Single mode fibre directional coupler manufacture

A method of making a single mode optical fibre directional coupler in which a pair of fibres (30,31) in side-by-side contact is subjected to a succession of drawing operations produced by traversing the fibres longitudinally through a flame (34) while stretching them between a pair of carriages (32,33) driven at slightly different speeds.

This invention relates to the manufacture of single mode fibre directional 
couplers. 
In a directional coupler the field associated with the propagation of 
energy in one waveguide is arranged to overlap that of energy propagating 
in an adjacent waveguide so that an exchange of energy can take place 
between the two guides. In an optical waveguide one of the functions of 
the optical cladding is normally to distance the optical field from 
anything that might interact with it and thus provide a potential source 
of additional attenuation. In the manufacture of an optical fibre 
directional coupler this distancing effect of the cladding needs to be 
partially suppressed over a certain distance in order to provide a 
coupling region. One way of achieving this that has been described in the 
literature is by physical removal of a proportion of the cladding by 
etching and/or polishing. An alternative way that has also been described 
in the literature consists of using a drawing-down operation to reduce the 
diameter of the optical core. This reduction causes the optical field to 
expand, and clearly a condition eventually reached when a significant 
proportion of the energy extends beyond the confines of the cladding. In 
this context it will be noticed that an ancillary effect of the drawing 
operation required to reduce the core diameter will have been a 
corresponding reduction in cladding thickness. The manufacture of single 
mode optical fibre directional couplers by this drawing technique has been 
described by B. S. Kawasaki et al. in a paper entitled `Biconical-taper 
single-mode fiber coupler`, appearing in Optics Letters Vol. 6 No. 7 pp 
327-8 (July 1981). The authors state the fabrication methodology to be 
essentially the same as that described by B. S. Kawasaki and K. O. Hill in 
a paper entitled `Low-loss access coupler for multimode optical fiber 
distribution networks` appearing in Applied Optics Vo. 16 No. 7 pp 1794-5 
(July 1977). This involves twisting together two fibres so that they are 
held in side-by-side contact with each other, mounting the fibres under 
spring tension in a jig, and then using a microtorch flame to soften and 
fuse the fibres so that the spring elongates the fibres in the softened 
region to form twin biconical tapers with a total length of approximately 
1 cm. 
We have tried to use this method for the production of single mode fibre 
directional couplers, but have found that the size and shape of the 
smallest part of the biconical taper is so critical in the determination 
of the resulting coupling characteristics of the directional coupler that 
the method appears far from suited to commercial scale manufacture. 
The present invention concerns an alternative drawing method of making 
fibre directional couplers that is more readily controllable than the 
above referenced method and produces a longer coupling region. The 
increased length of the coupling region means that its cross-sectional 
area is correspondingly larger. This is advantageous because the light is 
therefore less weakly guided and hence less susceptible to the effects of 
environmental strain. Moreover the effects of any applied environmental 
stress are not so heavily concentrated into a short zone. 
According to the present invention there is provided a method of making a 
single mode fibre directional coupler, wherein a plurality of glass single 
mode optical fibres held in side-by-side contact with each other are 
subjected to a plurality of progressive stretching operations to produce 
the requisite optical coupling strength between the fibres, in each of 
which operations the fibres are axially stretched at a controlled rate 
while being moved axially relative to a heat source that provides a 
localised zone within which a region of the fibres is sufficiently 
heat-softened to allow plastic flow stretching of the fibres to occur 
within this zone.

The confinement of the optical field of the fundamental guided mode of an 
optical fibre is related to the V-value of that fibre by the relationship 
EQU .lambda.V=2.pi.a(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2 
where .lambda. is the wavelength, a is the core radius and n.sub.1 and 
n.sub.2 are respectively the core and cladding refractive indices. From 
this formula it is clear that if a length of optical fibre is subjected to 
a drawing operation to reduce its diameter the V-value will diminish. Such 
a reduction in V-value is accompanied by an increase in the fundamental 
mode radius (defined as the radius enclosing 1-e.sup.-2 of the total 
power). These effects are depicted in FIG. 1 for a typical single mode 
fibre designed for operation at 1.3 microns. This fibre has a core 
diameter of 9 microns and a cladding diameter of 125 microns. At the 
operating wavelength of 1.3 microns its core and cladding refractive 
indices are respectively 1.480 and 1.447, and hence the V-value of this 
fibre lies between 2.1 and 2.2. FIG. 1 shows that the power does not begin 
to spread appreciably till, by drawing down the fibre to about half its 
original diameter (diameter draw down ratio of 2), the V-value has been 
reduced to about 1. However, it is also seen that, by the time a draw down 
ratio of 2.5 is reached, the fundamental mode radius is expanding very 
rapidly indeed and is about to rise above the diminishing value of the 
cladding radius. Clearly once this stage is reached quite small changes in 
draw down ratio will have a major effect upon the coupling between the two 
fibres of a directional coupler. 
