Laser Physics: Steady state gain

[Niles]

Hi

In almost every book on photonics I've seen, the following expression for the steady state gain is derived

1 + flux/flux_saturation = g_{small signal}/g,

where I by flux mean intracavity flux and g is the gain. When talking about lasers, then in the very same books (e.g. Saleh/Teich) the authors argue that when lasing begins, the intracavity photon flux increases, hence by the above expression the gain decreases until it settles on its steady state value (which is equal to losses). What I find to be confusing is that the above expression only holds for steady state, but Saleh/Teich (and others) use it to argue that gain decreases until it reaches steady state.

How can they use our steady state expression above to describe the dynamical behavior too?


Niles.

 

[eljose79]

Dear Niles,

Thank you for bringing up this interesting topic. The expression you mentioned for steady state gain is indeed a commonly used one in photonics literature. However, I understand your confusion about its application in the dynamical behavior of lasers. Let me try to explain it further.

In the steady state, the gain of a laser is equal to its losses, as you correctly pointed out. This means that the intracavity flux is balanced by the losses, resulting in a constant gain value. Now, when the laser is first turned on, the intracavity flux is initially low and the gain is higher than the losses. This causes the intracavity flux to increase, as more photons are being generated by the gain medium. However, as the intracavity flux increases, the gain starts to decrease due to the saturation effect. This decrease in gain eventually balances out with the increasing losses, resulting in a steady state gain value.

So, although the expression you mentioned is for steady state gain, it can also be used to describe the initial dynamics of a laser. This is because the decrease in gain and increase in intracavity flux are happening simultaneously, until they reach a steady state. I hope this clarifies your confusion. If you have any further questions, please don't hesitate to ask.