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.
In practice, the orthogonal polarizations may not be associated with degenerate modes. Synonyms monomode optical fiber, single-mode fiber, single-mode optical waveguide, unimode fiber.
Source: from Federal Standard 1037C