Fiber-Optic Techniques - LAN Wiring - page 258

The answer that many are realizing is that you can do both. The standard spec-
ifies a minimum of two cables to each work area outlet, one of which may be fiber.
However, it does not prohibit placing more than two cables to a work area.
Virtually all modular cover plate designs allow at least four module inserts. So, the
optimum solution for today and the future is to place two copper cables and two
fiber cables, one single-mode and one multimode, in each outlet box. If you use the
new SFF fiber connectors, and Category 5e or Category 6 copper modules, you
should be set for many application/years to come.
VCSEL Laser Source Multimode Problem
A popular laser source for high-speed networking is the vertical-cavity surface-emit-
ter laser (VCSEL). This semiconductor diode combines high bandwidth with low
cost and is an ideal choice for the gigabit networking options. In a single-mode fiber
environment, the laser source works flawlessly. Its highly collimated beam of light
is easily aimed right down the center of the narrow single-mode core and propagates
in essentially a single mode of transmission, with all the attendant advantages.
However, as we just covered, much of the installed fiber in the local network
world is actually multimode fiber. Back when we were using LED sources for the
lower-speed technologies, such as 10BaseFL and 100BaseFX, we simply aimed the
LED at the transmit end of a multimode fiber and let things happen. In effect, the
excess light that did not fill the core was lost, but the light that did enter the core of
the fiber propagated along the core in all modes (thus the term multimode ). The
method of applying the source light to the fiber is called the launch , and applying
more light than is needed is an overfilled launch.
Unfortunately, when multimode fiber was made in the past, little attention was
paid to minor defects in the glass core. If multiple modes are launched into the fiber,
a tiny aberration here and there causes little harm, because all the other modes (rays)
of light get through just fine. At most, these defects might cause a small increase in
cable attenuation. Nothing to be worried about, unless you launch with a laser source!
Remember that a laser is an extremely narrow beam of coherent light, on the
order of 1 µm from most VCSELs, that is launched precisely down the center axis of
the fiber core. This means that any minor defect may deflect a substantial amount of
the available light energy, causing an excessive amount of attenuation. This may, in
turn, cause a link failure if the system cannot tolerate an extra 2 to 10 dB of loss.
Worse yet, minor deflections of the fiber, which might be caused by mechanical move-
ments or even temperature changes, can cause the link problem to be intermittent.
Two solutions exist to what has been dubbed the VCSEL problem. One is to
get better fiber. Certainly, if you are installing new cable, this is the best option. You