Note: Descriptions are shown in the official language in which they were submitted.
CA 02639818 2012-05-30
OPTICAL FIBER CABLES
Field of the Invention
This invention relates to optical fiber cables specially adapted for drop line
applications.
Background of the Invention
Fiber-to-the-premises (FTTP) from local telephone and cable service providers
is rapidly being implemented. This service requires a broadband optical fiber
distribution network comprising local optical fiber distribution cables
installed in
neighborhood and city streets. The local distribution cable is a large fiber
count
(multi-fiber) cable. Single fiber or few fiber cables are used for the "drop"
line from
the street to the premises. In many cases, aerial drop lines are used, and
these have
special requirements. In other cases, cables are buried in earth or installed
in
conduit. These installations have different requirements.
Most current optical fiber drop cables are "universal", i.e., have a single
construction designed for a universe of drop applications. However, as applied
to
many current applications the universal designs are excessively large, and are
difficult to connectorize. An example of a robust optical fiber cable design
is
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shown in Fig. 1, the OFS Mini LT. The cable 11 comprises optical fiber subunit
12, abutted on both sides with strength members 13 and 14. This cable has a
design tensile strength of 300 Ibs, compliant with the Telcordia GR-20 and
ICEA-
S-717 standards for Outside Plant optical cables. It is also designed to mimic
s earlier copper cable versions so that the external cable appearance matches
that
of existing copper versions, and standard hardware and installation equipment
may be used for both. However, for some important drop installations,
typically
indoor applications, this cable is either overdesigned or under designed in
the
following particulars.
io These cables are rigid and stiff, and difficult to bend or handle. They
have
a preferred bending axis due to the ribbon shape, making bending difficult in
other than the preferred axis. The 300 lb. tensile requirement leads to a
large
cable footprint, typically about 4 x 8mm.
The non-circular cross-section of the cable makes it difficult to
15 manufacture and handle. The non-circular cross section is partly for
hardware
compatibility in outside installations, which is not relevant to many current
applications. A non-circular cross section also makes the cable difficult to
connectorize. The cable is not flame retardant, and thus not suitable for
indoor
applications.
20 Some optical fiber cables contain gel-filling compounds for preventing
water incursion in the cable. Filled cables are not necessary for indoor
applications.
Universal drop cable designs used in aerial installations may be subjected
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to movement and sag due to wind and ice build-up, and due to mechanical strain
caused by differential thermal expansion. Accordingly some universal drop
cables
commonly have a loose fiber design. In this design the optical fibers are
loosely
received, "floating" within the cable encasement. Again, this is an overdesign
for
optical fiber cables used in less hostile environments.
New designs for FTTP drop cable that offer compact size and low cost, and
ease in connectorizing, are continually being sought.
Statement of the Invention
We have designed an optical fiber cable adapted for drop cable applications
that has a dual jacket, dual reinforcement layers, a round cross section, and
a tight
buffered construction. The optical fiber cable of the invention is a unitary
compact
coupled fiber assembly with a small profile, and is light in weight, while
still sufficiently
robust for many indoor/outdoor drop cable installations. The small profile and
round
construction make the cable easy to connectorize.
Certain exemplary embodiments can provide a method for installing a dual-
jacketed optical fiber cable where the optical fiber cable comprises: (a) a
tight
buffered optical fiber subunit comprising at least one optical fiber encased
in a
polymer layer, the optical fiber subunit having an essentially round cross
section,
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CA 02639818 2012-05-30
(b) an inner reinforcement layer comprising a polymer wrap surrounding the
optical
fiber subunit, (c) an inner jacket comprising a polymer layer surrounding the
inner
reinforcement layer, wherein elements (a), (b) and (c) constitute a subcable
cordage,
the optical fiber cable further comprising: (d) an outer reinforcement layer
surrounding the subcable cordage, (e) an outer jacket comprising a polymer
layer
surrounding the outer reinforcement layer, the method comprising the steps of:
(1) removing the outer jacket and the outer reinforcing layer of a portion of
the cable
thereby exposing a portion of the subcable cordage, (2) routing the portion of
the
subcable cordage to a terminal point, and (3) attaching the portion of the
subcable
cordage to the terminal point.
Brief Description of the Drawings
Fig. 1 is a sectional view of a conventional optical cable designed for
universal
drop cable applications;
Fig. 2 is sectional view of the optical fiber cable of the invention.
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Detailed Description
The dual-jacketed, all dielectric, self-supporting cable of the invention is
shown in Fig. 2. The design comprises an optical fiber subunit with optical
fiber
21, surrounded by a tightly buffered layer 22. The tight buffered optical
fiber
subunit is a 250 micron fiber buffered up to a diameter of 0.9 mm (buffer
layer
thickness 650 microns). Other tight buffered optical fiber subunit diameters,
typically 0.4 mm to 1.2 mm may be used. This allows termination with piece
parts of standard optical connectors. The tight buffer layer completely
surrounds
and encases the optical fiber, meaning that the buffer layer contacts the
optical
1o fiber coating of the optical fiber. The tight buffer layer is a polymer,
for example,
PVC, nylon, polyolefins, polyester thermoplastic elastomers, fluoropolymers,
UV-
curable acrylates, or a combination of these materials. While the preferred
optical fiber subunit contains a single optical fiber, equivalent cable
designs may
have optical fiber subunits with 1-3 optical fibers.
