There is a new multimode fiber standard TIA 492-AAAD that has just been published by TIA. This standard applies to class Ia, graded-index, 50/125 µm multimode optical fiber that is designated as OM4 fiber in the industry.
The main benefit is that it provides a higher bandwidth for optical signal transmission at 850 nm wavelength. Both the IEEE and Fiber Channel standards committees are incorporating it into their current standards development efforts, enabling longer transmission distances than currently achievable with OM3 and lower grade multimode fiber.
The performance of conventional 62.5/125 µm multimode fiber and the 50/125 µm multimode fiber are designated as OM1 and OM2 respectively.
The bandwidth of these fibers is specified using an overfilled launch test method. OM3 and OM4 are referred to as "laser-optimized" multimode fibers because they are specifically designed for optimum performance when used with low-cost vertical-cavity surface-emitting lasers (VCSELs).
The fiber's refractive index profile is precisely controlled to minimize differential mode delay (DMD). DMD of the fiber is specified as a function of the radial distance from the center of the fiber in fractions of picoseconds per meter.
The combination of the modal structure of the lasers, the modal delay structure of the fiber, and the coupling between the laser and the fiber modes are used to determine the effective modal bandwidth (EMB) for OM3 and OM4 fibers.
The EMB value is aligned with the assumptions of the IEEE P802.3ae (10GBASE-SR) link budget model for a channel including attenuation and inter-symbol interference (ISI).
What are the performance differences between OM1, OM2, OM3 and OM4 fibers and what do these performance values mean for current and future optical fiber applications, such as Fiber Channel and Ethernet?
In answer to this question, the key performance specifications and the maximum transmission distances for these applications are summarized in the table on the right.
What are the main reasons to consider OM4 fiber for new installations?
• Multimode optical fiber systems enabled by 850 nm VCSEL technology are significantly less expensive (2x to 3x) than those used to power single mode fibers at 1300 nm
• OM4 provides the highest bandwidth among available multimode fiber alternatives
• OM4 fiber can support distances of 300 metres and potentially up to 550 metres for 10 GbE
• OM4 fiber can support distances up to 125 metres minimum for the 40/100 GbE IEEE 802.3ab standard under development (see note). The additional 25 m compared to OM3 fiber will support most access-to-distribution and distribution-to-core links in large data centers
• OM4 fiber offers the potential to increase the distance for 40/100 GbE when combined with tighter transceiver specifications. Distances between 150 m to 250 m may be feasible in the future
OM3 "Laser optimized" fiber is currently widely used to support 10 Gigabit applications for intra-building backbone and data centre applications for distance up to 300 metres. OM4 fiber provides additional benefits for longer distances, potentially up to 550 metres and is well positioned to support future 40 GbE and 100 GbE applications.
(Note: A related CNS magazine standards update article published January/February 2009 on "40 Gb/s and 100 Gb/s Ethernet" discusses multimode implementations using parallel optics. These implementations (40GBASE-SR4 and 100GBASE-SR10) require the use of four or 10 pairs of multimode fibers respectively.)