Multimode fiber optic cable is commonly used in data transmission. It has a larger diameter core with a common diameters in the 50 to 100 micron that allows multiple modes of light transmission. Because of this, multimode fiber optic cable has higher “light-gathering” capacity. However, the quality of the signal is reduced over long distance for high dispersion and attenuation rate with this type of fiber. Compared to single-mode fiber, the multimode fiber bandwidth-distance product limit is lower. So it is mostly used for communication over short distances, such as within a building or on a campus. There are many types of multimode fiber optic cables and different type has different transmission rate.
Multimode fiber cable is described by their core and cladding diameters. It is usually 50/125 um and 62.5/125 um in construction. The 50/125 um multimode fiber has a core size of 50 um and a cladding diameter of 125um; while the 62.5/125 um multimode fiber has a core size of 62.5 um and a cladding diameter of 125um (shown as the figure). The transition between the fiber core and the cladding should be sharp, which is called a step-index profile, or a graded-index profile. These two types have different dispersion characteristics, so they have different effective transmission distance. Multimode fibers can be constructed with graded or step-index profile, and those fibers with graded-index are better in accuracy and performance. The specified size 50/125um and 62.5/125um include the diameter and the cladding. The cladding limits the light to the core because of its lower refractive index. The cable construction is shown in the following figure and indicates the cable core, cladding and outer jacket diameters.
Multi-mode fibers are described using a system of classification determined by the ISO 11801 standard — OM1, OM2, and OM3 — which is based on the modal bandwidth of the multimode fiber. OM4 (defined in TIA-492-AAAD) was finalized in August 2009 and was published by the end of 2009 by the TIA. The letters “OM” stand for optical multimode.
OM1 specifies 62.5μm cable and has been the most popular multimode fiber choice throughout the 1980’s and 1990’s. By the early 21st century, it has become the most widely used multimode fiber. However, compared to other multimode fibers, it has the lowest data transmission rate and the shortest transmission distance. The performance of OM1 (62.5 μm) fiber has reached its peak at this time.
OM2 specifies 50-micron cable. For many years, 62.5/125 μm (OM1) and conventional 50/125 μm multimode 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) which are not suitable though for today’s higher-speed networks. And because of their relatively large core size, they were ideal for use with LED transmitters.
OM3 is laser optimized multimode fiber. This technology allows the laser transmission system to use multimode optical fiber for the first time without the use of mode adjustment cables. OM3 together with new low-cost vertical cavity surface emitting laser realizes 10G transmission.
OM4 fiber has been on the market since 2005 and it is the optimized product of OM3 fiber. OM4 fiber and OM3 fiber are fully compatible with each other, and both of them use the same light green outer jacket. Like OM3, OM4 was designed specifically to work with VSCEL. But the difference is that OM4 has a longer transmission distance of 550m in 10G transmission while OM3’s is 300m in the same condition. In addition, the effective mode bandwidth of OM4 is 4700MHz.km which is more than double that of OM3 (the effective mode bandwidth is 2000 MHz.km of OM3). And OM4 has a greater advantage in transmission distance ans cost control.
Followings are the tables for types and specifications of multimode fiber optic cables:
The type of multimode fibers is up to hundreds and different different types have different transmission constant and rate which result in narrow bandwidth, high dispersion and high loss of fiber optic. Therefore, multimode fiber cable is only suitable for middle short transmission distance and fiber optical communication system with small capacity.