Modes Step Index

Consequently, extremely little or no energy is lost to heat through the leakage of the higher modes into the cladding, because they are not present. All energy is confined to this single, lowest-order mode. Because the higher-order mode energy is not lost, attenuation is not significant. Also, because the input signal is confined to a single ray path, that of the lowest-order mode, very little chromatic dispersion occurs.

Single-mode propagation exists only above a certain specific wavelength called the cutoff wavelength. The cutoff wavelength is the smallest operating wavelength when SMFs propagate only the fundamental mode. At this wavelength, the second-order mode becomes lossy and radiates out of the fiber core. As the operating wavelength becomes longer than the cutoff wavelength, the fundamental mode becomes increasingly lossy. The higher the operating wavelength is above the cutoff wavelength, the more power is transmitted through the fiber cladding.

As the fundamental mode extends into the cladding material, it becomes increasingly sensitive to bending loss. Comparing the output pulse and the input pulse, note that there is little attenuation and time dispersion. Lower chromatic dispersion results in higher bandwidth. However, single-mode fiber-optic cable is also the most costly in the premises environment. For this reason, it has been used more with metropolitan- and wide-area networks than with premises data communications. Single-mode fiber-optic cable has also been getting increased attention as local-area networks have been extended to greater distances over corporate campuses. The core diameter for this type of fiber-optic cable is exceedingly small, ranging from 8 microns to 10 microns. The standard cladding diameter is 125 microns.

SMF step index fibers are manufactured using the outside vapor deposition (OVD) process. OVD fibers are made of a core and cladding, each with slightly different compositions and refractive indices. The OVD process produces consistent, controlled fiber profiles and geometry. Fiber consistency is important, to produce seamless spliced interconnections using fiber-optic cable from different manufacturers. Single-mode fiber-optic cable is fabricated from silica glass. Because of the thickness of the core, plastic cannot be used to fabricate single-mode fiber-optic cable. Note that not all SMFs use a step index profile. Some SMF variants use a graded index method of construction to optimize performance at a particular wavelength or transmission band.

August 25, 2011

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