This is probably a stupid question but I'm having a small problem with an optics issue, and it keeps coming up in past exams. Most of the formulae I know for things like coherence length and number of modes in a laser involve the frequency spread (linewidth) of the laser [tex] \Delta \nu [/tex] However the question give me the linewidth or spectral width as a wavelength e.g [tex] \Delta \lambda = 0.002 nm [/tex] along with the working wavelength [tex] \lambda = 633 nm [/tex]. Can anyone tell me how I can get from the linewidth to the frequency spread, I've tried various things (such as dividing c by the variance etc) but can't get anything that matches the answers when I follow through. Thanks Incidently does anyone know an online source (or possible a good textbook) for basic non-linear optics/lasers? There have been questions on deriving equations for growth of second harmonics etc and it polarisability, and I'm lost.