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. It can mean single-transverse-mode operation, where a laser operates on a single kind of transverse resonator mode, which is almost always a Gaussian mode (although operation on a single higher-order mode is also possible, e. by using some diffractive element in the laser resonator). If operation. Within the laser community, one of the most overused and often miscommunicated terms is the phrase “single mode. ” This is because a laser beam when traveling through air takes up a three-dimensional volume in space similar to that of a cylinder; and just as with a cylinder, a laser beam can be. The mechanism responsible for keeping light confined within the fiber's core is known as Total Internal Reflection (TIR). In the following, we discuss the basic propagation characteristics of optical fibers. where n1 and n2 (< n1) represent the refractive.