Note: Descriptions are shown in the official language in which they were submitted.
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PptlO16
MOUNTING OPTICAL WAVEGUIDES
The invention relates to methods for mounting optical
waveguides, such as optical fibres, in a support tube.
Many optical devices are fabricated in a substrate, such as
lithium niobate. These devices typically comprise an optical
waveguide whose transmission characteristics can be varied by
applying electrical or acoustic pulses to the substrate. In order
to make use of such a device, it has to be coupled to other optical
15 waveguides and in the past this has been achieved by mounting
an optical waveguide, such as an optical fibre, in a metal tube
and then welding the tube and fibre to the substrate in
alignment with the waveguide in the substrate. This is described
for example in JP-A-5758369.
More recently, optical devices have been developed in
which at least two waveguidès are fabricated in the device, the
ends of the waveguides terminating relatively close together. An
example of such a device is a directional coupler in which the
2 5 waveguides are 250 microns apart. Since typical optical fibres
have a diameter of 125 microns it is very difficult to use the
conventional technique described above to connect optical fibres
separately to the ends of each waveguide.
3 0 A more practical alternative to connecting optical fibres
singly to each waveguide is to pre-assemble the ffbres into an
array, and then to align and fix the whole array as a unit. This
imposes strict tolerances on the position of each fibre in the
array. In the case of lithium niobate devices, the core-to-core
3 5 separation of the fibres in the array must be maintained to a
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tolerance of i 1 ,um or better to avoid excessive optical coupling
losses.
EP-A-0274222 describes a method in which a single optical
5 waveguide is mounted in a metal tube. A glass bead is inserted
in the tube which is subsequently heated by applying heat
through the metal tube. The glass bead then melts and on
cooling rehardens securing the optical fibre within the tube. This
method cannot be readily adapted to solve the problem outlined
10 above since on melting the glass bead tends to alter the set
spacing of the fibres. However, in order to ensure low loss
coupling to and from the fibres it is important to maintain the
set spacing between the fibres within the tube.
According to a first aspect of the present invention, a
method of mounting a plurality of optical fibre waveguides in a
support tube comprises the steps of inserting the fibres into the
tube, locating their free ends in a jig capable of holding them at
their desired separation, applying a sealant to be drawn under
2 0 the influence of capillary action along the fibres and the tube,
and causing the sealant to harden thereby to provide a seal
fixing the fibres in the tube.
Where the fibres are silica glass fibres, the sealant is
2 5 preferably glass which flows and can be drawn along the tube
and fibres at a temperature above its melting point but low
enough to avoid damaging the optical fibres, an appropriate
temperature profile along the tube being established by heating
an extended region of the tube.
Conveniently, the fibres are temporarily secured in their
desired configuration before insertion into the tube.
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At least one of the fibres may carry a spacer element to
assist maintaining correct fibre spacing before the sealant has
hardened.
The tube may be heated for the temperature at a first
position to be above the melting point of the sealant and to be
near to but below the melting point at a second position.
According to a second aspect of the present invention, a
method of mounting an optical waveguide in a support tube
comprises heating an extended region of the tube with a
waveguide inserted therein; causing sealant to be drawn along
the tube; and terminating the heating step to allow the sealant to
harden and fix the optical waveguide in the tube, wherein the
temperatures during the heating step are selected to be such
that they do not damage the optical waveguide.
The drawing of sealant along the tube, caused by capillary
action, requires that the tube is heated for a much longer time
2 0 than in the conventional method. If this were achieved by
applying heat to a single position, the temperature required
would be so high, due to the significant heat loss into the
remainder of the tube, that the waveguide, in the case of an
optical fibre, would become brittle, or be damaged in some other
2 5 way. However, by heating an extended region of the the tube,
for example by applying heat at two spaced positions, heat loss is
considerably reduced thus reducing the overall time required
and therefore the total heat input.
The temperature will be selected so as to be high enough to
melt the sealant but be low enough to prevent damage to the
optical waveguide.
