Some Question And Answer About OM4 Multimode Fiber


The burst out in demand for bandwidth in enterprise networks is driving an urgent need for higher Ethernet network speeds. There are several factors, including broadband penetration fueled by video-rich content, Data Center demands, and exponential growth in super computer and R&D computing activities.

Laser Optimized Fiber:
Laser optimized multimode fiber is recognized as the medium of choice to support these high speed data networks. With next-generation 40 and 100 Gigabit Ethernet speeds on the horizon, the industry is developing a new type of multimode fiber, called OM4, there is now a standard that is specifically targeted at this product. Prior to the standardization of OM4, these higher bandwidth fibers were sold as a part of OM3.Which will offer a minimum effective modal bandwidth of 4700 MHz-km at 850 nm, compared with 2000 MHz-km for OM3 fiber optic cable.

What is OM4 fiber?
OM4 fiber is a 50 μm laser-optimized fiber with extended bandwidth. It is designed to enhance the system cost benefits enabled by 850 nm VCSELs for existing 1 and 10 Gb/s applications as well as future 40 and 100 Gb/s systems.
OM4 fiber supports Ethernet, Fiber Channel, and OIF applications, allowing extended reach upwards of 550 meters at 10 Gb/s for ultra long building backbones and medium length campus backbones. With an Effective Modal Bandwidth (EMB, also known as laser bandwidth) of 4700 MHz-km (more than double the IEEE require-ment for 10 Gb/s 300 meter support), OM4 fiber is also especially well suited for shorter reach data center and high performance computing applications.
Why is it called OM4 and the relationship with OM1,OM2,OM3?
Multimode fibers are identified by the OM (“optical mode”) designation as outlined in the ISO/IEC 11801 standard:
• OM1, for fiber with 200/500 MHz-km over filled launch (OFL) bandwidth at 850/1300 nm (typically
62.5/125 μm fiber)
• OM2, for fiber with 500/500 MHz-km OFL bandwidth at 850/1300 nm (typically 50/125 μm fiber)
• OM3, for laser-optimized 50 μm fiber having 2000 MHz-km effective modal bandwidth (laser band-width), designed for 10 Gb/s transmission.
For many years 62.5/125 µm (OM1) and conventional 50/125 µm multi-mode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from Ethernet (10 Mbit/s) to Gigabit Ethernet (1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125 µm multi-mode fiber (OM3). Fibers that meet this designation provide sufficient bandwidth to support 10 Gigabit Ethernet up to 300 meters. Optical fiber manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 GbE up to 550 meters. Laser optimized multi-mode fiber (LOMMF) is designed for use with 850 nm VCSELs. Today, this evolution continues with the development of OM4 multimode fiber as the industry prepares itself for speeds of 40 and 100 Gb/s.

What are the standards that define the use of OM4 fiber?
There are a number of standards under development that will define the use of OM4 fiber for high-speed transmission. Within the TIA, work is progressing on TIA-492AAAD, which will contain the OM4 fiber performance specifications. Similarly, IEC is working in parallel to adopt equivalent specs that will be documented in theinternational fiber standard IEC 60793-2-10 as fiber type A1a.3.

What role will OM4 fiber play in next-generation speeds?
IEEE continues to work on standards for next-generation speeds, where OM4 fiber is likely to play a large role.For short reach 40 Gb/s and 100 Gb/s applications on multimode fiber, it appears the IEEE 802.3ba Task Force has defined a Physical Medium Dependent (PMD) solution involving already proven parallel optics technology. This will help preserve the low cost advantage of today’s 850 nm VCSEL light sources. These parallel systems will transmit one 10 Gb/s signals on each of 4 or 10 fibers (for 40 Gb/s and 100 Gb/s, respectively). Each 10 Gb/s signal will be aggregated in an arrayed transceiver containing 4, or 10, VCSELs and detectors.
For these parallel systems, IEEE set an objective of a minimum reach of 100 meters (m), specifically on OM3 fiber (OM1 and OM2 fibers will not be supported in the 40 Gb/s and 100 Gb/s standard). Because the 100 m distance is expected to cover only about 85 percent of data center links, the Task force subsequently adopted OM4, capable of reaching 125 m. Although the additional 25 m may seem insignificant, it will support the majority of the remaining access to distribution and  distribution to core links in large data centers.

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