1. In an optical fiber, a dimensionless quantity, V , given by where a is the core radius, λ is the wavelength in vacuum, n _{1} is the maximum refractive index of the core, and n _{2} is the refractive index of the homogeneous cladding[?].
Note 1: In multimode operation of an optical fiber having a powerlaw refractive index profile[?], the approximate number of bound modes, i.e., the mode volume, is given by where V is the normalized frequency greater than 5 and g is the profile parameter[?].
Note 2: For a step index fiber, the mode volume is given by V ^{2}/2. For singlemode operation, V < 2.405. Synonym V number.
2. The ratio between an actual frequency and a reference value.
3. The ratio between an actual frequency and its nominal value.
Source: from Federal Standard 1037C and from MILSTD188
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