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optical multimode (fiber optics) (OM)

OM classes (Optical Multimode) are the standardized classification ofoptical fibers for FO cabling, comparable to the classification of TP cables( category).

The standardization of multimode fibers in OM classes supports the cabling of Gigabit Ethernet, 10 Gig abit Ethernet( 10GbE), 25 Gigabit Ethernet( 25GbE), 40 Gigabit Ethernet( 40GbE), 50 Gigabit Ethernet ( 50GbE) and 100 Gigabit Ethernet ( 100GbE). It is carried out by ISO/ IEC and until 2008 provided three classes for multimode fibers (OM1, OM2, OM3) and two OS classes for single- mode fibers. In 2008, IEEE 802.3ba specified OM4 as the fourth OM class for 40- gigabit Ethernet and 100-gigabit Ethernet, which met the increasing bandwidth requirements in data centers. In 2016, another OM class was added, OM5, which supports short wave division multiplexing( SWDM) and wideband multimode fiber(WBMMF) with transmission rates up to 400 Gbps. Transmission is via eight multimode fibers and MPO connectors over a distance of 150 m.

The standardization of the OM classes takes into account different specifications for single-mode fibers and gradient fibers, as well as the optical windows used with their different excitation conditions by light-emitting diodes(LEDs ) and lasers. The four optical classes have minimum transmission lengths between 300 m and 2,000 m.

OM classes for optical transmission

OM classes for multimode fibers

OM classes for multimode fibers

Wavelengths of 850 nm and 1,300 nm are available for optical transmission in different OM classes. The attenuation values are the same in all classes, but the bandwidth length products are very different.

For the OM1 class, a bandwidth length product of 200 MHz x km is specified at 850 nm, the OM2 class has one of 500 MHz x km, and the OM3 class is suitable with a modal bandwidth of 2 GHz x km. The OM" and OM3 fibers are available in a plus version (OM2+, OM3+) with improved characteristics. They are laser-optimized and more insensitive to bending.

OM3 operates at wavelengths of 850 nm with a VCSEL laser, which is much more efficient than other lasers used in conjunction with single-mode fibers. In this configuration, distances of 300 m can be bridged for 10 Gigabit Ethernet. Whereby this is only ensured by adhering to the extremely tight limits for the Differential Mode Delay( DMD).

Standardized colors for OM cables

Standardized colors for OM cables

OM4 class multimode fibers are for 40- and 100-gigabit Ethernet and have an Effective Modal Bandwidth( EMB) of 4.5 GHz x km. The Telecommunications Industry Association( TIA) and the International Electrotechnical Commission (IEC) are working on standardizing optical fiber to OM4. Like OM3, OM4 is specified exclusively for a wavelength of 850 nm.

Since the distances that can be bridged with OM4 cable are relatively short and four or ten multimode fibers have to be used for the transmission of 40GbE (4x10Gbit/s) or 100GbE (10x10Gbit/s or 4x25Gbit/s), OM class 5 relies on wideband optimized multimode fiber (WBMMF). In contrast to conventional multimode fibers, this has a wide first optical window in the wavelength range between 850 nm and 950 nm. Multiple wavelengths can be transmitted in this range. The wavelength division multiplexing used for this is Short Wave Division Multiplexing (SWDM), which transmits signals at four different wavelengths between 850 nm and 950 nm.

Use with 10- and 100-gigabit Ethernet

IEEE 802.3ba has specified a Physical Medium Dependent( PMD) with parallel optical transmission using multiple VCSEL lasers with a wavelength of 850 nm. With this parallel technology, four or ten 10 Gbit/s signals are transmitted in parallel for 40 or 100 gigabits, respectively. In this constellation, OM3 multimode fibers can bridge 300 m at 10 Gbit/s and 100 m at 100 Gbit/s. However, since this only covers about 80 % of the applications in data centers, the OM4 fiber was provided for these applications, which brings it to 550 m and 125 m respectively. The 25 additional meters are crucial to cover the remaining applications.

Englisch: optical multimode (fiber optics) - OM
Updated at: 21.08.2019
#Words: 602
Links: classification, fibers, fiber optics (FO), cabling, terminal portability (ISDN) (TP)
Translations: DE

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