A characteristic of the optical fiber cable design of the invention, and one
that contrasts with the optical fiber cable of Fig. 1, is that cable strength
is
provided by two separate strength members that are concentric with the optical
fiber subunit. The two concentric strength layers are alternated with two
concentric jacket layers.
The inner strength layer 23 in Fig. 2 is a wrap of aramid yarn. This
provides reinforcement, and allows an optical connector to be crimped on the
inner cordage using industry-standard techniques. For outdoor applications,
the
aramid yarn may be coated with a waterswellable finish, or the core may be
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dusted with waterswellable powder, so as to provide waterblocking. Other high
strength polymer tapes or yearns may be used. A polymer wrap refers to any
polymer tape, yarn, ribbon, or the like, made of high strength polymer
material.
The inner jacket 24 is a polymer layer with an outer diameter of less than
3.2 mm, and preferably 2.9 mm, the diameter of industry standard simplex
cordage. The combination of the buffered fiber 21, 22, the inner reinforcement
layer 23, and the inner jacket 24, produces an optical fiber subcable cordage
that
in some applications can be separated from the remaining cable for moderate
cable spans. For example, the main cable can be routed to a connection area
io such as a cable closet or enclosure, and the outer layers of the cable
stripped
leaving only the subcable cordage to be routed to the optical fiber connection
point. The OD of the subcable can have a relatively small standard cordage
diameter, e.g. 2.5mm, 2.4mm, 2.0mm or 1.6mm, to reduce both the overall size
of the drop cable, and produce a small diameter subcable cordage. Thus a
suitable range for the diameter of the subcable cordage is 1.2 mm to 3.2mm.
The inner jacket can advantageously be made flame-retardant when required for
inddor, or indoor/outdoor applications. Suitable materials for the inner
jacket are
PVC, polyolefins such as polyethylene or polypropylene, flame-retardant
polyolefins, polyurethanes, or other suitable materials.
The subcable cordage is enclosed in outer reinforcement layer 25, and
outer jacket 26. Outer reinforcement layer 25 may be made out of any suitable
linear strength member. Aramid yarns are preferred due to low weight and high
specific strength (strength per unit area). However, glass yarns, glass rods,
and
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aramid rods, and combinations of these, may also be used. A ripcord may be
added so as to provide easy access to the inner jacket. For outdoor
applications,
waterblocking may be provided, which includes waterswellable coatings on the
reinforcements, or waterswellable powders, yarns, or tapes applied to the
outer
reinforcement layer. Outer jacket 26 may be made of any suitable material for
the application. For outdoor applications, polyethylene with carbon black may
be
used. If low temperature functionality is required, a UV-resistant
polyurethane
may be deployed. If flame retardancy is required, a PVC, non-halogen flame
retardant polyolefin, or fluoropolymer may be used. Resistance to UV
io degradation or flame retardancy may be incorporated as needed. PVC is a
preferred choice for the outer jacket material as it is easy to process, and
is a
proven material that provides a flexible jacket with some flame retardancy.
The
thickness of the combination of the outer reinforcement layer and the outer
jacket
will typically be in the range of 1.5 to 3.0 mm.
As mentioned earlier, a significant characteristic of the optical fiber cable
of the invention is a small cable diameter and small cross section area. Even
with a relatively complex design, i.e. two reinforcement layers and two jacket
layers, the cable can be produced with an overall cable cross section area of
less
than 25 mm2. The preferred cable diameter is 4.5 mm or less.
An important advantage of the optical fiber cable design of the invention is
that it is easily terminated with standard connectors. To create factory-
terminated `pigtail' (connector on 1 end) or factory-terminated `jumper'
(connector
on both end) cables, the outer jacket and outer reinforcement is stripped
back,
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exposing the inner jacket of the subcable cordage. A length of heat-shrink
tubing
may then be slipped over the end of the cable, providing a seal for the
transition
between the outer jacket of the cable and the stripped end of the subcable
cordage. The subcable cordage may then be terminated using standard
procedures for cordage that will be familiar to those skilled in the art.
Connectors
that may be used will depend on the specific application. If the connectorized
cable is intended for installation indoors, it may be terminated with standard
indoor connectors such as SCs, LCs, STs, FCs, MT-RJs or combinations thereof.
This list is given by way of example and is not limiting. If the cable is to
be
io installed outdoors, but ends of the cable are to be installed in outdoor
distribution
frames or terminals that are sealed so as to be weatherproof, standard
connectors may be used. Combinations of indoor only, `shrouded' indoor
connectors, and hardened outdoor connectors may be used as appropriate.
As noted earlier, the cross section of the cable is essentially round.
However, some degree of ovality can be tolerated. The term "essentially round"
is intended to include oval shapes.
Various additional modifications of this invention will occur to those skilled
in the art. All deviations from the specific teachings of this specification
that
basically rely on the principles and their equivalents through which the art
has
been advanced are properly considered within the scope of the invention as
described and claimed.
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