# Laser Question - Axial Modes & Gain Saturation

• ulver48
In summary, the laser output consists of the central laser mode and some axial modes due to constructive interference in the optical cavity. 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, at some point g(vo)=g_th, where vo is the central frequency. However, this does not result in the disappearance of other modes as they do not move under the gain threshold. Instead, the gain curve becomes flatter at the top when steady state operation is reached, resulting in power broadening. This is confirmed by experiments and is explained in the textbook "Optoelectronics: An Introduction" by
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.

ulver48 said:
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.

ulver48 said:
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.

## 1. What are axial modes in a laser?

Axial modes refer to the longitudinal standing waves that are formed within the laser cavity. These modes are determined by the length of the cavity and the wavelength of the laser light.

## 2. How do axial modes affect laser operation?

Axial modes can affect the output of a laser by causing interference patterns and fluctuations in the laser intensity. They can also contribute to mode hopping, where the laser output switches between different modes.

## 3. What is gain saturation in a laser?

Gain saturation refers to the point at which the amplification of light within the laser medium reaches its maximum, and any additional input of energy will not result in a significant increase in output power.

## 4. How does gain saturation impact laser performance?

Gain saturation can limit the maximum output power of a laser, as well as affect the beam quality and stability. It is an important consideration in designing and operating a laser system.

## 5. What factors can cause gain saturation in a laser?

Gain saturation can be caused by various factors such as high input power, high temperature, and the physical properties of the laser medium. It can also be affected by the design of the laser cavity and the optical components used.

• Optics
Replies
1
Views
831
• Optics
Replies
1
Views
2K
• Optics
Replies
44
Views
14K
• Optics
Replies
1
Views
2K
• Atomic and Condensed Matter
Replies
3
Views
2K
Replies
2
Views
827
Replies
1
Views
4K
• Atomic and Condensed Matter
Replies
3
Views
6K
• Atomic and Condensed Matter
Replies
1
Views
3K
• Quantum Physics
Replies
1
Views
2K