The invention is particularly suitable for mounting two or
3 5 more optical waveguides in the support tube. Thus, in
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accordance with a third aspect of the present invention, there is
provided a method of mounting two or more optical waveguides
in a support tube in which an accurately controlled separation
between the waveguides is established by using a jig to maintain
S the waveguide positions while a sealant is allowed to harden and
fix the waveguides in position.
The mounting of two or more optical waveguides within
the same tube is very simple and straightforward a~ the sealant is
10 drawn into the tube by capillary action rather than having to be
provided in situ in the form of a glass bead, for example.
The tube may be heated at a first position to a temperature
above the melting point of the sealant, and at a second position
15 to a temperature below its melting point.
In accordance with a third aspect of the present invention,
a method of mounting an optical waveguide in a support tube
comprises heating the tube with a waveguide inserted therein at
2 0 two positions spaced along the tube; causing sealant to be drawn
along the tube; and terminating the heating step to allow the
sealant to harden and fix the optical waveguide in the tube,
wherein the temperatures at the two positions are selected such
that the amount of heat supplied to the tube during the heating
2 5 step does not damage the optical waveguide.
Preferably, the sealant is introduced through an aperture
in the tube positioned between the two heating positions.
Typically, the sealant will comprise glass particularly where a
3 0 hermetic seal is to be achieved between the waveguides and the
device to which they are to be mounted. However, other forms
of sealant such as an epoxy could be used in other applications.
In some cases, where more than one optical waveguide is
3 5 to be mounted in the tube, there is a risk that they could twist
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together within the tube and/or they could diverge as they
emerge from the tube. To overcome this problem, the optical
waveguides may be joined together at one or more points
adjacent their ends.
The support tube will typically be of approximately
circular cross section or have at least circular aperture; however,
where appropriate, the tube may be of different shape, have a
different cross section, or both. For instance, even though the
10 aperture may be oval or otherwise elongate in cross section to
accommodate several fibres arranged in one plane, the tube will
often be externally of circular cross section for greater ease of
mounting in device housings.
l S An example of a method of mounting optical fibres in a
support tube according to the invention will now be descried
with reference to the accompanying drawings, in which:-
Figure 1 is a plan of the support tube;
Figure 2 is a perspective view of the mounting apparatus;
Figure 3 is a plan of the support tube with the heating
elements;
Figure 4 is a longitudinal section of the apparatus shown in
Figure 3, and
Figure S is a schematic cross section of a modified
30 embodiment of the support tube of Figure 1.
Referring first to Figures 1 and S, the illustrated support
tube 1 is made from stainless steel. The support tube 1 has a
wide diameter bore portion 2 and a narrow diameter bore
3 S portion 3 connected by a tapering portion 4. The internal
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diameter of the portion 3 is drilled out to 0.4 mm over a length
of about 3 mm to accommodate two optical fibres 18, 19, with
the required separation. Two holes 5, 6 are drilled in the side of
- the tube 1 in the tapering portion 4 and the wide diameter bore
portion 2 respectively. The hole S is to allow access for glass
sealant to fix the aligned fibres, and the hole 6 is to allow access
for adhesive to protect the uncoated fibres to the rear of the
glass seal.
Prior to use, the tube 1 should be cleaned in
trichloroethylene, then placed in an ultrasonic bath of soap
solution, rinsed in deionised water and finally cleaned in
methanol.
The tube 1 is crimped onto a length of PTFE sleeving 7 as
shown at 16. Referring now also to Figure 2, the tube is then
loaded into an assembly jig 19. The principal components of the
jig 19 are an aluminium mould 8 with impressions of two fibre
ends 9, 10 to position the fibres to be fixed accurately with their
2 0 cores 250 microns apart; and two platinum heater coils 11, 12.
The heater coils 11, 12 are provided around the narrow portion
3 and the end of the wider portion 2 adjacent the tapering
portion 4 of the tube 1 respectively.
