Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to optical cable.
Optical cables are now being used for ~elecommunication
purposes as replacements for conventional cables using electrical
conductors. These optical cables comprise optical waveguides housed
within protective sheaths to protect them from damage which may be
caused by surrounding physical or environmental conditions. These
protective sheaths hence form protection against crushing or cutting
into the waveguides by hard objects such as rocks as such damage
directly increases the attenuation of the waveguides.
While the presence of water within an optical cable is
not detrimental to the performance of waveguides, passage of the water
along the inside of cable should be prevented as its presence at
connection points or terminals may cause problems. More importantly,
the formation of and retention of ice around the waveguides creates a
crushing effect which is known to increase the attenuation to very
serious proportions. Thus, the protective sheaths must also be water
impermeable to prevent or minimize ingress of water. However, even
when efforts are made to prevent water ingress, sheath damage may
provide pathways for water into cable and upon freezing, attenuation
problems will still result. Thoughts have been given, there~ore, to
the provision of a means which will prevent the crushing action of ice
upon the waveguides by the formation of ice, but attempts to solve the ; ~`
proble~ have so far been unsuccessful.
The present invention is concerned with an optical cable
having a medium within its sheath which successfully prevents the
crushing problem upon waveguides in the presence of water when the
temperature drops below freezing.
Accordingly, the present invention provides an optical
cable comprising at least one optical waveguide, a protective sheath
surrounding the wav~guide, and a mix~ure of a hydrophobic and
hydrophilic powder within the sheath and contacting the waveguide, the
hydrophilic powder in contact with water being substantially
non-swellable and forming a viscous solution and preventing the
formation of ice crystals.
With the above invention, the formation and expansion of
a solid mass of ice is prevented and crushing of the waveguides does
not occur.
One particular hydrophilic powder is a high molecular
weight resin which may be an anionic polyacrylamide resin. This may
have an average molecular weight between 2.5 x 106 and 7 x 106 and with
between 12% and 28% acrylamide groupings which have been converted to
acrylic acid groupings. The acrylic acid groupings prevent
destabilization of the viscous solution when the water is not pure as
under such circumstances the polyacrylamide wollld itself be unstable.
Suitable materials are sold under the trade mark "Separan" by Dow
Chemical Corporation or under the trade mark "Magnifloc" by Cyanimid
Company. Alternatively to the acrylic acid groupings, the
polyacrylamide resin may be crosslinked by irradiation to increase its
molecular weight.
As further alternatives, the resin is an irradiated
crosslinked polyethylene oxide or the resin is a hydrolized starch
graft polymer of polyacrylonitrile. ~ ;
The hydrophobic powder is preferably calcium carbonate
surface treated with a hydrophobic material. The calcium carbonate
itself is inert but when surface treated, the powder acts in
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hydrophobic fashion. The surface treatment is preferably by way of a
coating of a water repellent long chain fatty acid or mod;fied fatty
acid selected from the group consisting of lauric acid, myristic acid,
palmitic acid, stearic acid and arachidic acid. Alternatively, the ;~
hydrophobic powder is inherently hydrophobic and is not surface coated.
It has been discovered that for any given cable construction, a
determinable amount of the powder, to provide a given density, may be ~-
used and the coaction of the powder with water does not cause noticable
crushing of the optical waveguide. On the other hand, the powder is
soluble in wa~er to cause a viscosity build-up and, it is believed,
that ice crystal formation is prevented because the water penetration
in this viscous s~stem, is in the form of microbore channels which
break up the ice crystals as they tend to form. Whatever is the true
explanation may not be known, but the effect is, remarkably, that a
solid ice block does not form and no crushin~l of the waveguide takes
place when the viscous solution becomes frozen.
As already stated, the amount of powder, i.e. its
density, is determinable for any particular cable design and this
determination may be made experimentally. If too much powder is
included, this will result in dry powder itself crushing the waveguide
and causing an attenuation increase. Alternatively9 if too little
powder is used, then ice formation into a solid block will not be
prevented and the ice will crush the waveguide. It is found that ~he
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use of the two powders together enables the desired requirements of the
invention to be obtained without completely filling the spaces around
the waveguide with powder whereby the crushing effect by dry powder is
avoided.
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The hydrophobic powder is instrumental in blocking
further penetration by water along the cable after the hydrophilic
powder has formed into a viscous solution in contact with the water
whereby water penetration into the cable is restricted. Suitable ; ~ -~
proportions of the materials to provide a viscous solution to prevent ;~
ice block formation and to restrict penetration of water into the
cable, lie between 5% and 30~ of the hydrophilic powder and between 95X
and 70% of the hydrophobic powder.
