Effective Modal Bandwidth (EMB)

What Is Effective Modal Bandwidth?

EMB is the figure of merit that separates one grade of multimode fibre from another. It states, in megahertz-kilometres, how much modulation bandwidth the fibre can carry over a given distance before the different light paths arriving at different times smear adjacent pulses into each other.

The unit multiplies out, which is what makes it useful. 4700 MHz·km is roughly 4700 MHz over a kilometre or 47,000 MHz over 100 metres. Set that against the bandwidth the signal needs and the answer is a distance.

Two bandwidth figures appear on a multimode datasheet and they are not interchangeable. Overfilled launch bandwidth — 1500 MHz·km for OM3, 3500 for OM4 — describes the fibre under an LED-style launch that fills every mode. EMB describes it under the restricted, off-centre launch a real VCSEL produces, which is why EMB and not OFL is what the reach tables are built from.

How It Is Measured

EMB is derived from a differential mode delay measurement. A narrow probe pulse is launched at a series of radial offsets across the core and the arrival times are compared; a fibre whose index profile was made precisely shows little spread between the offsets, a poor one shows a lot.

The DMD data is then weighted by ten standardised source templates to give a calculated minimum, minEMBc, and the reported EMB is 1.13 × minEMBc. The multiplier exists so that a fibre passing the calculation will also pass a direct bandwidth measurement.

What It Is Not

EMB is not attenuation, and this is the distinction that costs people the most time. It cannot be improved by better connectors, cleaner ferrules or a stronger transmitter. Once a link is past the distance its modal bandwidth allows, it fails on pulse overlap no matter how much light arrives.

That is the fundamental reason multimode standards publish metres while single-mode standards publish decibels. On OS2 there is only one mode, so the limits are loss and chromatic dispersion and you can trade one against the other in a link budget. On multimode you cannot.

Where You Meet It

On every multimode fibre datasheet, and implicitly inside every reach number in the reach calculator. It is also the reason OM5 exists: OM5 adds a second EMB specification at 953 nm so that short-wavelength WDM has something to rely on. Compare the grades in OM3 vs OM4 vs OM5.

Related Terms

OM3, OM4, OM5, VCSEL, chromatic dispersion.