Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the coupling of optical fibres
and in particular to optical fibre couplers of the type
in which a number of fibres converge in a coupling zone,
either so as to terminate at a common end face for coup-
ling to a further fibre or group of flbres or some otheroptical component, or prior to diverging after transfer
of light between the various fibres.
In order to achieve good perormance it is important that
the numerical aperture of the fibres be maintained so far
as possible throughout the coupler, and misregistration
and losses in the coupling zone be kept to a minimum. To
achieve these objectives it is desirable that the fibres
converge in the coupling zone to a greater extent than
normal packing of the fibres would permit. Various
approaches to this problem have been attempted, exempli-
fied by the disclosures o United States Patents Nos.
4,083,625 (Hudson)and 4,449,781 (Ligh~tstoneet al~, and
the prior art considered in those patents. In Hudson,
the fibres are prepared so that end surfac~sof the cores
of fibres to be joined are in abutment, and the coupling
zone is then fused and shaped in a fused condition to pro-
vide a tapered transition, or the fibres are shaped prior
to fusion to provide the desired taper. The process is
complicated by the desirability of maintaining continuity
of the fibre cladding, whilst the small size oE the fibres
and the precise ma-ting and orientation required of the ex-
posed core surfaces would make implementation o~ the pro-
cess very exacting and labour intensive.
An alternative approach is disclosed in the Lightstone et
al patent, in which the fibres are etched so that the major
portion of the cladding layer is removed, and are then
fused together whilst being drawn out so that the fibre
cores are biconically tapered adjacent the fusion zone.
There is no direct contact between the cores of adjacent
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fibres, but the theory is that the biconical taper will
increase the reflection angle of the light through the
fusion zone, thus permitting light to pass between the
cores of adjacent fibres in the fusion zone. Obviously
the taper and the degree of etching of the cladding must
be very carefully controlled, and the method disclosed
involves continuous monitoring of light transfer between
the fibres during the fusion and drawing process to
achieve a desired degree of coupling. Since the arrange- I,
ment is dependent upon promoting the escape of light from
the fibre cores into the cladding, some leakage is in-
evitable. The results obtained are somewhat unpredict-
able, as is illustrated by the widely varying transmis-
sion losses shown by the examples, and dis~arities can
occur in the performance o~ di~erent ports o~ the same
coupler. The technique is also dificult to implement
because of problems in controlling etching of the fibres
so that a uniform thin layer of cladding remains through-
out the portions of the fibres to be tapered and fused.
United States Patent No. 4,566,753 to Mannschke shows a
coupler using groups of tapered fibres coupled through a
graded index rod lens. This patent utilizes the lens to
collect and avoid loss of the light escaping from the
tapered portions of the fibres.
Uni~ed States Patent No. 3,933,455 to Chown discloses a
method for processing the ends of optical fibre bundles
by tapering the fi~re ends by etching so as to improve
their packing, inserting the ends in a low refractive in-
dex glass tube, and heating and drawing down the fibres
in the tube to form a solid glass core. There is ho~ever
no disclosure of the application of this technique to a
reflective or transmission type coupler for individual
fibres.
An object o the present invention is to provide couplers
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through which a consistent numerical aperture may be
maintained, in which losses can be maintained at a low
level in which a very high level of uniformity of optical
signal strength can be obtained in all parts and which can
be readily manufactured on an industrial scale.
Accordingly the invention provides a method of
manufacturing couplers for optical fib:res, comprising
etching the cladding glass of fibres to be soupled along
lengths of the fibres from which the coupling is to be
formed so as to remove most of the cladding layers of the
fibres, selectively increasing the intensity of etching in
multiple spaced zones of the etched lengths so as to remove
all of the cladding glass from the core glass in those
zones, bringing the fibres inko general longitudinal
alignment and positioning the fibres so that their
selectively etched zones interengage to provide close
abutment between the fibres in those zones, fusing khose
zones toqether whilst holding the fibres to prPvent drawing
of their cores, so as to form a fused coupling zone in
which the core and cladding glasses are blended, and
directly surrounding the peripheral surface of the fused
coupling zone with means to prevent loss of light
therefrom. Preferably the intensity of etching is
selectively increased in a plurality of longitudinally
spaced zones along each fibre so as to provide the fibre
with a substantially sinusoidally undulating exterior
profile, and the fibres are longitudinally positioned
relative to one another so that the profiles o~ adjacent
fibres interlock. The peripheral iayer around the fused
coupling zone may be provided either by a ferrule or by a
layer of a transparent substance having a re~ractive index
such as to maintain a substantially constant numerical
aperture through the coupler.
