Fourier Analysis: What is the Difference with (w-wo)?

[hanson]

Hi all.
I am learning a numerical method that involves Fourier transform.
As far as I know, I think Fourier transform is tool to find the frequency spectrum of a signal.
And the usual form shall be
"Integrate from negtive infinity to positive infinity, f(x)*exp(i*w*x)dx"

However, when i search this in wikipedia, it is defined with (w-wo), where wo is the central frequency.

http://en.wikipedia.org/wiki/Split-step_method

I don't really know what this definition is doing and what changes it would bring.

Why is it necessary to do so?

Please help.

 

[fresh_42]

Second, it is necessary to Fourier transform back and forth because the linear step is made in the frequency domain while the nonlinear step is made in the time domain. An example of usage of this method is in the field of light pulse propagation in optical fibers, where the interaction of linear and nonlinear mechanisms makes it difficult to find general analytical solutions. However, the split-step method provides a numerical solution to the problem.