What parameters impact gigabit transmission in multimode fiber? We explore important factors that anyone working with multimode fiber should be aware of and understand.
There are six parameters influencing gigabit optical transmission technology used for LAN electronics. These are:
The first two here, Channel loss and Intersymbol Interference (ISI), typically make up the bulk of gigabit transmission loss budgets, and both are parameters that are reliant on the quality and performance of fiber optic cabling.
The other 4 factors are relatively small in LAN systems, but can play more of a role where transmission distances are longer.
The total path loss or attenuation between the transmitter and receiver is the summation of various loss mechanisms: scattering, microbending, macrobending and interconnection. The total attenuation of the optical signal between transmitter and receiver limits the maximum system length and the number of connections allowed.
The intrinsic fiber loss is dominated by what is known as Rayleigh scattering, which results from variations in density and compositions of the glass. This loss varies with the wavelengths of light applied.
In the LAN environment, the most cost effective electronics operate in the 850 nm wavelength using LED (Light Emitting Diode) and VCSELs (Vertical Cavity Surface Emitting Laser). 850nm is not the area of minimum loss for a fiber but allows sufficient distances required in the LAN.
The power coupled into the fiber core is also attenuated by microbending and macrobending losses. A microbend is a local deflection of the fiber axis with amplitude much less than the fiber diameter.
A ray of light propagating within the core will remain confined to the core unless microbends are sufficient to cause it to strike the core-cladding interface at an extensive angle, which allows the ray to escape and increase loss.
A macrobend is a bend or loop in the fiber with a radius of curvature of several millimeters or more. The effect of macrobends is to cause power to be lost from the core and thereby induce additional loss.
Interconnection loss associated with splices and connectors can be divided into two components- intrinsic and extrinsic. Intrinsic mechanisms relate to the fiber itself and are a direct function of manufacturing tolerances on fiber core diameter, ovality, eccentricity and numerical aperture.
Extrinsic mechanisms depend on the connection hardware and its ability to control separation between the fiber ends. Variations in the alignment and axial tilt and fiber transverse offset normalized to core diameter, result in connector ‘insertion loss’.
The specific characteristics of the LC connector, including wide fiber spacing, better fiber alignment, precision mating, easy connector cleaning and simplex/duplex configuration combine to produce overall superior performance for factory-terminated and field installed connectors.
Physical fiber contact and proper cleaning of fiber optic connectors are also critical factors in connector performance. With individually spring-loaded ferrules for each fiber, LC connectors mate more precisely and ensure that fibers are always in physical contact. The individual ferrules also make fiber cleaning much simpler.
The LC connector meets the tight loss budget requirements of the high bit rate applications such as gigabit ethernet, and exhibits loss performance of 0.1 dB, far better than competitive products and connector standards.
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