IFFT function for dispersion

Hi there, i got a formula for chromatic dispersion and its in frequency response, so i need to get it to time domain and insert it into my simulation.

I try using the IFFT function in Matlab, but its not working, can anyone advice on proper way of using it, i went through the matlab help and did everything i could, but its not working.

The formula is

, so when i make it like

, im getting the same result when solve it in frequency domain, i mean its equivalent. Anyone, please advice me on this.

Transfer function for chromatic dispersion= exp(j/2 ω2β2 L)
where β=β2=Dλ^2/2πc;
L-length of the fiber;
You have to multiply the envelope of input signal with this transfer function in frequency domain .
In time domain the transfer function is given by
Transfer function = 1/(2√((-2πj Lβ )) )exp((t^2j)/2βL )
This transfer function must be convoluted with the envelope function in time domain