Cisco 7609-S Installation Manual page 110

Appendix B Connector and Cable Specifications
Cable Specifications
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 cable. (See Figure B-9.)
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 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, exciting 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 300 meters, you can omit the patch
cord (although there is no problem using it on short links).
For link spans less than 984 feet (300 meters), you can omit the patch cord.
(We do not recommend using the LX/LH GBIC and MMF without a patch cord
for very short link distances of 33 to 328 feet (10 to 100 meters.) The result could
be an elevated bit error rate [BER]).
Cisco 7609 Internet Router Installation Guide
B-16
OL-5079-04