This applet models the interaction between electrons (shown as blue dots) and photons (red squiggles). Five processes are modelled: absorption, stimulated emission, spontaneous emission, pumping and feedback; the frequency of occurrence of these processes may be set using the text fields provided.
The absorption rate is the probability that a photon will be absorbed by any one electron; similarly for the stimulated emission rate. The other rates are the probability per time interval T of the event occurring, where T is the time taken for a photon to travel from one side of the display to the other.
The feedback rate is the proportion of photons leaving the simulation (by reaching the far right hand side) which will be returned to the start. The rate at which photons are leaving the simulation is displayed in the upper right corner (photons leaving per interval T), and is plotted against time on the graph below the simulation.
The population inversion (proportion of electrons at the upper level) is displayed in the top corner of the applet. Using the "inversion" field, this value may be set. When the "apply inversion" button is clicked, the electrons will be randomly distributed according to the specified inversion. In addition, an inversion of 0 or 1 may be set using the "none excited" or "all excited" buttons.
The (graph below the display) plots the number of photons leaving. With appropriate parameters (including sufficient feedback), the graph should show a series of evenly-spaced spikes.