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Relative Modal Distribution of Multimode Fibers: (Right)
A fully filled fiber means that all modes carry equal power, as shown by the line across the top of the graph. A long length of fiber loses light in the higher order modes faster, leading to the gently sloping "EMD" curve. Mode filtering strips off the higher order modes, but provides only a crude approximation of EMD. The microlensed LED , often thought to overfill the modes, actually couples most of its power in lower order modes. The E-LED (edge-emitting LED) couples even more strongly in the lower order modes. Connectors are mode mixers, since misalignment losses cause some power in lower order modes to be coupled up to higher order modes.
In an actual operating communications system, such controlled conditions obviously do not exist. In fact some work presented by Corning at an EIA Standards meeting shows how far the real world is from what we expected it to be.
It has been accepted as "common knowledge" that microlens LEDs (as used with most multimode datacom systems) overfill fibers, and when we use them as test sources, we are testing with an overfilled launch. Not so. Tests on microlens LEDs indicate that they underfill compared to EMD. And edge-emitter LEDs (E-LED), typical of the high speed emitters at 1300 nm, concentrate their power even more into the lower order modes.
Other facts that come out of the Corning project shows that connectors mix some power back into the higher order modes due to angular misalignment and switches strip out higher modes . In a simulated FDDI system using 8 fiber optic switches and 20 pairs of connectors, with fiber lengths of 10 to 50 meters between them, the majority of system power was concentrated in the lower order modes.
What conclusions can we draw ? The most significant conclusions is that it may not be prudent to design datacom and LAN systems on the worst-case loss specifications for connectors and switches. In actual operation, the simulated system exhibited almost 15 dB less loss than predicted from worst case component specifications (obtained with fully filled launch conditions). In most of today's high speed systems, LEDs are too slow to be used as transmitters, so a special type of low cost 850 nm laser called a VCSEL (vertical cavity surface-emitting laser) is used as a transmitter. VCSELs couple light tightly into the core of a multimode fiber, similar ot a eLED in the diagram above.