The ε" < 0 approximation vs semi-classical approach to active media

We analyze applicability of the ε" <; 0 approximation to wave propagation in active material, comparing the results to the FDTD-based calculation employing more general Maxwell-Bloch model. It is shown that the slab thickness should be less than some critical value, corresponding to the divergence of the Airy series and to the start of the laser generation.