Cisco Catalyst 6500 Series Installation Manual page 124

Determining Cable Distances
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. (See
Figure 2-3
LED
Laser
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
The solution is to condition the laser light launched from the source (transmitter)
so 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
Catalyst 6500 Series Switch Module Installation Guide
2-12
LED Transmission Compared to Laser Transmission
Laser transmission
Figure 2-3.)
Chapter 2
LED transmission
Preparing for Installation
Figure 2-3.)
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