These are called as single mode fibers because they support on one mode of propagation(TE,TM or TEM). The optical signal traveling inside this fiber has only one group velocity. Due to single mode traveling, the amount of dispersion is less than than introduced in multimode fibers. These fibers can have either step index or graded index profile. They are high quality fibers used for wide band long haul communication and they are fabricated from doped silica to reduce internal attenuation.
A highly focused light beam is produced by the light source. This beam travels almost horizontally. The critical angle is close to 90º. As the critical angle is close to 90º, the propagation of different beams is more or less similar, the delays are negligible and the signal reconstruction is easier.