# Laser noise spectrum

1. May 27, 2012

### Niles

Hi

I have read the following online (http://tf.nist.gov/general/pdf/1819.pdf):

"Regardless of the application, the basic goal of locking the frequency of a laser to a cavity is to reduce the frequency fluctuations between the laser and cavity. The noise spectrum of the laser’s frequency fluctuations leads to an effective “linewidth” of the laser, which conceptually describes the broadening of the laser’s spectrum around its central frequency".

What I don't understand 100% is the bolded part. So what they are trying to tell me is that the noise spectrum of the (e.g.) Lorentzian width of a laser is what makes it Lorentzian?

Best,
Niles.

2. May 27, 2012

### M Quack

That the laser's frequency has a noise spectrum means that statistically the frequency can be a bit higher or lower and the "ideal" frequency.

I don't know what the distribution of the noise is, but Gaussian is always the first guess. Loentzian usually result from finite life time, not noise.

In any case, the spectral function will have some width. That is the effective line width of the laser frequency.

Nothing fancy. If you have a spread of frequencies, the width of that is effectively a bandwidth. (This assumes that the frequency changes many times during the measurement.)

3. May 27, 2012

### Niles

Thanks, I sometimes overcomplicate matters. Your explanation is very down-to-earth.