# Power Spectral Density of a laser

1. May 15, 2012

### Niles

Hi

I am confused about the concept of Power Spectral Density (PSD) of a laser. Say I hook up my laser error signal to a PSD: Is it correct to say that the PSD basically gives me the Fourier transform of the signal?

Best,
Niles.

2. May 15, 2012

### mathman

As a general statement a power spectrum is the Fourier Transform of a signal. I am not familiar with your context.

3. May 16, 2012

### f95toli

Sort of. The PSD gives you slightly different information than the FT; if you plot the PSD you are showing the (average) distribution of POWER in the frequency band.
So it is slightly different than a FT.

Most spectrum analysers can show both the FFT and the PSD.

In your context the PSD is important for several reasons, not least because it allows you to figure out which type of noise dominates for a certain frequency (=what the controller is having to compensate for); this can be seen by looking at the slope of the PSD.

Last edited: May 16, 2012
4. May 16, 2012

### DrDu

Not my field but isn't the PSD the FT of the autocorrelation function of the signal?

5. May 16, 2012

### Cthugha

Yes, it is.

6. May 16, 2012

### f95toli

Indeed

At work I use a book called "Spectral Analysis of Signals" by Stoica as a reference book.

http://www.amazon.co.uk/Spectral-An...9568/ref=sr_1_7?ie=UTF8&qid=1337198030&sr=8-7

However, you don't really need to understand all the theory to use a PSD (unless you need to understand how e.g. different window functions will affect the shape of the spectrum). PSDs are extremely common in some fields, and when people talk about spectrum it is frequently the PSD (and not the FT) they are talking about.

Last edited by a moderator: May 6, 2017
7. May 16, 2012

### Niles

Thanks for all your help. Generally it seems to me that the litterature on noise in diode lasers, PSDs, drift, etc.. is sparse. If you know of any good references, I would be very interested.

Best wishes,
Niles.

8. May 16, 2012

### f95toli

Have a look at Enrico Rubiola's homepage

http://rubiola.org/

His stuff is mainly related to oscillators, but much of the material will be relevant to e.g. stabilized lasers as well.
He once sent me a reference to a book dealing specifically with lasers, I will try to find the reference when I'm back at work next week,

9. May 17, 2012

### Niles

Thanks, I would be very interested in that.

Best wishes,
Niles.