The different modes are distinct by the patterns of the propagating electric and magnetic fields (see insert in Figure 04).
Multi-mode fibers generally have a wider core diameter and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,000 meters.
Modes with azimuthal dependence have longitudinal components of both E and B. Although the mathematics is more involved, the general features are similar to the simplified case above.
Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of material absorption, scattering, and connection losses. Although material absorption for pure silica is only around 0.03 dB/km (modern fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques.