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.