The invention extends to an optical fibre coupler formed
from fibras having a glass core and a glass cladding
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layer of lower refractive indcx, wherein the fibres
coupled have portions with a greatly reduced thickness
of cladding layer and untapered cores`which converge into
a coupling zone formed from the glass of the cores and
claddings of the fibres, blended by fusion so that glass
in the coupling zone has a refractive index intermediate
between that of the core and the cladding, and wherein
means are provided peripherally surrounding the coupling
zone to prevent escape of light therefrom.
Further features of the invention will become apparent
from the following description of preferred embodiments
with reference to the accompanying drawings, in which:
Figure 1 is a diagrammatic cross section through an
etching bath for the preparation of fibres for use in
couplers;
Figure 2 is a diagrammatic cross section through a
prepared fibre;
Figure 3 shows prepare~ fibres assembled together prior
to fusion;
Figure 4 shows a group of fibres following fusion;
Figure 5 illustrates a coupler completed by the applica-
tion of a replacement cladding layer to the fusion zone;
Figure 6a shows how the coupler of Figure S may be cut
in half to provide two similar units, each having an end
1 25 face as illustrated in Figure 6b;
- Figure 7 illustrates how the fibre assembly of Figure 4
may be cut in two; and
Figure 8 illustrates how units such as those shown in
Figure 6amay be coupled.
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Referring to Figure 1, optical fibres to be formed into
star or dlrectional couplers are prepared firstly by
stripping the outer protective jacket normally applied
to such fibres over a distance of, typically, about 4
centimetres. A group of fibres F so prepared is arranged
in parallel and then draped over a bath 2 of etchant 4 so
that the portions from which the jacket has been removed
dip into the etchant, which is selected to attack the
fibre material, normally silica. Typically, the etchant
is a blend of hydrofluoric and nitric acids.
In order to etch the fibres to a desired profile, as shown
in Figure 2, the etchan~ is locally agitated by streams 6
of air bubbles emerging from nozzles 8 located in the
bottom o the bath and fed by a small air pump 10. The
amount of air required is readily supplied by a pump
similar to those used for the aeration of domestic fish
tanks. For clarity of illustration, no attempt has been
made in the drawings to show components to scale. Typic-
ally the nozzles 8 are about 1.25 cm apart, about 2 mm in
diameter, and located about 1-2 cm below the fibres. The
intense local agitation of the etchant provided by the
bubbla streams results in etching of the fibres proceed-
ing faster in the areas of impingement of the bubbles
than elsewhere, so that the etched fibres assume a cross
sectional profile as illustrated in Figure 2. The fibres
are~withdrawn from the bath and washed clean of excess
acid when etching has proceeded to the point such that
substantially all the cladding 11 has been removed from
the fibre core 9 in spaced zones 12 equal in number and
spacing to the number and spacing of the nozzles 8, and
the cross sectional profile of the fibre surface shows
approximately sinusoidal undulations. The number of
bubblestreams can be varied, and whilst three is in most
cases~preferred, two or four or more could be utilized.
By staggering the longitudinal positions of adjacent
,
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fibres as illustrated in Figure 3, and by causing the
fibres F to follow slightly curved paths, the sinusoidal~
ly undulating portions of the fibres can be brought into
close abutment in a zone 14. They can be retained in
this relationship during subsequent processing by braid-
ing the fibres in the approaches to the abutment zone
using further interweaving of the fibres similar to that
shown in Figure 3, and by retaining the fibres on sup-
ports 16 using clamping members or adhesive tape 180 The
zone 14 is then subjected to gas or electric heating,
using a gas fired ring burner, an annular electric heat-
ing element or a gas flame moved around the zone, so as
to fuse the glass and allow the core glass and the remain-
ing cladding glass to fuse together into a more or less
homogeneous mass, resulting in A fibre assembly as shown
in Eigure 4, in which the individual fibres merge into a
fusion zone 20 in which the glass has a refractive index
intermediate between that of the fibre cores and that of
the fibre cladding, according to the amount of cladding
glass remaining after etching. There is no drawing and
therefore tapering of the fibre cores entering the fusion
zone. It will of course be understood that the number o
fibres utilized will depend on the applicatlon and may be
any number from two upwards rather than the four or five
fibra assemblies shown in the drawings.
Subsequent treatment of the assembly depends upon the type
of coupler to be produced. If a coupler having the same
number of input and output fibres is required, then it
may be treated as shown in Figure 5 by the application of
a replacement cladding layer 22 to the fusion zone 20.
