Also referred to as the V number in fiber optics; the normalized frequency is a dimensionless quantity, denoted by the symbol V, given by:
where a is waveguide core radius, l is wavelength in vacuum, and n1 and n2 are the maximum refractive index in the core and refractive index of the homogeneous cladding, respectively. (Note that the difference between the square root of the squares of n1 and n2 is by definition the numerical aperture of the fiber.) For a multimode fiber, the mode volume of that fiber, which is the approximate number of bound modes within that fiber is directionally proportional to the normalized frequency. For a single-mode fiber, it is required that the normalized frequency, satisfies the condition V < 2.4048. For a step index fiber, the mode volume of that fiber is directionally proportional to the square of the normalized frequency, that is V2 .