As the core diameter decreases, the number of modes which can propagate in optical fiber decreases. For the core diameter smaller than a specific value, only the fundamental mode can exist. This kind of optical fiber is singlemode optical fiber (SMF). In SMF, it is theoretically proved that the light intensity distribution is close the Gaussian distribution.

Since the mode dispersion can not be zero even for GI-MMF, SMF is employed for the long-distance optical fiber communication. For a ultrashort pulsed light source, the SM propagation is the noly effective mode. This is because the MM propagating light is affected by the mode dispersion and mode noise.

chapter2-08

Fig. Light propagation and refractive index distribution in SMF and MMF.
(a) SI-SMF, (b) SI-MMF, (c) GI-MMF