In an optical fiber, a dimensionless quantity, V, given by
where a is the core radius, is the wavelength in vacuum, n1 is the maximum refractive index of the core, and n2 is the refractive index of the homogeneous cladding. Note 1: In multimode operation of an optical fiber having a power-law 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 single-mode 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.