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Definition: a normalized frequency parameter, which determines the number of modes of a step-index fiber
The V number is a parameter which is often used in the context of step-index fibers. It is defined as
where λ is the vacuum wavelength, a is the radius of the fiber core, and NA is the numerical aperture.
The V number can be interpreted as a kind of normalized optical frequency. For V values below ∼2.405, a fiber supports only one mode per polarization direction (→ single-mode fibers). Multimode fibers can have much higher V numbers. For large values, the number of supported modes of a step-index fiber can be approximately calculated as
There is also the so-called Marcuse formula for estimating the mode radius of a step-index fiber from the V number, see the article on mode radius.
For certain types of photonic crystal fibers, an effective V number can be defined, where ncladding is replaced with an effective cladding index. The same equations as for step-index fibers may then be used for calculating quantities like the single-mode cut-off, mode radius, splice losses, etc.
Bibliography
| [1] | A. W. Snyder and J. D. Love, "Optical Waveguide Theory", London: Chapman and Hall (1983) |
See also: fibers, step-index fibers, fiber core, numerical aperture, single-mode fibers, multimode fibers
Category: fibers and other waveguides
