What are saturable absorbers and how do they impact the behavior of lasers?

[AmyJ]

I'm about to do some study on bifurcations and oscillations in lasers with saturable absorbers. however, my knowledge on lasers is fairly basic, so I'm not sure where to begin or find more information. If anyone could give me some advice, it would be very helpful :). What type of lasers usually have saturable absorbers and in what way do they differ from other lasers? :)

 

[Cthugha]

Saturable absorbers are a way to realize passive Q-switching. A saturable absorber is any material, which absorption decreases with increasing intensity of the incident light. How well high intensity builds up inside a cavity depends on its Q-factor. The fewer losses you have, the higher the Q-factor will be. If you are using saturable absorbers you usually have high losses (due to the strong absorption) at first. A weak lasing process happens, but the buildup of intensity inside the resonantor is rather slow. Nevertheless a lot of energy is stored in the gain medium as the pumping produces an inversion. As the intensity inside the cavity slowly increases, the material becomes more transparent and the losses go down. Stimulated emission is now very likely and the intensity builds up quickly. The losses are quite low, but the number of photons inside is so high that nevertheless a significant portion of the photons will be coupled out and emitted as a pulse. Now the intensity is lower again, the energy stored in the gain medium has been released in the lasing process, the absorption rises and the process begins again. So saturable absorbers are one way to produce pulsed emission. It is a rather cheap method and the repetition rates are quite high. One drawback is, that the pulses can't be triggered externally.

Saturable absorbers can be found quite frequently in Nd:Yag lasers.

 

[AmyJ]

Thank you. That is really helpful. Much appreciated :D

 

[Lambduh]

To repeat cthugha a bit:

For some general information on pretty much any laser subject i use this online encyclopedia. Has a decent article on saturable absorbers which you can take to branch off on pretty much any other laser subject.

http://www.rp-photonics.com/saturable_absorbers.html