Optical fiber service cable

An optical service cable includes a core tube containing a stack of optical ribbons and a filling compound. The filling compound may include a flame-retardant material, such as magnesium hydroxide or aluminum trihydrate.

BACKGROUND OF THE INVENTION 
This invention relates to an optical fiber service cable. The cable may 
serve as a buried service cable in a local area network to extend from the 
outdoor environment to customer premises. 
Optical service cables, sometimes called drop cables, must differ somewhat 
in design from typical indoor cables and outdoor cables in that they must 
be operable both in the outdoor environment, where the optical fibers must 
be protected from deleterious effects resulting from water ingress, and 
the indoor environment, in which the cable must pass tests to ensure that 
it does not rapidly spread flames from one part of the building to another 
through a riser duct. The UL-1666 vertical flame test is commonly used for 
this purpose. The fluid waterblocking compositions often used in cables 
for outdoor applications must be compensated for in order for a cable to 
pass the required flame test. A service cable designed to meet these 
problems is U.S. Pat. No. 4,892,282 which issued on Jan. 9, 1990 in the 
names of Christopher A. Story and Susan M. Cooper. The Story et al. cable 
includes a core tube made of a flame-resistant material such as a 
thermoplastic rubber enclosing individual coated optical fibers and a 
filling compound. The core tube is surrounded by a tensile layer of 
densely packed loose fiberglass yarns, and water swellable powder may be 
applied thereover. A layer of fiber reinforced plastic members for 
compressive strength surrounds the loose fiberglass yarn layer, and is 
enclosed in a sheath of flame-resistant material. 
Another service cable is disclosed in U.S. Pat. No. 5,050,957, which also 
provides a flame-retardant outer sheath, a core tube holding individual 
coated optical fibers, and a strength member system between the sheath and 
the core tube. Waterblocking yarns are also present. An alternate 
embodiment includes a waterblocking tape and a metallic tape layer. 
U.S. Pat. No. 5,343,549, which issued Aug. 30, 1994 in the names of Samuel 
D. Nave and Harvey R. McDowell, III, includes an outer flame resistant 
jacket over multiple layers of buffer tubes containing individual optical 
fibers and filling compound which are stranded about a central tensile 
member. Between the outer buffer tube layer and the outer jacket is a 
system including a flame resistant tape between layers of aramid yarns. 
Optical fiber ribbon cables are also used in today's telecommunication 
networks. Ribbon cables are sometimes thought to be desirable, especially 
where it is important to gang splice many optical fiber connections in a 
short time. U.S. Pat. No. 5,293,443, which issued Mar. 8, 1994 in the 
names of Christopher K. Eoll et al. discloses an optical fiber cable 
having buffer tubes stranded around a central member, the buffer tubes 
holding optical ribbons and a filling compound. The Eoll et al. cable is 
not disclosed to be suitable for an indoor environment and does not 
disclose flame retardance as an object of the cable. 
What is desired and seemingly is not provided by the prior art is an 
optical fiber ribbon service cable including both flame retardance and 
waterblocking provisions. The prior art also seemingly has not provided 
filling compounds including flame-retardant materials in optical service 
cables. 
SUMMARY OF THE INVENTION 
The foregoing problems of the prior art have been overcome with the optical 
fiber ribbon cable of this invention, which is suitable for buried and 
riser or plenum use in a local area network. The cable includes a plastic 
core tube enclosing at least one optical ribbon. Typically, the core tube 
holds a stack of up to eighteen optical ribbons each holding up to twelve 
coated optical fibers surrounded by a common ribbon coating, which may be 
formed of a material curable by ultraviolet light. The core tube also 
encloses a waterblocking filling compound, which may be a grease 
composition. The waterblocking filling compound effectively blocks entry 
of water into the core while minimizing optical loss to the cable. The 
core tube loosely surrounds the ribbon stack and preferably is of a single 
layer construction of polypropylene. A dual layer construction in which 
the outer layer is polybutylene terepthlate (PBT) and the inner layer is 
polycarbonate may also be used. Stranded about the core tube is a layer of 
discrete resin impregnated fiberglass strength members. Surrounding the 
strength members is a system comprising a polyimide tape, which acts as a 
flame barrier, between optional aramid fiber layers. A tape comprising a 
superabsorbent material may optionally be included in the system as well. 
