In step index single mode fiber, the core diameter is extremely small, that it allows only one mode to propagate through it. This means that only single light ray propagates through the step index fiber. Due to this the transmitted ray does not experience distortion due to delay. In fiber-optic communication, a single-mode optical fiber, also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining. In this regime, the fiber is called a single-mode fiber. Higher-order modes like LP 11, LP 20 etc.