unconditioned laser source designed for operation on an SMF cable is directly
coupled with an MMF cable, differential mode delay (DMD) might occur. DMD may
degrade the modal bandwidth of the fiber-optic cable. This degradation causes a
decrease in the link span (the distance between the transmitter and the
receiver) that can be reliably supported.
Ethernet specification (IEEE 802.3z) outlines parameters for Ethernet
communications at a gigabit-per-second rate. The specification offers a
higher-speed version of Ethernet for backbone and server connectivity using
existing deployed MMF cable by defining the use of laser-based optical
components to propagate data over MMF cable.
Lasers function at
the baud rates and longer distances required for Gigabit Ethernet. The 802.3z
Gigabit Ethernet Task Force has identified the DMD condition that occurs with
particular combinations of lasers and MMF cable. The results create an
additional element of jitter that can limit the reach of Gigabit Ethernet over
With DMD, a single
laser light pulse excites a few modes equally within an MMF cable. These modes,
or light pathways, then follow two or more different paths. These paths might
have different lengths and transmission delays as the light travels through the
cable. With DMD, a distinct pulse propagating down the cable no longer remains
a distinct pulse, or in extreme cases, might become two independent pulses.
Strings of pulses may interfere with each other making it difficult to recover
DMD does not occur
in all deployed fibers; it occurs with certain combinations of worst-case
fibers and worst-case transceivers. Gigabit Ethernet experiences this problem
because of its very high baud rate and its long MMF cable lengths. SMF cable
and copper cable are not affected by DMD.
MMF cable has been
tested for use only with LED sources. LEDs can create an overfilled launch
condition within the fiber-optic cable. The overfilled launch condition
describes the way LED transmitters couple light into the fiber-optic cable in a
broad spread of modes. Similar to a light bulb radiating light into a dark
room, the generated light that shines in multiple directions can overfill the
existing cable space and excite a large number of modes.
Figure 12. LED
Transmission Compared to Laser Transmission
Lasers launch light
in a more concentrated fashion. A laser transmitter couples light into only a
fraction of the existing modes or optical pathways present in the fiber-optic
The solution is to condition the
laser light launched from the source (transmitter) so that it spreads the light
evenly across the diameter of the fiber-optic cable making the launch look more
like an LED source to the cable. The objective is to scramble the modes of
light to distribute the power more equally in all the modes and prevent the light
from being concentrated in just a few modes.
An unconditioned launch, in the
worst case, might concentrate all of its light in the center of the fiber-optic
cable, exiting only two or more modes equally.
A significant variation in the
amount of DMD is produced from one MMF cable to the next. No reasonable test
can be performed to survey an installed cable plant to assess the effect of
DMD. Therefore, you must use the mode-conditioning patch cords for all uplink modules
using MMF when the link span exceeds 984 feet (300 meters).
For link spans less than 984 feet (300 meters), you can omit the
patch cord. We recommend that you do no use the LX and LH GBIC and MMF without the patch
cord for very short link distances of 33 to 328 feet (10 to 100 meters) because
it may result in an elevated BER.