Reference Guide To Fiber Optics
|Topic: Fiber Optic Cable||Table of Contents: The FOA Reference Guide To Fiber Optics|
Fiber Optic Cable
Cable Types: (L>R): Zipcord, Distribution, Loose Tube, Breakout
Cable provides protection for the optical fiber or fibers within it appropriate for the environment in which it is installed.
Fiber optic "cable" refers to the complete assembly of fibers, other internal parts like buffer tubes, ripcords, stiffeners, strength members all included inside an outer protective covering called the jacket. Fiber optic cables come in lots of different types, depending on the number of fibers and how and where it will be installed. It is important to choose cable carefully as the choice will affect how easy the cable is to install, splice or terminate and what it will cost.
job is to protect the fibers from the environment encountered in an
installation. Will the cable become wet or moist? Will it have to
withstand high pulling tension for installation in conduit or continual
tension as in aerial installations? Does it have to be flame-retardant? Ultra flexible?
Will the cable be exposed to chemicals or have to withstand a wide
temperature range? What about being gnawed on by a woodchuck or prairie
dog? Inside buildings, cables don't have to be so strong to protect the
fibers, but they have to meet all fire code provisions. Outside the
building, it depends on whether the cable is buried directly, pulled in
conduit, strung aerially or whatever.
cables are comprised of layers of protection for the fibers. Most all
start with standard fiber with a primary buffer coating (250 microns)
Tight buffer coating
(tight buffer cables like simplex, zipcord, distribution and breakout
types): A soft protective coating applied directly to the 250 micron
coated fiber to provide additional protection for the fiber, allowing
easier handling, even direct termination on the fiber.
Loose Tubes (loose
tube cables): Small, thin plastic tubes containing as many as a dozen
250 micron buffered fibers used to protect fibers in cables rated for
outside plant use. They allow the fibers to be isolated from high
pulling tension and can be filled with water-blocking materials to
prevent moisture entry.
Strength members and stiffeners:
Usually aramid yarn, the same used in bulletproof vests, often called
by the duPont trade name Kevlar, which absorbs the tension needed to
pull the cable and provides cushioning for the fibers. Aramid fibers
are used not only because they are strong, but they do not stretch. If
pulled hard, they will not stretch but eventually break when tension
exceeds their limits. This ensures that the strength members will not
stretch and then relax, binding the fibers in the cable. The proper
method of pulling fiber optic cables is always to attach the pull rope,
wire or tape to the strength members.
The outermost layer of protection for the fibers that is chosen to
withstand the environment in which the cable is installed. Outside
cables will generally be black polyethelene (PE) which resists moisture
and sunlight exposure. Indoor cables use flame-retardant jackets that
can be color-coded to identify the fibers inside the cable. Some
outdoor cables may have double jackets with a metallic armor between
them to protect from chewing by rodents or kevlar for strength to allow
pulling by the jackets. Indoor-outdoor cables have a PE outer jacket
that can be removed to expose a flame-retardant inner jacket for use
Protection Against Water and Moisture
Look at the pictures below to see how each type of cable incorporates these components.
Simplex and zip cord:
Loose tube cables:
Another "cable" type is not really cable at all. By installing a
"cable" which is just a bundle of empty plastic tubes, you can "blow" fibers into the tubes
using compressed gas as needed. If you need to upgrade, blow out the
old fibers and blow in new ones. Both indoor and outdoor versions of
air-blown fiber cables are available and its even been used for FTTH. Special fibers are required that
have been coated for easier blowing through the tubes, but any
singlemode or multimode fiber is available. It's more expensive to
install since the tubes must be installed, special equipment and
trained installers are needed but can be cost effective for upgrades.
Cables that contain two types of fibers, usually multimode and
singlemode. These cables are often used in campus and premises
backbones where the singlemode fibers may be used in the future.
Cables that contain both fibers and electrical conductors. Underwater
tethered vehicles use cables like this, as do some cables used for
remoting wireless antennas or CCTV cameras. These cables must be
properly grounded and bonded for safety.
Cable Design Criteria
Choosing a cable requires consideration of all the environmental factors involved during installation and during the cable's lifetime. Here are some of the most important factors.
Pulling Strength: Some cable is simply laid into cable
trays or ditches, so pull strength is not too important. But
other cable may be pulled thorough 2-5 km or more of conduit. Even
with lots of cable lubricant, pulling tension can be high. Most
cables get their strength from an aramid fiber (Kevlar is the
duPont trade name), a unique polymer thread that is very strong
but does not stretch - so pulling on it will not stress the other
components in the cable. The simplest simplex cable has a pull
strength of 100-200 pounds, while outside plant cable may have
a specification of over 800 pounds.
Bending Limits: The normal recommendation for fiber optic cable bend radius is the minimum bend radius under tension during pulling is 20 times the diameter of the cable. When not under tension, the minimum recommended long term bend radius is 10 times the cable diameter.
Water Protection: Outdoors, every cable must be protected from water or moisture. It starts with a moisture resistant jacket, usually PE (polyethylene), and a filling of water-blocking material. The usual way is to flood the cable with a water-blocking gel. It's effective but messy - requiring a gel remover (use the commercial stuff - it's best- -but bottled lemon juice works in a pinch!). A newer alternative is dry water blocking using a miracle powder - the stuff developed to absorb moisture in disposable diapers. Check with your cable supplier to see if they offer it.
Crush Loads or Rodent Penetration: Armored cables are used because their strong jackets withstand crushing and rodent penetration. Direct burial OSP cables are usually armored or installed in conduit. Armored indoor cables are available with NEC rated jackets for placement with other cables under false floors, as in data centers.
Fire Code Ratings: Every cable installed indoors must
meet fire codes. That means the jacket must be rated for fire
resistance, with ratings for general use, riser (a vertical cable
feeds flames more than horizontal) and plenum (for installation
in air-handling areas. Most indoor cables use PVC (polyvinyl chloride)
jacketing for fire retardance. In the United States, all premises
cables must carry identification and flammability ratings per
the NEC (National Electrical Code) paragraph 770.
These ratings are:
Cables without markings should
never be installed indoors as they will not pass building inspections! Outdoor
cables are not fire-rated and can only be used up to 50 feet
indoors. If you need to bring an outdoor cable indoors, consider
a double-jacketed cable with PE jacket over a PVC UL-rated indoor
jacket. Simply remove the outdoor jacket when you come indoors
and you will not have to terminate at the entry point.
a fiber optic cable for any given application requires considering two
issues, installation requirements and environmental or long-term
requirements. Installation requirements include where and how the cable
will be installed, such as pulled in conduit outdoors or placed in
cable trays in a building. Long term requirements need to consider
moisture or water exposure, temperature, tension (aerial cables), or
other environmental factors.
You should contact several cable manufacturers (two minimum, three preferred) and give them the specs. They will want to know where the cable is going to be installed, how many fibers you need and what kind (singlemode, multimode or both in what we call "hybrid" cables.) You can also have a "composite" cable that includes copper conductors for signals or power. The cable companies will evaluate your requirements and make suggestions. Then you can get competitive bids.
Since the cable plant design will call for a certain number of fibers, consider adding spare fibers to the cable - fibers are cheap compared to installing more cables. That way, you won't be in trouble if you break a fiber or two when splicing, breaking-out or terminating fibers. And consider future expansion needs. Most users install many more fibers than needed, especially adding singlemode fiber to multimode fiber cables for campus or premises backbone applications.
General Guidelines For Installing Fiber Optic Cable