An optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest. Note 1: 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 . Note 2: 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. Note 3: 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. Note 4: 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.