| in an optical fibre, a mode whose field decays monotonically in the transverse direction everywhere external to the core and which does not lose power to radiation
NOTE 1 – Where the refractive index decreases with distance from the axis and where there is no central refractive index dip a bound mode is one for which :
n(a)k ≤ β ≤ n(0)k
where β is the imaginary part (phase coefficient) of the axial propagation coefficient,
n(a) is the refractive index at r = a, the radius,
n(0) is the refractive index at r = 0,
k is the angular free space wavenumber 2 π/λ and λ is the wavelength.
NOTE 2 – Bound modes correspond to guided light rays in the terminology of geometric optics.
NOTE 3 – In a multimode fibre, the power in bound modes is predominantly contained in the core of the fibre.