Since the refractive index of the fusion zone has been
decreased relative to that of the fibre cores by the ad-
mixture of glass from the cladding layer, the cladding
layer is formed from a substance, conveniently a trans-
parent silicone compound, selected so as to have a
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refractive index relative to that of the glass of the
fusion zone such that the numerical aperture of the
fusion zone is substantially equal to that of the origi-
nal fibres. To complete the coupler unit, a close fit-
ting jig ox casing is secured around the layer 22 andadjacent portions of the fibres and secured in place with
optical epoxy resin which may also form the layer 22 as-
suming availability of a resin which can provide approp-
riate refractive index matching. The fibre strands lead-
ing to the fusion zone are then provided with protectivesleeving and the central portion of the unit potted with
suitable material such as epoxy resin within an outer
casing. The external packaging does not have any effect
on the optical characteristics of the unit and may be
varied to suit re~uirements.
If the coupler is to be connected to another optical
component then the unit of Figure 5 may be cut in half
and the ends ground and polished to provide two identical
units, as shown in Figure 6a, each having an end face as
shown in Figure 6b, in which the cladding layer 22 sur-
rounds the fused zone 20. Such units may be cemented
directly into ferrules 24 as shown in Figure 8. The
ferrules are then positioned in a coupling block 26, for
example by threading the ferrules so that they may be
screwed into the block. Such an arrangement is particu-
larly useful when the number of input and output fibres
required is different at the front and so that fibres are
of different configuration or diameter/ or when a single
fibre F is to be coupled to multiple fibres. If a single
fibre is used, a prepared end of the fibre is cemented
directly into a ferrule 24. Each ferrule incorporates
a lens 28 mounted at its inner end, coupled to the end
face of the fibre by a liquia typically a silicone, which
is selected to match the refractive index of the fused
zone 20 (or the fibre coxe in the case o a single fibre).
The lenses associated with the ferrules face each other
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on a commor~ axis within a housing 30 in the coupling
member 26, which is also filled with the refractive index
matching fluid to submerge the inner ends of the ferrules.
The lenses are selected, and khe ferrules are positioned
in the housing so that the focal points of the two lenses
coincide, this being readily achieved by moving the fer-
rules in the housing so as to obtain maximum light trans-
mission through the unit. The ferrules 24 prevent light
from escaping the fused cores 20, and therefore the layer
22 may be unnecessary, although its presence is advantage-
ous in appIications where optimized coupling coefficiency
and minimi~ed back-scattering are desired. In applica-
tions where the layer 22 is considered unnecessary, the
unit of Figure ~ is cut in hal~ prior to the application
of that layer to pro~ride units as shown in Figure 7,
the fused zone 20 of which may be cemented directly into
the ferrules 24, in a manner similar to that shown in
Figure 8. The use of the carefully selected refractive
index matching fluid within the housing 30 prevènts
losses due to unnecessary refractive index transi~ions,
whilst the selection of lenses of appropriate focal length,
so that the fused cores 20 subtend equal angles at the
common focus, avoids loss of light when connecting units
have cores of different diameter.
The techni~ues descxibed above not only provide couplers
which can be made to have very low losses, but provide
considerable manufacturing advantages. The etching of
the fibres is made deliberately non uniform, the undulat-
ing profiles facilitating packing of the fibres in the
fusion core, and providing interlocking between the fibres
which reduces the risk of the extremely delicate fibres
being damaged during subse~uent handling in the course of
the manufacturing process. The etching process i5 much
easier to control, since rather than attempting to leave
the thinnest possible layer of cladding without anywhere
attacking the cores, as in the Lightstone et al patent,
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the et~hant may be allowed to reach the core in the zones
12 without the risk of the core being attacked outside
what will become the fusion zone. Since the cladding o~
the etched fibres through the undulating zones is
discontinuous, the cladding and core glasses tend to merge
on fusion, rather than remaining in discrete layers as in
the Lightstone et al process. This means that very high
coupling coefficients can be rsadily achieved with
consistently low lossesO No biconical tapering of the
fibre cores in required, with the risk o~ light loss and
extreme physical fragility associated with such tapers;
both outright ~ractures and localized microfractures have
an increased incidence in tapered fibres. Furthermore,
the absence of core taper means that the present invention
15 i5 applicable to the manufacture of couplers for single
mode as well as multi mode applications. Although the
foregoing description has assumed the use o~ step-index
fibres, the invention is equally applicable to graded
index fibres.
Although the bubble stream technique described above is
simple and effective in providing the desired selective
etching o~ the fibres r other techniques may be employed.
For example, an ultrasonic standing wave pattern set up in
the etching bath around the relevant æone of the fibres
will produce a similar effect.
