Four Aspects About Multi-mode Fibers
Data centers are never ceased their steps to bring greater speed and efficiency to telecommunication and datacoms industries. An enormous amount of data is transmitted, gathered and analyzed everyday, all which requires a vast number of high-bandwidth interconnections between data centers, and people. During these interconnections, fiber optic cables see their heaviest use.
Fiber optic cables can deliver more bandwidth for voice, video and data applications, and carry thousands of times more information than copper wire. With fiber optic cables, reliable and secure data transmission is ensured. Fiber optic cables are available in single-mode and multi-mode versions based on transmission mode standard. This article puts its focus on the latter version: multi-mode fiber (MMF), discussing MMF from its core size attenuation, bandwidth and manufacturing ways.
MMF: Larger Core Size
It’s known that MMF has a much larger core size and cladding diameter, whose different types are distinguished by jacket color: for 62.5/125 µm (OM1) and 50/125 µm (OM2), orange jackets are recommended, while aqua is recommended for 50/125 µm "laser optimized" OM3 and OM4. MMF’s larger core endows it greater light gathering capacity, allowing multiple modes of light to propagate through the fiber simultaneously. Thus, MMF is more suitable for relatively shorter-reach application, usually less than 600m. When it’s deployed in GbE applications, the maximum reach is 550m in combination of 1000BASE-SX SFP (ie. 1783-SFP1GSX).
MMF: Attenuation/Signal Loss
Attenuation refers to the reduction of signal loss when light travels through the fiber optic cable, which is measured in decibels per kilometer (db/km). Insertion loss is the total attenuation from all sources plus any reflection losses over a specific fiber length. Such attenuation is often caused by absorption of optical energy by tiny impurities in the fiber such as iron, copper, or cobalt. Sometimes, the scattering of the light beam as it hits microscopic imperfections, called Rayleigh scattering can also lead to signal loss phenomenon. Attenuation problem is a commonplace in MMFs.
MMF: More Bandwidth
Bandwidth quantifies the complicated data-carrying capacity of MMF, given in units of megahertz-kilometer (MHz·km). Bandwidth behavior of MMF arises from multi-modal dispersion (multi-path signal spreading) which happens as the result of light traveling along different modes in the core of fibers. The bandwidth specification of performance of a MMF is verified through optical measurements during fiber manufacture. Actual system performance and data-rate handling rely heavily on bandwidth, affected by transceiver technology and device characteristics.
MMF: Manufacturing Ways
MMF can be manufactured in two ways: step-index or graded index.
Step-index fiber has an abrupt change or step between the index of refraction of the core and the index of refraction of the cladding. Multi-mode step-index fibers have lower bandwidth than other fiber designs.
Graded index fiber is designed to reduce modal dispersion inherent in step index fiber. This design maximizes bandwidth while maintaining a larger core diameter for simplified system assembly, connectivity and lower network costs. Graded index fiber is made up of multiple layers with the highest index of refraction at the core. Each succeeding layer has a gradually decreasing index of refraction as the layers move away from the center. High order modes enter the outer layers of the cladding and are reflected back towards the core. Multi-mode graded index fibers have less attenuation (loss) of the output pulse and have higher bandwidth than multi-mode step-index fibers.
MMF related transceivers: Multi-mode Transceivers
A fiber optic transceiver is a package, usually a plable module, comprising of a receiver on one end of the fiber and a transmitter on the other end. Over the years, multi-mode bandwidth specifications and measurement methods have evolved along with the transceiver technology, so as to keep up with delivery of higher transmission speeds. The combination of transceiver and fiber optic cable plays an important role in fiber’s practical link length. As for multi-mode transceivers which have larger core, they are often used in short-reach applications with 850mn wavelength. Listed below are several commonly-used multi-mode transceiver ports: 1000BASE-SX, 10GBASE-SR, 10GBASE-LRM, among which 10GBASE-SR port type enjoys widely deployment in 10GbE applications when the required distance is not so long. Take F5-UPG-SFP+-R for example, this F5 compatible 10GBASE-SR SFP+ transceiver listed in Fiberstore takes OM3 MMF as its transmission medium for 300m reach.
Besides what have been discussed above, there is also another MMF feature that comes into your mind: that is the affordability. MMF is less expensive than its counterpart single-mode fiber (SMF). Because of this, more people prefer MMF to SMF when the required distance is not so long. Thus, this big saving can be re-invented in other projects.
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