A fully microscopic model is used to calculate absorption/gain and spontaneous emission for GaInNAs quantum-well laser gain media. It is demonstrated how this approach can be used to derive the optical properties for the regime of semiconductor laser operation from low density photo luminescence spectra which can be obtained from simple experiments. Numerical results are presented showing that increased well depth leads to strongly increased differential gains and gain amplitudes and pronounced shifts of the gain maximum with increasing density. On the basis of a quantum Blotzmann model for the incoherent carrier dynamics it is shown, that high carrier confinement can lead to unusually long carrier capture times. Furthermore, temperature dependent bandstructure parameters for GaInNAs for the applied 10-band k·p-model are presented that have been derived from comparison to recent experimental data.