# More optics

1. Jan 7, 2006

### Beer-monster

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 $$\Delta \nu$$

However the question give me the linewidth or spectral width as a wavelength e.g $$\Delta \lambda = 0.002 nm$$ along with the working wavelength $$\lambda = 633 nm$$.

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.

2. Jan 7, 2006

### George Jones

Staff Emeritus
What happens if you use $\nu = c \lambda^{-1}$ and

$$\Delta \nu = \left| \frac{d \nu} {d \lambda} \right| \Delta \lambda.$$

Regards,
George

Last edited: Jan 7, 2006
3. Jan 7, 2006

### Beer-monster

That worked, thanks George.

Just trying to identify and fill the gaps in my optics knowledge before my upcoming exam.

There's a few gaps but less than I expected

4. Jan 7, 2006

### George Jones

Staff Emeritus
Good luck!