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- Fundamental mode has no nodes and oscillation pattern is half-wavelength. This mode is main mode for single-mode optical fibers. It corresponds to the direct perpendicular angle of light incidence into the cable, so that single ray of light propagates in the center of the core, as most of the energy is concentrated in the center (where is the biggest standing wave amplitude, l=0,m=1 on the picture)
- Higher order modes describe behavior of light, when it enters core at an angle (acceptance cone). Standing waves with bigger amount of peaks are considered to be of higher order. Peaks are distributed along cross section according to the mode indexes: one for angular distribution and other for circular distribution.
Higher the order of the mode, more oscillation happens partially in the cladding (see 1st picture) due to partial refraction of light into the cladding. In angular distributed modes, energy tends to "stick" to the outer layers of the core. This behavior indicates larger angle of incidence, when in optical ray representation, it bounces off the core walls.
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