure is drawn to collapse the inner layer to for~ a solid core sur-
rounded by pure fused silica. This process requires the use of a glass tube
of high optical purity which is quite costly and the drawing step introduces
undesired contamination.
Summary of the Invention
Parent application 172,ôl7 relates to a method of making an optical
fiber lncluding the steps of: heat treating the bore of a hollow silica glass
tube under vaccuu~ to remove moisture and prevent formation of oxygen-hydro-
gen compounds in the fiber; coating the heat treated bore with a first silica
glass layer in a hydrogen-free atmosphere;coating a second silica layer on
said first silica layer~ said second silica layer having a higher refractive
index than said first silica layer; and collapsing said tube and coated bore
to form a solid str~cture having a core and cladding layer, said core having ;
- a higher index of refraction than said cladding layer.
' According to the present inventiop there ls provdded a :ethod of
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making a coated tube suitable for drawing into an optical fiber including the
steps of: depositing a core glass layer on the inside of a tube by introduc-
ing g~ass ~orming vapor reagents into the bore of said tube; and heating said
vapor reagents in said tube by means of a vapor reaction heating excitor mov-
ing relative to said tube whereby the heated vapor reagents proximate the vapor
reaction heating excitor are converted to glass on the inside of the tube, the
relative movement between the tube and excitor causing a relatively uniform
layer of glass to be deposited throughout the length of the tube to thereby
form a coated tube.
The tube is not necessarily a self supporting structure but may take
the form of a deposited layer lining the bore of another tube. Methods suit-
able for depositing the glass layer include evaporation, radio frequency (r.f.)
sputtering, and r.f. excited vapour reaction.
Brief Description of the Drawings
In ~he accompanying drawings which illustrate two coated tubes and
an exemplary embodiments of the present in~ention:
~igure 1 shows a structure of a known type having a single core
glass coating;
Figure 2 depicts a twice coated tube before drawing into a fibre;
Figure 3 depicts apparatus for coating the bore of a tube.
Description of the_Preferred Embodiments
Figure 1 depicts a structure of a known type produced by the deposi-
tion of a layer 10 upon the bore of a hollow tube 11. The core of the com-
pleted fibre is provided by the material of the deposited layer 10 while the
cladding is provided by the material of the tube 11.
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Figure ? dcpicts a structure of the type produced according to the
parent application. The bore of a ho~low silica glass tube 20, ~hich can be
slightly impure and lossy, is lined with a first layer 21 of low absorption
silica cladding glass and then a further layer 22 of doped silica core glass.
The core of the completed fibre is provided by the material of the second de-
posited layer 22, while the inner and outer regions of the cladding are pro-
vided respectively by the material of the first deposited layer 21 and the
material of the tube 20. The tube is thus spaced from the core and is not in-
volved to any extent in light propagation. The outer region of the cladding
can safely be made ~ore lossy and of a less expensive glass than the other
remainder of the fibre, but energy should not be coupled into it from the
core. Therefore the outer tube refracti~e index is equal to or less than and
no greater than that of the inner cladding. me refractive index of this in-
ner cladding is in its turn, less than that of the core which is of a higher
refractive index. It may be noted that the refractive index of an absorbing
medium is strictly a complex quantity and that it is the real part of the
refractive index of the tube 20 ~hich must be not greater than the refractive
index of the layer
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21.
The manufacturing process can also be used to produce
a succession o~ layers upon -the bore of a tube whose composition
is chosen to produce a substantially quadratic grading of refract-
ive index required in -the production of` a self-focusing multi-
mode fibre. -~
In the manufacture of single mode fibre a dep~sited
layer only 0.5~ thick may be required. This is within the known
art of deposition of oxides without need for close matching of
the expansion coefficients of the substrate and deposited layer
glasses. More generally thicker layers are required, typically in
the range 5 to lO~. in which case attention has to be paid to
matching the various expansion coefficients. Suitable compositions
can be selected from a wide range of known glasses, and in particular
it is known that a variety of high silica content glasses can be
adequately matched to a pure silica glass substrate.
A preferred manufacture for an optical fibre to carry
GaAs laser radiation employs a silica tube 30, as shown in Figure 3,
typically 7 mm external diameter with 1 mm wall thickness. The ~ '
20 bore of the tube is flame polished and then vacuum baked to remove ~ ;
any traces of moisture. The presence of moisture may produce
-OH groups in the completed fibre with their attendant undersi
able absorption in the region of 0.9~ . After the tube has been ~ ~
baked it is placed so as to pass through the centre of an r.f. ~ ;
induction coil 31 and its ends are located in seals 32.
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The higher refractive index core glass coating generally ~ ;
first deposited upon the surface of the bore of the -tube is the
product of a known r.f. vapour reaction process producing a silica
glass containing a few percent titania. The chemical reagents for
this process are silicon tetrachloride, titanium tetrachloride, and
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oxygen. Both chlorides are liquids a-t roorn temperature, but
are introduced to the reaction zone in vapour f'orm by bubbling
dry nitrogen carrier gas through the liquid reagents maintained
at constant temperature. The two liquid reagents are kept sep-
arate, and two independent gas streams are used for the entrain-
ment. This enables the relative proportions of the two vapours
at the reaction zone readily to be controlled merely by altering
the relative flow rates. In the bore of the tube 30 the two
vapours become mixed with a supply of dry oxygen gas. The
reaction does not proceed spontaneously at room temperatures
but is promoted in the localized region of the r.f. excited
glow discharge.
A uniform coating along the length of the bore of the
tube is provided by progressive movement of the tube 30 through the
coil 31 or by movement o~ the coil along the length of the tube.
Uniformity of the deposited layer may be enhanced by rotating the
tube about its axis during the deposition process. A small recipro~
cating movement of the tube or coil in the axial direction may als~
be superimposed.
The drawing of the coated tube into a fibre in such a way
as to collapse its bore is performed as a separate manufacturing
step. The tip of the tube is introduced into a hot zone to soften
it for pulling into a fibre. Surface tension alone will suffice to `;-
convert the softened hollow tube into a solid structure, but may be
assisted by maintaining the inside of the tube at a reduced pressure.
In the present case, where the quality of the initial
silica tube is less pure, the supply of titanium tetrachloride vapour
is shut off while a first cladding glass layer of pure silica
of equal or higher refractive
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inde~ than the tube is deposited in situ. The deposition process is then
repeated using both titanium and silicon tetrachloride vapours so as to
produce a second doped silica layer of higher refractive index than that of
the first layer and which will form the core of the completed fibre.
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