Influence of Gain on Mode Structure and Loss
In the previous discussion of the output power we did not take the mode structure into account. The volume of the beam inside the medium was only characterized by an effective cross sectional area A b resulting in a fill factor γ =A b /A, with A being the cross sectional area of the active medium. The question we want to answer in this Chapter is how the beam cross section has to be defined to get a realistic expression for the output power of a laser resonator. In Part III we defined the mode radius w by using the second intensity moments. A first approximation, therefore, would be to use this mode radius for the definition of the mode cross section A b = πw 2 . For transverse multimode lasers this is a very good approach that provides output powers close to the observed ones.
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