FIG. 2 is a graph showing the observed variation in coupled power during 
the making of a directional coupler by symmetrically tapering down a pair 
of twisted fibres using a static flame method. In the making of this 
coupler the twisted pair of fibres was locally heated with a stationary 
microtorch flame while the ends were moved apart at a constant rate to 
produce a biconical taper. Light was launched into one end of one of the 
fibres, and the light output from the other end of that fibre was 
monitored to provide a trace which records the power output as a function 
of time. The flame was applied at time 1, after which the drawing down 
operation proceeded at a linear extension rate of between 100 and 200 
microns per second until time t.sub.2, at which the output had dropped by 
just less than 3.5 dB, whereupon the extension was halted and the flame 
extinguished. As predicted, the initial stages of extension produced 
substantially no coupling between the fibres. However, once coupling did 
become apparent, it proceeded at an ever increasing rate at least as far 
as the 3 dB point. At this stage the tapered region rapidly cooled upon 
extinction of the flame, and the loss increased by a further 1 dB. It is 
the rapidity of onset of coupling that makes this particular method of 
making a coupler so difficult to control in such a way as to terminate the 
drawing operation at the appropriate end point. This problem of control 
becomes progressively worse as couplers are pulled that require the 
stronger coupling factors necessary to exploit their spectral properties 
in the manufacture of wavelength multiplexers and demultiplexers. Simply 
slowing down the extension rate used in the drawing process does not help 
matters to any significant extent because the longer the heat-softened 
thinned fibre is left in the flame the greater is the risk of it sagging 
or becoming blown awry by the flame. Any localised bending of this nature 
causes unacceptably high losses resulting from the weak guiding associated 
with low V-value in this region. Associated with this problem is the 
further problem resulting from the significant change in coupling that 
occurs on removal of the flame and the consequent cooling of the coupling 
from about 1800.degree. C. to room temperature. 
An attempt to reduce the scale of the first problem by trying to lengthen 
the region of the draw down zone by the use of a fishtail flame was found 
to be largely ineffective. It is believed that the reason for this was 
that the temperature profile of a fishtail flame will evitably produce one 
or more local hot spots at which the fibres draw down faster than at other 
points. The resulting regions of smaller diameter cannot conduct the heat 
away as fast as larger diameter regions, and hence a runaway condition is 
created with the result that once again the coupling is effectively 
confined to a short region of strong coupling. 
