Modeling CO2 Laser as a 4-Level Laser

[chimay]

I am not sure this is the right place where I should post, I hope I've chosen the right section.

I'm studying laser's fundamentals on Svelto's "Principles of Lasers". Here I find the description of a generic four level laser by means of rate equations; it can be represented like this: (first picture)

Then, I read that the CO2 laser, whose basic scheme is in the second picture, can be modeled as a four-levele laser too; I don't understand why this is true. In the CO2 laser the radiative transition is between level 3 and 2, while in the first photo it is between 2 and 1. Moreover, level n.3 in the first picture should be pratically empty, while in the CO2 laser it isn't so.

Can anyone explain me how I can model a CO2 laser like a four-level laser like the one in the first picture?

Thanks.

 

[kspace]

I am not a laser expert, but maybe this helps. The pictures are describing somewhat different processes. The CO2 laser is a four level system:

v0: ground state
v1: symmetric vibrational mode
v2: bending mode
v3: anti-symmetric vibrational mode

In the CO2 laser you have a lasing from the anti-symmetric (v3) vibrational mode to the symmetric vibrational (v1) mode with the release of energy as a photon. Such a transistion satisfies conservation laws. However, you can also have relaxations between the antisymmetric mode (v3) and the "bending mode" (v2), yet this is not an optically active transition. The v3-v2 transitions are due to collisions in the gas.

the first picture (left) also shows a four level system with lasing between levels (v2) and (v1). A system like this would have a molecule excited to (v3). The molecule then relaxes to (v2) and lases from (v2 to v1). Finally the molecule may relaxes from v1 to the ground state v0 via collisions.

 

[chimay]

So your point is "there are 4 levels in both cases, they just differ from the radiative transition, 3->2 instead of 2->1", is it?

 

[kspace]

Yes - I think that is what is going on