Information Technology Reference
In-Depth Information
multimode fiber, so millions of meters of fiber were installed with no specification
for DMD. The problem does not occur in single-mode fibers, because they propa-
gate only one mode. However, a cure exists for legacy multimode fiber to enable the
gigabit operation. The solution is a short length of specially constructed fiber cable
that has been designed to “spread” the laser optic's single mode into multiple modes
before launch down the fiber.
This length of cable is called a
conditioned launch adapter
(Fig. 12.3), and it
may be used for LX transceivers to assist in operation over multimode fiber. LX
operation can be over either single or multimode fiber, so the external adapter must
be used. The conditioned launch mode-spreading function can be contained within
the short wavelength transceiver for SX operation, because SX only uses multimode
fiber. Many manufacturers recognize this problem and now provide a type of higher-
quality multimode fiber that minimizes the glass imperfections that cause DMD.
You can generally recognize these advanced fiber cables by their claims to operate
well at gigabit speeds with the “vertical-cavity surface-emitter launch” VCSEL laser
diodes. These VCSEL optics are relatively inexpensive, as lasers go, and have been
a key component in providing cost-effective gigabit optics.
Fiber attenuation is the final factor that affects gigabit performance. In Chapter
5, we saw that the attenuation of fiber ranges between 1.0 and 3.5 dB for multimode
fiber at 1300 and 850 nm and between 0.4 and 0.5 dB for single-mode fiber at 1550
and 1310 nm. This attenuation affects the distance one can successfully transmit on
a fiber, because the light output power of the laser transmitting optics is limited for
safety reasons, and because the receive power must be sufficient for signal recovery
by the receive optics.
Multimode fiber
VCSEL
source
(a)
Conditioned
launch adaptor
Multimode fiber
VCSEL
source
(b)
FIGURE 12.3
Conditioned launch adapter. (a) Without adapter, glass flaw blocks narrow modes; (b) adapter
(mode spreader) allows most modes to bypass glass flaw.