Turning attention now to the preferred method of the present invention, and 
referring to FIG. 3, two single mode glass optical fibres 30, 31 from 
which any traces of plastics cladding have been removed are twisted 
together to hold their surfaces in side-by-side contact over at least a 
portion of a region between two independently driven blocks 32 and 33 to 
which the fibres are securely clamped. The clamping is arranged so that 
the axis of the twisted pair of fibres is accurately aligned with the 
direction of motion of the two blocks. Between these two blocks is located 
a microtorch 34 whose flame provides localised heating of the fibres 30, 
31. This microtorch is fixed in position. 
The blocks 32, 33 holding the ends of the fibres are driven in the same 
direction but at different speeds. The leading block is always driven 
slightly faster than the trailing block so that the fibres are subjected 
to a progressive extension as they are scanned through the flame of the 
microtorch. This process is analogous to the pulling of optical fibre from 
a preform, but whereas in fibre production the draw down typically 
involves a linear scale reduction in the region of 200, in the present 
instance a scale reduction of about 1% or not much more than a few percent 
is desirable. Several traverses are then required to produce the requisite 
amount of coupling. The blocks are driven at speeds typically lying in the 
range of 5 to 10 mm per minute. For good control of speed to about 0.25% 
it is preferred to drive each block with its own digitally controlled 
servo-motor incorporating a shaft-encoder in its feedback loop. The 
preferred way of providing successive traverses is, on termination of each 
traverse, to reverse the rotation sense of both motors and, at the same 
time to change their relative rotational rates so that the block that was 
formerly the trailing block is now driven faster than the other block 
because it has assumed the function of the leading block. Thus successive 
traverses take place in opposite directions. Usually the rotational rates 
will simply be interchanged at the end of each traverse so as to give the 
same rate of extension for the succeeding traverse, but at least one 
change of extension rate may be employed. This is so that a relatively 
faster rate may be used for the first few traverses before the onset of 
any appreciable coupling, and then a slower rate of extension is employed 
to permit greater control over determining precisely when to halt the 
process. Flame ignition is approximately synchronised with motor start-up, 
and similarly flame extinction is approximately synchronised with the 
halting of the motors. In this context it should be appreciated that the 
motor start-up can safely lead the flame ignition by a few moments since 
the extension rate is small having regard to the elastic strain that the 
cold fibres can tolerate. Similarly flame extinction can safely lead motor 
shut-down. 
The momentary dwell of the flame that is associated with each reversal of 
the motors may produce the trace of an undesirable neck in the draw-down 
profile. The optical effects of any potential neck of this sort can be 
reduced or eliminated by ensuring that the later traverses are made 
sufficiently longer than the initial ones to terminate well up the 
shoulders produced by the ends of these initial traverses in regions where 
there is effectively no coupling between the fibres. (The traverses must 
however be confined to the region where the two fibres are in contact with 
each other, so as to avoid any risk that the flame would allow a swan-neck 
to form in either fibre.) A convenient way of monitoring the extension 
process is to position, before the start, a pointer 35, 36 on each block 
so that they meet in the plane of the microtorch flame. At the end of the 
first traverse, assumed to be in the direction of arrows 37, the tips of 
the two pointers will have moved apart, and the pointer 35 will intersect 
the plane of the microtorch. On the second traverse the flame reaches the 
end of the reduced diameter portion produced by the first traverse when 
the tip of pointer 36 again reaches the plane of the microtorch. 
In the production of a typical simple 3 dB coupler the blocks may start 5 
to 8 cm apart, and typically four or five traverses are employed to 
produce an extension of between 2 to 5 cm. The production of a coupler for 
multiplex applications requires tighter optical coupling between the 
fibres, and will therefore generally involve more traverses and a greater 
extension. 
The manufacturing process is monitored by directing light of a particular 
wavelength into one end of one of the fibres and observing the changes in 
light output from the other end of either or both fibres as the extension 
proceeds. It is found that light launched into one end of one fibre, fibre 
A, is initially transferred virtually exclusively to the output from fibre 
A because the coupling is too weak for there to be any appreciable 
transfer of power into the second fibre, fibre B. Then, as the extension 
proceeds, and the coupling gets stronger, the output from fibre A 
decreases while there is a corresponding increase in the power output from 
fibre B. In due course the 3 dB point 40 of FIG. 4 is reached, at which 
the power output is equally divided between the two fibres. Then as 
coupling is increased still further to enter the domain of over coupling, 
the output from fibre A is diminished until the power is transferred 
totally to fibre B as represented by the points 41. A further increase of 
coupling brings more power back into the output from fibre A until, after 
passing through a second 3 dB point 42, power emerges exclusively from the 
output of fibre A as represented by points 43. After this, a continued 
increase in coupling starts the whole cycle over again, with the power 
output oscillating between output from fibre A and output from fibre B. 
The coupling strength of any particular configuration of coupler depends 
upon the coupler geometry, the V-values of the fibres within the coupling 
region, and the length of that region. The V-values depend not only upon 
fibre geometry and refractive indices, but also upon wavelength. 