The use of the hydrophilic powder and hydrophobic powder
is particularly suitable for an optical cable in wh1ch a plurality of
optical waveguides are housed within grooves provided in a central
core, the powder occupying the voids in the grooves.
The invention also includes a method of making an optlcal
cable having optical waveguides extending along grooves provided within
a core, wherein after the waveguides have been located within the
grooves, a mixture of a hydrophilic and hydrophobic powder is placed ;~
within the grooves in contact with the waveguides prior to the
provision of a protective sheath, the hydrophilic powder in contact
with water being substantially non-swellable and forming a viscous
solution to prevent formation of ice crystals.
In a preferred method, the core with the waveguides
located in the grooves is passed through a fluidized bed of the powder
to powder fill the grooves~ that is the core is passed beneath the
fluidized surface of the bed and powder flows into the grooves and
around the waveguides by reason of its fluid flow characteristics.
Thus, this process is distinct from an electrostatic process in which
particles are electrostatically attracted from a fluidized bed onto the
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surface of an article disposed over the bed. An electrostatic process
is unsuitable for present requirements because it would not result in
filling of the grooves but merely in covering the groove surfaces and
waveguide surfaces. -
It is found that the powder flowing into the grooves by
the Fluidized bed process, remains substantially completely within the
grooves for a short time after removal of the core from the bed, such
time being sufficient to locate a covering material, e.g. a core wrap
around the core so as to prevent the powder from leaving the grooves.
This is surprising in view of the fact that some of the grooves are
downwardly facing and may have openings at the core surface which are
around 3mm wide. Any powder which does drop from the lower grooves
drops onto the core wrap which then replaces the powder in the grooves
as it is raised against the core preparatory to wrapping.
One embodiment of the invention will now be described
wi~h reference to the accompanying drawings, in which:-
Figure 1 is a cross-sectional view of an optical cable
according to the embodiment;
Figure 2 is a graph illustrating the effect of different
~uan~ities of powder in the cable of the embodiment; and
Figure 3 is a diagrammatic side elevational view of
apparatus used in the construction of the cable.
As shown in Figure 1, an optical cable 10 comprises a
crush resistant core 12 surrounding a crush resistant strength member
13, the core having been formed from extruded plastics material such as
high densi~y polyethylene. The core is extruded with a plurality, i.e.
six, circumferentially spaced ribs 14 which extend longitudinally of
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the core~ The ribs may be strictly axial or helically formed. The
ribs have circumferentially wide outer ends and define tapering grooves
16 between them. These grooves may be up to 3mm or more in
circumferential width and have a base diameter of about 6mm. The
outside diameter of the ribs is approximately 10mm.
Each of the grooves 16 carries a plurality of optical
waveguides 18 which are sufficiently loosely contained therein to avoid
any external pressure upon the waveguides, created by heat expansion or
shrinkage of the cable, such as may increase the attenuation of the
waveguides.
The cable structure is as described in greater detail in
copending Canadian patent application Serial No. 362,479 filed October 3,
1980 in the name of R.J. Williams and entitled "Optical Cable" in that
a core wrap 19 of .003mm thick polyester and protective sheath
comprising a metal inner layer 20 and an outer water impermeable
polymeric layer 22 are provided around the core and waveguides.
Additionally, however, the present embodiment includes,
according to the invention, a mixture of hydrophilic powder and a
hydrophobic powder within each of the grooves. This material is
composed oF a hydrophilic powder which is high molecular weight
polyacrylamide powder, e.gO as sold under the trade mark "Separan" by
Dow Chemical Corporation, in admixture with a hydrophobic powder such
as calcium carbonate powder, the particles of which are surface coated
with stearic acid which provides the hydrophobic properties to the
powder.
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As will now be discussed, in the event that moisture or
water penetrates through the sheath to occupy the grooves, the
polyacrylamide powder prevents the waveguides from being subjected to
compression by any ice formed upon lowering in tempera~ure whereby
attenuation in each of the waveguides is not increased. This effect is
thought to be rather surprising in view of the fact that it may be
considered that the hydrophilic powder itself would assist in the
compressive force applied by the ice. However, if the amount of the
powder mixture per unit volume of the grooves is controlled, i.e. its
density, then the required degree of prevention in increase in
attenuation is achieved. The hydrophobic powder, is used for the
purpose of blocking the advance of water along the cable whereby the
water is localizecl at or around its point of entry. It is determined
that suitable proportions of the hydrophilic and hydrophobic materials
to provide a viscous solution to prevent ice block formation and to
restrict penetration of water along the cable mqy lie between 5% and
30% of the hydrophilic material and, correspondingly, between 95% and
70% of the hydrophobic material. The percenl:ages chosen are dependent ;~ ;
upon what is most suitable for particular requirements of cable design
and this may be determined experimentally. The final dry density of
the powder within the grooves depends upon the percentage of
hydrophilic powder being used in the mixture suitable for prevention of
the ice crushing effect upon the waveguides. Hence, this final density
is partly determinable b~ any actual mix ratio that is decided upon.
