If the incident angle of light is larger than the critical angle (θc), light can propagate with different optical paths in optical fiber (the number of optical paths is determined by the wavelength, quality, and property of propagating light). Figure schematically represents light propagations in optical fiber at different incident angles. As is shown in Fig., in comparing the optical paths represented in the solid and dashed line, the optical path of dashed line is obviously longer, and the propagation time differs between these two light paths.

Each optical path for light propagation is called mode. The modes are discrete in optical fiber. This is because mode can not stably exist when the standing wave with nords at the core/clad boundary is not formed in the radical direction. Therefore, the mode in optical fiber is the transverse mode in a precise sense.

Fig. Modes of optical fiber.