There are different categories of graded-index multimode glass fiber cable, including OM1, OM2, OM3 and OM4 cables (OM stands for optical multi-mode). OM1 specifies 62.5-micron cable and OM2 specifies 50-micron cable. These are commonly used in premises applications for shorter reach 1Gb/s networks. But OM1 and OM2 cable are not suitable for today’s higher-speed networks. OM3 and OM4 are both laser-optimized multimode fiber (LOMMF) and were developed to accommodate faster fiber optic networking such as 10, 40, and 100 Gbps. Both are designed for use with 850-nm VCSELS (vertical-cavity surface-emitting lasers) and have aqua sheaths.
OM3 specifies an 850-nm laser-optimized 50-micron cable with an effective modal bandwidth (EMB) of 2000 MHz/km. It can support 10-Gbps link distances up to 300 meters. OM4 specifies a high-bandwidth 850-nm laser-optimized 50-micron cable an effective modal bandwidth of 4700 MHz/km. It can support 10-Gbps link distances of 550 meters. 100 Gbps distances are 100 meters and 150 meters, respectively.
What Makes OM4 Different?
OM4 fiber, with its higher bandwidth, has an extremely precise refractive index profile, virtually free of perturbations or defects. Just like OM3, OM4 is a 50-micron, considered to be laser-optimized multimode fiber for use with VCSELs. The key difference of OM4 from OM3 is in the refractive index of the fiber, which is more precisely graded to better equalize the arrival time of the light modes traveling at various speeds along the core of the fiber. Mode equalization depends on how well the graded index profile is constructed during fiber manufacturing. The better the modes are equalized, the higher the bandwidth of the fiber. This translates to higher bandwidth and a 550-meter reach for 10Gb/s (with some vendors claiming a 600-meter reach) and a 150-meter reach for 40/100 Gig, compared to 300 and 100 meters respectively for OM3.
You can use OM2 fiber with VCSELs, but its performance is limited to 550 meters at 1 Gb/s and only 82 meters at 10 Gb/s, compared to OM4 fiber’s reach of over 1000 meters at 1 Gb/s and 550 meters at 10 Gb/s.
OM3 and OM4 fibers are selected as the only multimode fiber for 40G/100G applications, which are the development trends of fiber optic communication. The 40G and 100G speeds are currently achieved by bundling multiple channels together in parallel with special multi-channel (or multi-lane) connector types. This standard defines an expected operating range of up to 100m for OM3 and up to 150m for OM4 for 40 Gigabit Ethernet and 100 Gigabit Ethernet. The OM3 and OM4 fibers are optimized for 850-nm transmission and have a minimum 2000 MHz.km and 4700 MHz.km effective modal bandwidth (EMB), respectively. Two EMB measurement techniques are used today for the bandwidth measurement. The minimum effective modal bandwidth calculated (EMBc) method, in our opinion, offers a more reliable and precise measurement compared to the differential mode delay (DMD) mask technique. With minEMBc, a true scalable bandwidth value is calculated that can reliably predict performance for different data rates and link lengths. With a connectivity solution using OM3 and OM4 fibers that have been measured using the minEMBc technique, the optical infrastructure deployed in the data center will meet the performance criteria set forth by IEEE for bandwidth.
OM4 cable is also especially well suited for shorter reach data center and high performance computing applications, which is the best option for the small percentage for users running 10Gb/s over links between 300 and 550 meters (or the even smaller percent who anticipate running 40 or 100Gb/s between 100 and 150 meters).