# Laser Question - Axial Modes & Gain Saturation

• ulver48

#### ulver48

I am studying laser physics at the moment and there is something that confuses me. The laser output consists of the central laser mode and some axial modes due to constructive interference in the optical cavity. At first there is a high enough population inversion and while the laser works this population inversion decreases until a threshold is reached defined by internal losses and the losses due to the mirrors. So at the steady state the relation g(v)= g_th holds , where g(v) is the gain curve of the laser. If the initial gain is decreased due to a decrease of the population inversion then at some point g(vo)=g_th, where vo is the central frequency. Shouldn't the other modes move under the gain threshold due to the gain decrease and thus disappear with time? If that's not true then the gain curve must become flatter when continues wave (CW) operation is reached. Thanks for your time.

I am studying laser physics at the moment and there is something that confuses me.

Your question is difficult for me to parse, but if I understand you correctly, saturation of the lasing transition is accompanied with broadening of the linewidth ('power broadening'). Does that answer your question?

• ulver48
Sorry if I wasn't very clear. I try to understand which of the following statements is right and why :
a) The other modes move under the gain threshold due to the gain decrease and thus disappear with time. (That's surely not the case according to the experiments but what about theory? )
b) The gain curve must become flatter at the top when steady state operation is reached and for some of the modes around the central mode the equation g(v_m)=g_th holds where v_m is the frequency of these modes. Maybe there is also some kind of linewidth broadening.
c) There is a different explanation.

Sorry if I wasn't very clear. I try to understand which of the following statements is right and why :
a) The other modes move under the gain threshold due to the gain decrease and thus disappear with time. (That's surely not the case according to the experiments but what about theory? )
b) The gain curve must become flatter at the top when steady state operation is reached and for some of the modes around the central mode the equation g(v_m)=g_th holds where v_m is the frequency of these modes. Maybe there is also some kind of linewidth broadening.
c) There is a different explanation.

I guess I don't understand #1: what do you mean 'move'? As for #2, I would answer that 'power broadening' is equivalent to your 'gain curve must become flatter'

What textbook are you using?

Sorry. Now I understand that the second answer must be correct. I am using the textbook " Optoelectronics: An Introduction" by Wilson and Hawkes. But this question comes from the book "Laser Electronics" by Verdeyen. It's the picture 8.3 that confuses me in case you have the book, from the chapter 8 Laser Oscillation and Amplification.I could post the figure if you don't have the book.