Single-mode optical fiber
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In fiber-optic communication, a single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest. Single mode fibers are best at retaining the fidelity of each light pulse over longer distances and exhibit no dispersion caused by multiple spatial modes; thus more information can be transmitted per unit time giving single mode fibers a higher bandwidth in comparison with multi-mode fibers. A typical single mode optical fiber has a core radius of 5 to 10 µm and a cladding radius of 120 µm. Currently, data rates of up to 10 gigabits per second are possible at distances of over 60 km with commercially available transceivers.
Data rates are limited by polarization mode dispersion and chromatic dispersion.
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.
There are a number of special types of single-mode optical fiber which have been chemically or physically altered to give special properties, such as dispersion-shifted fiber.
Equipment for single mode fiber is more expensive than equipment for multi-mode optical fiber, but the single mode fiber itself is usually cheaper in bulk.
Single-mode fibers are also known as monomode optical fibers, single-mode optical waveguides, and unimode fibers.
