Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: MAY 2016

i. Single-mode operation only occurs above a theoretical cutoff wavelength $λ_c$ given by:

$\lambda_c =\dfrac {2\pi an_1}{V_c}(2\triangle)^{1/2}------- (1)$

ii. Where $V_c$ is the cutoff normalized frequency? Hence $λ_c$ is the wavelength above which a particular fiber becomes single-mode.

iii. The normalized frequency may be expressed in terms of the numerical aperture NA and the relative refractive index difference Δ respectively, as:

$V=\dfrac {2\pi}{\lambda}a(NA) \\ V= \dfrac {2\pi}{\lambda}an_1(2\triangle)^{1/2} ----- (ii)$

iv. The normalized frequency is a dimensionless parameter and hence is also sometimes simply called the V number or value of the fiber. Where the core radius a, the relative refractive index difference Δ and the operating wavelength λ.

v. Dividing Eq. 1 by Eq. 2 for the same fiber we obtain the inverse relationship.

$\dfrac {\lambda_c}{\lambda} = \dfrac V{V_c}$

vi. Thus for step index fiber where $V_c= 2.405,$ the cutoff wavelength is given by

$\lambda_c =\dfrac {V\lambda}{2.405}$

vii. An effective cutoff wavelength has been defined by the ITU-T which is obtained from a 2 m length of fiber containing a single 14 cm radius loop.

viii. This definition was produced because the first higher order LP11 mode is strongly affected by fiber length and curvature near cutoff.

ix. Recommended cutoff wavelength values for primary coated fiber range from 1.1 to 1.28 μm for single-mode fiber designed for operation in the 1.3 μm wavelength region in order to avoid modal noise and dispersion problems.