By the way, I am not exactly sure how you obtained a frequency bandwidth range of 1.5e18 Hz for you wavelength bandwidth of 2e-10m (0.2 nm). According to my quick calculations, a frequency bandwidth range of 7e13 Hz corresponds 0.2 nm of wavelength bandwidth. It looks like you used the relation between a single frequency and a wavelegnth: f=c/lambda. In your case you are dealing with the bandwidth (multiple frequencies) you should consider that equqtion through differential.