What is WBMMF? - Wideband Multi-mode Fiber Explained

WBMMF – The Multi-mode Fiber Evolution Drive

Today data centers are supporting the heavy loads of our results of hours of work, online socialization and video entertainment. These loads are translated in Terabytes of storage and are demanding high speed and low latency applications. Everyone wants to drive on highways and avoid congestion. Data center’s highways are represented by the allocated bandwidths between the main components of the digital ecosystem: computational processing units, storage units, backup and restore units, defense units and so on. The layer 1 physical representation of these highways are optical fiber patches and optical fiber links. The roles are split: single mode fibers, with their thin cores (9 microns) and single mode transmission of light, allow huge data streams to flow between data centers (hundreds of kilometers, between 1270nm and 1625nm), while the multi-mode optical fibers although shorter in length (hundreds of meters) and with larger core diameter (65 microns or 50 microns) support the local interconnectivity (at 850nm lambda and in special cases also 1310nm lambda) and agile daily operation of a data center.

A general consideration about the comparison of the costs of single and multi-mode fibers are coming up. A lower cost of production for system components of a multi-mode optical fiber patch is driven from physical principles and components of conducting the light from one end to another. It is also sustained by the number of ports to be cabled in a data center. Particularly, the multi-mode fiber has large Numerical Aperture (NA), or cone of acceptance of light. Light entering outside of the cone escapes out of the core. Large NA allows easier, less precise transmitter & connector alignment and Lowers costs of transmitters, connectors and thus for the whole installation. Light travelling multi-modes through the fiber leads to the spread of light pulse width but excessive pulse spreading is the precondition for intersymbol interference (ISI) which leads to bit errors. In addition pulse spreading limits the bandwidth (transmission carrying capacity).

Is there any way to overcome these limitations and increase the bandwidth for the multi-mode fibers of up to 400Gbps?

Here are the main approaches for that:

• Reduce the core diameter from 65 to 50 microns
• Reduce the modal Dispersion
• Use as a light source the VCSL - Vertical Cavity Surface Emitting Lasers which have the small spot size approaching the fiber, excites fewer modes, bandwidth dependent on which modes happen to carry power. Disadvantage: Non-uniform, fluctuating, non-repeatable power profile. Different VCSELs give different bandwidth, even on the same fiber.

What is the bandwidth of a given multi-mode fiber? How is it measured?

There are two ways to compute the bandwidth of a multi-mode fiber: Effective Modal Bandwidth Calculated (EMBc) and DMD Measurement mask which represent difference in delay between the earliest and latest arriving pulses. There are standards that are applied: TIA FOTP-220 or IEC 60793-1-49.

The Effective Modal Bandwidth Calculated (EMBc) method:

The method is bandwidth calculated from interaction of the DMD with ten simulated VCSELs (weighting functions). Ten VCSELs that represent the range of compliant VCSEL specifications. The lowest of the 10 calculated bandwidths is minEMBc. minEMBc is multiplied by 1.13 for EMB.

The new WBMMF -Standard

OM3 and OM4 provide very high laser-optimized modal bandwidth at 850nm, the
predominant wavelength of multi-mode applications. But to provide equivalent performance
over a range of wavelengths needed to support low-cost wavelength division multiplexing
(WDM) requires a new fiber specification because the modal bandwidth of OM3 and OM4 can diminish quickly when operated at different wavelengths, making them less than ideal for
supporting WDM lane rates above 10 Gb/s per wavelength. Because of this the new WBMMF standard OM5 comes into the picture.

Why is the OM5 standard so important?

It gives IEEE 802.3, Fibre Channel and Infiniband input the possibility on how a standard defined multi-mode fiber will operate with the next generation systems with up to 400G speed, for example with SWDM components.

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