As an example of the above, Figure 2 is a graph showing
the effect upon the effectiveness of water blockage in the cable of the
first embodiment with the use of different densities of powder fill in
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the grooves. The density is expressed as weight in grams/ft run along
the cable.
To deter~ine the values for the graph, various lengths of
cable were filled with different densities of the powder mixture. The
mixture was the same in each case and consisted of 20% of
polyacrylamide powder and 80% of calcium carbonate coated with stearic
acid. A measured sample (e.g. one foot length) was taken from each
length and was weighed, and the sample separated into its constituent
par~s to remove ~he powder and the parts were weighed to thereby obtain
the weight of the powder per unit length.
Each length of cable was then subjected to a water
blocking test which involved the removal of a portion of the sheath and
core wrapping, loc:ation of a watertight gland across the exposed core
while sealing against the layer 22 at each side of the removed portion
oF sheath, and subjecting the exposed core to a 3 foot head of water
applied directly within the gland. After a specified time, each cable
length was opened to deterMine the distance travelled by the water from
the gland.
As shown in Figure 2a water penetration was clearly less
in cable lengths using more powder per unit length of cable~ This is
to be expected because of the ~reater amounts of hydrophilic and
hydrophobic powder used. Also, as the curve shows, the degree of water
penetration increased disproportionately to the decrease in powder
weight per unit length until at about 6.5gm/ft of powder, the
hydrophobic powder was incapable of forming an effective seal and water
penetrated along the cable as far as the quantity of water would cause
it to pass. An effective block for water occurred at a minimum powder
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content of about 7.2gm/ft. On the other hand, if the density oF the
powder was increased too far, it could cause compacting o-F the filling
medium and a compressive load ko be applied to the waveguides ln the
dry powder state whereby there was a noticeable and undesirable
increase in the attenuation o-f the waveguides. It was determined that
up to a weight of powder of about 9gm/ft length, the attenuation was
either not noticeable or was acceptable. Up to this weight, the powder
mixture also prevented the normal block formation of ice down to a
temperature of -40C whereby the ice itself also applied insufFicient
pressure to the waveguides to deleteriously affect the attenuation.
Severe overFilling caused compacting oF the filling medium and this in
itself increased the pressure on the fiber and caused an attenuation
i ncrease .
Hence, the above test shows that with the particular
cable described in this embodiment and with a powder mix of 20%
polyacrylamide and 80% calcium carbonate, a measure of between 7 and
9gm/ft of powder was effective in water blocking the cable while also
preventing the waveguide attenuation from increasing to unacceptable
levels. With different percentages of the powder materials and
different cable structures, the useful operating range of powder
weights may change. This is, however, easily determinable
experimentally and perhaps by the method described above.
The cable is powder filled in the following manner.
As shown by Figure 3, the cable core 12 with the
waveguides located in the grooves 16 is passed through a fluidi~ed bed
24 oF the powder mixture. The core is passed beneath the defined upper
sur-face of the bed so as ts be immersed within the powder itself. This
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is distinct -From passage of the core above the bed which would entail
the use of means, such as electrostatic attraction, to lift particles
from the bed towards the core surface. In the process of the
embodiment~ the use of electrostatic forces is avoided. It is found
that the powder in the bath flows as a liquid to fill the grooves and
thus surround the waveguides. The density of fill for the grooves may
be changed by changing the throughput speed of the core in the bed
until the desired density is obtained.
Upon leaving the bed, the core is immediately wrapped
with the core wrap 19 as shown in Figur~ 3. This is done by
conventional means and will be described no turther. There is a
surprising effect with this powder fill process, however, and that is
that although the grooves have openings about 3mm wide, no significant
amount oF powder falls from the grooves before the core wrap is
applied. This is even the case with the grooves which lie undermost
and open downwardly. If powder from the lower grooves does fall out,
i~ is caught upon the approaching core wrap and re-inserted in the
grooves upon core wrap application.
After the core wrap is applied, it is passed through
another Fluidized bed 26. The particular core wrap used has a lint
type exterior surface, i.e. one which is a fluffy or matt finish. This
surface is capable of absorbing and holding a surrounding layer of
powder around the core wrap to provide an initial barrier to water
penetration within the shield. The metal inner layer 20 and polymeric
layer 22 are then applied by means not shown, but which are
conventional in the manufacture of electrical cable.
