Laser-optimized Multi-mode Fiber (LOMF)

It occurred to me as I was writing the last post that many of you may not be aware of the different grades of multi-mode fiber and that for the purposes of this blog, it would be good to present the differences so many of my points can be thoroughly understood.

Right now, there are three standardized types of LOMF as well as what most of us in the industry call FDDI-grade fiber, which is not laser optimized. So first, what does laser-optimized actually mean? In basic terms, it just means that the fiber was designed to be used with lasers, and in the case of MMF, typically VCSELs. FDDI-grade fiber pre-dated the use of VCSELs so is not laser-optimized - it was intended for utilization with LEDs. Lasers were adopted as the light source of choice when scientists and engineers realized that LEDs became very unstable when trying to modulate them at data rates beyond 100 Mbps. They originally tried to use the same lasers that were being used in CD players, but these turned out to be unstable at Gigabit data rates as well. In the early 1990s, the development of the VCSEL enabled these higher data rates.

As the light sources evolved, the fiber progressed with them. So, for 850nm operation today we have four choices: 

1. OM1 (FDDI):  Minimum OFL Bandwidth of 200 MHz•km; 10G Minimum Optical Reach of 33m
2. OM2:  Minimum OFL BW of 500; 10G Minimum Optical Reach of 82m
3. OM3: Minimum OFL BW of 1500; 10G Minimum Optical Reach of 300m
4. OM4: Minimum OFL BW of 3500; 10G Minimum Optical Reach of 550m

As you can see, the bandwidth of the fiber is intimately tied to what type of light source is used and the optical reach is dependent on both bandwidth and data rate. And, while OM1 fiber wasn’t necessarily designed to be used with lasers, it works fine with them, albeit at a shorter distance than with LOMF. Of note as well is the fact that there are a few cable manufacturers that also provide what I would call OM1+ cable that is 62.5-micron, but is laser-optimized, so may have some improved bandwidth and reach.

All this leads to a very important point – when specifying a cabling system for your networks and data centers, it is important to understand not only the fiber you’re going to install, but also the equipment you’re trying to connect. Just because you're "only" installing Gigabit systems and you've used OM1 fiber for years, doesn't mean it's the best solution (or even the most economical) for today and tomorrow.