Single-mode optical fiberIn telecommunication, a single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest.
The lowest order bound mode is ascertained for the wavelength of interest by solving Maxwell's equations for the boundary conditions imposed by the fiber, e.g. , core (spot) size and the refractive indices of the core and cladding.
The solution of Maxwell's equations for the lowest order bound mode will permit a pair of orthogonally polarized fields in the fiber, and this is the usual case in a communication fiber.
In step-index guides, single-mode operation occurs when the normalized frequency, V , is less than 2.405. For power-law profiles, single-mode operation occurs for a normalized frequency, V , less than approximately where g is the profile parameter.
Source: from Federal Standard 1037C