# Homework Help: Converting laser fwhm bandwidth in m to hz

1. Apr 9, 2010

### melcicero

hi, pls can someone help me with converting FWHM bandwidth of a laser from metres to hertz

2. Apr 9, 2010

### mgb_phys

Just from the dimensions
frequency (1/s)= speed (m/s) / wavelength (m)

3. Apr 9, 2010

### melcicero

thank you but i didn't think you could directly convert fwhm bandwidth just using that, when i did it i got a bandwidth of region 10E17 Hz -does that seem a bit high from a ruby laser cavity length 0.1m?

If i post the whole question it might help. Also should i include the refractive index of the medium ie c/n=f lambda, if this is the right way to do it?

for the 0.1m cavity, n =1.8, FWHM gain bandwidth is 2E-9m
qu is: what is the longitudinal mode spacing and the number of lasing modes? i could give the spacing in either freq or wavelength but its easier to do i think in freq that's why i wanted to convert. many thanks if you can help.

4. Apr 10, 2010

### ahmedhelal238

you can use the following formula

Df=c*Dlambda/Lambda^2