Figure 1: Evolution of beam profile for the LP₃₇ mode. The intensity profile stays constant.

• For a lossless fiber, the phase fronts of modes are always plane. The Poynting vector, showing the direction of energy flow, is always parallel to the fiber axis throughout the mode profile.
• If there are propagation losses, e.g. due to a dopant in the fiber core, the modes are somewhat changed. The wavefronts may now get curved, indicating an energy flow in the radial direction. (For example, if we have absorption only in the core, energy must flow from the cladding to the core. In most cases, though, there are only marginal deformations of wavefronts.) The optical power then decreases exponentially along the propagation direction; the propagation constant obtains a real part, which is minus the amplitude absorption coefficient.
• Generally, the number of guided modes increases as the wavelength gets shorter. For long wavelengths, one may have a single guided mode only (→ single-mode fibers), or in fact even no guided mode at all. (A step-index profile always has at least the fundamental mode, but for some other profiles this is not the case.)
• The modes form a basis (in the mathematical sense), i.e., a complete orthogonal system, for any guided light. In other words, any complex amplitude distribution which is guided by the fiber can be expressed as a linear superposition of the modes. This mode decomposition can be done for monochromatic light as well as for polychromatic light; in the latter case, one just does the decomposition for each frequency component separately.

Note that a superposition of different modes is generally not a mode itself. Generally, different modes have different β values. Therefore, their phase evolution is different, and the overall intensity profile changes during propagation due to the changing interference conditions. If we have only two modes excited, we get a simple mode beating, where the shape of the intensity profile varies periodically; the period is inversely proportional to the difference in β values. If many modes with different β values are excited, the evolution gets very complicated.

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