PHYS4121: Laser Physics Resources

This demonstration illustrates the use of the Beer Law for a medium in the steady state. Narrowband weak probe radiation is propagated through the medium (i.e. the light doesn't modify the absorption and scattering properties of the medium it propagates through). The intensity after a propagation distance $z$ through a medium with optical cross section $\sigma(\omega)$ is $I = I_0 \mathrm{exp} (\sigma(\omega) N^* z $) ($\sigma$ is taken to be constant at $ 4.0 \times 10^{-17}$ m$^2$ in this code), where $N^* = N_2 - \left(\frac{g_2}{g_1}\right)N_1 $ is the population (inversion) density. $N_1$ and $N_2$ are the state occupancies per unit volume of the two levels of the laser transition, and $g_1$ and $g_2$ are the degeneracies of those levels (level 2 is higher in energy than level 1). You can vary these parameters to see how the intensity of the radiation in the medium is affected with distance as it propagates. When $N^*$ is positive we have a population inversion and the intensity in the cavity grows exponentially. Otherwise the intensity will decay.
The plot below is of the normalised transimitted intensity $I_T/I_0$ against propagation distance $z$.