2 5 The aluminium mould 8 has been manufactured by
impressing optical fibres of the same external diameter as the
fibre ends 9 and 10 into the mould metal, in a manner similar to
that described in GB-A-2046466 in relation to the manufacture
of optical fibre joints.
At this stage, the tube 1 is preheated for two to three
minutes at about 500 C to remove any residual fluid and to help
form an oxide layer onto which the glass will wet. In one case,
this temperature was obtained with currents of 2 amps and 3.4
3 5 amps on the coils 11, 12 respectively. In order to learn the
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temperature at the critical point within the narrow portion 4 of
the tube, a sub-assembly tube with a thermocouple mounted
inside it is loaded into the tube and the currents are adjusted
accordingly. At temperatures in excess of 600 C the fibre
5 becomes brittle so that care has to be taken to avoid too much
heat.
The two optical fibres 18, 19 to be mounted have about 50
mm of primary coating 13 along their length removed. The bare
10 fibres are cleaned with isopropyl alcohol, cleaved 10 mm from
the coating and inserted into a jig where the fibres are made to
lay parallel to one another with the cleaved ends in the same
plane. As indicated schematically in Figure 5 at 17, the fibres
are fixed together (Norland UV Curing Type 81 ) at two points,
15 about 30 mm and 200 mm from the cleaved ends, to keep the
fibres 18, 19 from twisting as they are inserted into the sleeving
and to keep the ends in the same plane.
The joined fibres 18, 19 are then loaded into the tube 1
2 0 through the sleeving 7 and located into the grooves 9, 10 under a
glass clamp 14 such that when the stainless steel tube 1 has
expanded with the heat required to melt the glass, there is about
600 microns of fibre protruding from the tube. The tube 1
should, when heated, be at least 200 microns from the
2 5 aluminium mould 8.
The coils 11, 12 are then energised to heat the tube 1 to
about 500 C as before and with the aid of an instrument
manipulator (not shown) a glass rod is offered up to the hole 5.
3 0 The heat of the tube 1 in this region causes glass at the end of
the rod to melt and this melted glass is then drawn into the hole
5 by capillary action. When the glass ceases to be drawn into the
hole, the current in the coil 12 is increased to 3.8 amps to
encourage the flow along the inside of the tube 1 and the fibres
18, 19. The current should be reduced again to 3.4 amps as soon
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as the glass begins to be drawn in or else the temperature will
cause the glass to flow around the hole as well as into it. When
the glass will not be drawn even at 3.8 amps on the coil 12, its
current is lowered to 3.2 amps and that on the coil 11 is
5 increased to 2.6-2.8 amps, causing the glass to be drawn to the
end of the tube. When a meniscus is observed around the fibre
ends , this indicates that the glass sealant
has passed throughout the length of the narrow portion 3 of the
tube whereupon the coils 11, 12 should be cooled down slowly
10 (0.4 amps/minute) until the coil 11 is at 1.0 amps and the coil 12
is - at 2.0 amps, when both currents can be switched off.
Referring now specifically to Figure 5, a modification of the
method just described consists in slipping a spacer in the form of
15 a sleeve 15 over one of the fibres. The sleeve 15 is chosen to
have a wall diameter corresponding to the required gap between
the outsides of the fibres. Thus, if a separation of 25011 m is
desired between the fibre cores, and the fibres are 125~m in
diameter, the sleeve wall will need to be also 12511 m in
2 0 thickness. The effect of the sleeve 15 is to reduce the tendency
of the fibre ends to splay apart after the sealant has set and the
fibre ends are removed from the mould 8.
After carefully removing the assembly from the jig, a few
2 5 drops of Epotek 301 are drawn into the hole 6 to protect the
uncoated fibres behind the glass seal. The adhesive is cured for
60 minutes at about 65C.
This finished assembly can then be connected to an optical
3 0 device such as a directional coupler by offering up the exposed
ends of the fibres to the device and then welding the tube to the
device in a conventional manner.
*trade-mark
