The first generation of LAN to consider LED, is very cheap. The spot is very very big, VCSEL light source is also large. So the multimode fiber is researched a lot in 80-2000 year, diameter is 50, 62.5 microns.

Today we will talk about the multimode fiber, the research is mainly used for LAN optical transmission at that time(now extended in data center).

Single mode fiber, there are restrictions on the mode of optical transmission. Small dispersion, high requirement for light source, transmit far.

Multimode fiber, big dispersion, low requirement for light source, can’t transmit far.

In the standard of multimode fiber, OM1 OM2 mainly is LED design, wit a window of 1300nm wavelength. Later found that LED is not an ideal light source, OM4 OM3 began to turn to the transmission of VCSEL, that is 850nm wavelength.
For LED light source, called overfilled launch(OFL) bandwidth
For VCSEL light source, called the effective modal bandwidth(EMB)
First to analyze the bandwidth statement. Unit is MHz.km, is the product of distance and bandwidth.

Simple understanding is that high speed signal transmit short distance, low speed signal transmit long distance.

OM1, 2, 3, 4 is multimode fiber generation upgrade. Performance is getting better and better.

For the initial OM1,2, hope that the cheap LED can be used, called OFL bandwidth. LED's light is big, like a flashlight, one part is filled with core output.

Later found that VCSEL is the better multimode light source, then index of EMB appears.
The optical spot of VCSEL is much smaller than LED.

(Of course VCSEL is much bigger than DFB, VCSEL uses dozens of micron fiber, DFB uses a few microns):

Compared with the cost, single mode fiber is cheaper, and the multimode fiber is expensive.