Therefore, neglecting the effects of material dispersion, coupling 
strength can be expected to increase with wavelength. This wavelength 
dependence can be employed, by suitable device of geometry, to form 
devices for wavelength multiplexed optical systems. Thus FIG. 5 shows the 
spectral characteristics of a coupler produced by the method described 
with particular reference to FIG. 3 and designed for multiplexing or 
demultiplexing signals at 1.33 and 1.5 microns. To obtain these 
characteristics light from an incandescent filament was directed into a 
grating monochromator, and the input end of one fibre was held in fixed 
position at the monochromator output. A cladding modes stripper was 
interposed between this end of the fibre and the coupling region because 
this light launching arrangement inevitably involves the launching of 
unwanted cladding modes into the fibre in association with the wanted core 
mode. This particular arrangement of incandescent filament and 
monochromator provided a light source that could be tracked in wavelength 
from its short wavelength cut off in the region of 0.8 microns to its long 
wavelength cut off in the region of 1.8 microns. Trace 50 was obtained by 
monitoring the light output from the other end of the fibre into which the 
light was originally launched, while trace 51 was obtained by monitoring 
the output from the other fibre of the coupler. The first 3 dB point is 
seen to occur at a wavelength in the region of about 1.0 microns, though 
the picture is slightly confused by the effects of higher order modes 
which are not cut-off till about 1.2 microns. (This coupler is thus seen 
to be an overcoupled single mode fibre coupler, i.e. a coupler for which 
the coupling is so strong that the first 3 dB coupling point occurs at a 
wavelength shorter than the higher order modes cut-off). In second 3 dB 
point occurs in the region of 1.22 microns, and almost no power is seen to 
be transferred from the first fibre to the other in the region of 1.32 
microns. At about 1.34 microns the picture is again slightly confused, 
this time by a spurious spike appearing in the output of the 
monochromator. At a wavelength of about 1.5 microns the launched power is 
seen to be transferred to be virtually completely transferred from the 
first fibre to the second. Thus this device is seen to be capable of 
acting as a multiplexer or as a demultiplexer in a system operating at the 
two wavelengths of 1.32 and 1.55 microns. The position on the spectrum of 
the first 3 dB point and the spacing between this and subsequent 3 dB 
points can be altered by modifying the geometry and length of the coupling 
region. Thus whereas the coupler of FIG. 5 has 3 dB points separated by 
approximately 0.2 microns that of another coupler whose spectral 
characteristic is depicted in FIG. 6 exhibits a separation of 
approximately 0.1 microns. 
In the manufacture of these couplers the main advantages of the use of the 
differential pulling technique of the present invention include the 
feature that the rate of tapering can be made very slow, being limited 
only by the degree of control over motor speed, and hence good control of 
the end point is possible. In this context it is to be noted that the 
coupling region can be spread over a significant length of uniformly 
pulled fibre, and hence the application or removal of the localised 
heating provided by the microtorch has a proportionately smaller effect 
upon the end point since at any one time the flame is heating not the 
whole coupling region but only a small proportion of it. Additionally the 
extended coupling region is to be preferred because the coupling strength 
per unit length is less. This means that the V-values of the component 
fibres are higher, and hence the fibres are less susceptible to bending 
loss. Also it means that there is no sharply localised neck in the 
structure at which strain resulting from misalignment is liable to be 
concentrated. Both these factors ease the problems of designing a housing 
for the coupler that will make the optical performance of the resulting 
package relatively insensitive to strains of mechanical or thermal origin. 
Although the foregoing specific description has been related exclusively to 
couplers made from pairs of optical fibres, it will be evident that the 
invention is applicable also to couplers made from more than two fibres. 
Thus the invention is applicable to the manufacture of three-fibre 
couplers for the type of application described in the paper by K. P. Koo 
entitled `Performance Characteristics of a Passively Stabilised Fibre 
Interferometer using a (3.times.3) Fibre Directional Coupler` appearing in 
Proceedings of 1st International Conference on Optical Fibre Sensors, 
London 26-28 April, 1983. 
It has already been explained that the reason for requiring the fibres to 
be in side-by-side contact with each other over the entire drawing-down 
region is to ensure that the application of localised heating does not 
induce the formation of a swan-neck. In the foregoing specific description 
of couplers the necessary contact has been achieved by twisting the 
component fibres together, but it should be understood that this is not 
the only way of achieving this end. Thus an alternative method involves 
threading the fibres through a length of glass sleeving, and then locally 
heating that sleeving with a traversing heat source, such as the 
microtorch flame, to cause the sleeving to soften and collapse around the 
fibres under the effects of surface tension. 
It will also be evident that it is not essential for the localised heat 
source to be stationary in order to achieve the relative movement required 
for the performance of the invention. However, in general a stationary 
heat source is preferred because this simplifies the drive arrangements 
for the other components of the drawing apparatus.