Module 5 : Signal Distortion on Optical Fibers - Dispersion
Lecture 1 : Signal Distortion on Optical Fibers - Dispersion
Waveguide Dispersion
The waveguide dispersion is due to the bound structure of the optical fiber. While analyzing waveguide dispersion we assume that the material dispersion is absent, that is the refractive indices of the core and the cladding are independent of wavelength.
For analyzing waveguide dispersion we need relation. As we have seen in lecture 3 and 4, that the whole exercise of analyzing modal propagation was to obtain the phase constant for a particular mode at a particular frequency. There is no analytical expression which can describe relation. This relation is rather expressed in the form a numerical plot.
Generally we define two normalized parameters, -number and which can substitute and respectively. These parameters are characteristic parameters of a fiber.
V-number of an optical fiber
The V-number is a comprehensive parameter of an optical fiber as it takes into account the size of the optical fiber, , Numerical aperture (NA), and the wavelength of operation.
For a given fiber, and NA are constant and the V-number is proportional to frequency. The V-number therefore is also referred to as the Normalized frequency .
As we have earlier, the propagation constant has bound,
Where is the propagation constant in medium having refractive index , and is the propagation constant in medium having refractive index .
The bound on is independent of the frequency of operation, and the mode. One can therefore define a quantity called the Normalized Propagation Constant, as
for a particular mode at a particular frequency. There is no analytical expression which can describe 
relation. This relation is rather expressed in the form a numerical plot. 
-number and 
which can substitute 
and 
respectively. These parameters are characteristic parameters of a fiber. 
, Numerical aperture (NA), and the wavelength of operation. 
and NA are constant and the V-number is proportional to frequency. The V-number therefore is also referred to as the Normalized frequency . 
has bound, 
is the propagation constant in medium having refractive index 
, and 
is the propagation constant in medium having refractive index 
. 
is independent of the frequency of operation, and the mode. One can therefore define a quantity called the Normalized Propagation Constant, 
as 
