The usual fiber specifications
you will see are size, attenuation and bandwidth. While manufacturers
have other specs that concern them, like numerical aperture (the
acceptance angle of light into the fiber), ovality (how round
the fiber is), concentricity of the core and cladding, etc.,
these specs do not affect you.
- Fiber Itself
Fiber Optics, as we said, is sending signals down hair-thin strands
of glass or plastic fiber. The light is "guided" down
the center of the fiber called the "core". The core
is surrounded by a optical material called the "cladding"
that traps the light in the core using an optical technique called
"total internal reflection." The core and cladding
are usually made of ultra-pure glass, although some
fibers are all plastic or a glass core and plastic cladding.
The fiber is coated with a protective plastic covering called
the "primary buffer coating" that protects it from
moisture and other damage. More protection is provided by the
"cable" which has the fibers and strength members inside
an outer covering called a "jacket".
- Multimode & Singlemode
- Multimode & Singlemode fiber
are the two types of fiber in common use. Both fibers are 125
microns in outside diameter - a micron is one one-millionth of
a meter and 125 microns is 0.005 inches- a bit larger than the
typical human hair.
- Multimode fiber has light traveling
in the core in many rays, called modes. It has a bigger core
(almost always 62.5 microns, but sometimes 50 microns ) and is
used with LED sources at wavelengths of 850 and 1300 nm (see
below!) for slower local area networks (LANs) and lasers at 850
and 1310 nm for networks running at gigabits per second or more.
- Singlemode fiber has a much
smaller core, only about 9 microns, so that the light travels
in only one ray. It is used for telephony and CATV with laser
sources at 1300 and 1550 nm.
- Plastic Optical Fiber (POF)
is large core ( about 1mm) fiber that can only be used for short,
low speed networks.
Step index multimode was the first fiber design but is too
slow for most uses, due to the dispersion caused by the different
path lengths of the various modes. Step index fiber is rare -
only POF uses a step index design today.
Graded index multimode fiber uses variations in the composition
of the glass in the core to compensate for the different path
lengths of the modes. It offers hundreds of times more bandwidth
than step index fiber - up to about 2 gigahertz.
Singlemode fiber shrinks the core down so small
that the light can only travel in one ray. This increases the
bandwidth to almost infinity - but it's practically limited to
about 100,000 gigahertz - that's still a lot!
Fiber, as we said, comes in two types, singlemode and multimode.
Except for fibers used in specialty applications, singlemode
fiber can be considered as one size and type. If you deal with
long haul telecom customers, you may have to work with manufacturers
on specialty singlemode fibers.
Multimode fibers originally came
in several sizes, optimized for various networks and sources,
but the data industry standardized on 62.5 core fiber in the
mid-80s (62.5/125 fiber has a 62.5 micron core and a 125 micron
cladding.) Recently, as gigabit and 10 gigabit networks have
become widely used, an old fiber has been revived. The 50/125
fiber was used from the late 70s with lasers for telecom applications
before singlemode fiber became available. It offers higher bandwidth
with the laser sources used in the gigabit LANs and can go longer
distances. While it still represents a smaller volume than 62.5/125,
it is growing.
Fiber Types and Typical Specifications
| 50/125 OM2
|| 3/1 dB/km
for GbE LANs
| 50/125 OM3
for 850 nm VCSELs
| 62.5/125 OM1
|| 3/1 dB/km
|| 160/500 MHz-km
|| Most common LAN fiber
- ~100 Terahertz
high speed LANs
|| 4-6 dB/km
|| 50 MHz-km
|| Slow LANs
POF (plastic optical fiber)
|| @ 650 nm
| 1 mm
|| ~ 1 dB/m
|| ~5 MHz-km
|| Short Links
CAUTION: You cannot mix and match
fibers! Trying to connect singlemode to multimode fiber can cause
20 dB loss - that's 99% of the power. Even connections between
62.5/125 and 50/125 can cause loss of 3 dB or more - over half