Stranded about the tape and superabsorbent material is a layer of discrete 
resin impregnated fiberglass antibuckling members. If desired, a metallic 
tape for protection against gnawing by rodents, mechanical protection, and 
underground location may be included. Completing the cable is an outer 
jacket made of a flame retardant material. 
Cables according to the invention may also include optical transmission 
members such as individual coated light waveguides in the core tube 
accompanied by modern filling compounds containing materials such as 
magnesium hydroxide or aluminum trihydrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a dielectric cable designated generally by the numeral 10 and 
having a core which includes a stack of one or more optical ribbons 17. 
Each optical ribbon 17 comprises a planar array of a plurality of optical 
fibers each having a core and a cladding and having individual coatings as 
is well known to the art, the coated optical fibers in an array surrounded 
by a common coating which preferably is made of ultraviolet light curable 
material. In the preferred embodiment, each ribbon contains twelve optical 
fibers and the ribbon stack contains up to eighteen ribbons. The ribbon 
common coating material may be urethane acrylate. 
Plastic core tube 11 in the preferred embodiment loosely holds the optical 
ribbon stack and is of a single layer construction made of polypropylene. 
Single layer tubes made of polybutylene terephthalate (PBT) or 
polyethylene also may be used. A dual layer construction with the outer 
layer made of PBT and the inner layer made of polycarbonate (PC) may be 
used as an alternative. The inner diameter of tube 11 may be 9.4 mm and 
its outer diameter may be 10.2 mm. Tube 11, as well as other plastic tubes 
12 and 22, is made using crosshead extrusion. Tube 11 may be manufactured 
to have a length less than, equal to, or greater than the length of 
ribbons 17. A waterblocking filling compound 14 is inserted by a filling 
needle within tube 11. Filling compound 14 may be grease based gel-like 
material, or other filling compounds can be used as desired such as that 
disclosed in U.S. European patent application 484 744 A2, incorporated by 
reference, disclosing a filling compound comprising an oil and a 
flame-retardant material, including magnesium hydroxide and aluminum 
trihydrate, for optical cables. 
Tensile strength members 18 are arrayed about core tube 11. Members 18 are 
made of aramid yarn, fiberglass, or impregnated fiberglass. Members 18 may 
be impregnated or coated with water swellable material. Members 18 may be 
helically stranded or stranded with reverse oscillating lay. 
Surrounding members 18 is a spirally wrapped fire resistant tape 13, which 
is made of a material which may be a polyimide such as Kapton or 
fiberglass or Teflon, and may be 0.001 inch thick. Tape 13 may be between 
layers 16 of a water absorbent material, which may be in the form of 
longitudinally wrapped tapes, yarns, or loose powder. Surrounding tape 13 
is layer of antibuckling members 15 contacting and coupled to outer jacket 
22, which is pressure extruded over members 15. Antibuckling members 15 
are made of impregnated yarn and may be obtained from Owens-Corning or 
Pittsburgh Paint and Glass. Flat glass-reinforced plastic or aramid fiber 
rods may also be used. Members 15 may be helically stranded or stranded 
with reverse oscillating lay. 
Cable 10 has a flame-retardant outer jacket 22 which may be made from the 
material Gary Smokeguard II 6920, a flame retardant polyvinyl chloride, or 
a fluoropolymer such as PVDF or FEP. A rip cord 19 underlies jacket 22 of 
FIG. 1. FIGS. 3 and 4 show a cable 20, an armored version of cable 10. 
Metallic armor tube 21, which may be corrugated, surrounds an inner 
plastic polyethylene jacket 12, which need not be made of a flame 
retardant material. Water swellable powders or tapes may be added to the 
cable between tube 11 and jacket 12 if desired.