If the barriers in the multiple quantum well structure are thick enough, it is a good approximation to consider quantum wells uncoupled, that is, to solve for the wave functions considering one quantum well at a time. This can significantly reduce the length of the simulation domain and make the gain simulation much faster, while producing similar results as in the full MQW structure.
If all quantum wells in a multiple quantum well structure have the same geometry and material composition and there is no potential drop across the active region, which is a good approximation for laser simulations, it is possible to simply define geometry with a single quantum well instead of multiple quantum wells. Now we can perform the gain simulation on a single quantum well. The average charge density at the input to the gain simulation should be scaled accordingly, in order to represent the same local charge density as in the full MQW structure. Similarly, the final single quantum well gain and spontaneous emission results should be scaled onto the full MQW structure.
Here is the script file with an MQW gain example that allows easy switching between coupled and uncoupled cases and their comparison: mqwgain_InGaAsP_laser.lsf (10.2 KB) . The control option (boolean) is named uncoupled_wells and defined near the top of the file. The figure comparing the fully coupled simulation to the uncoupled well simulation (several times faster) is shown below. We can see that the gain curves are approximately similar, where the coupled case curves are a bit wider, which is due to the miniband formation in a multiple quantum well structure. This miniband is wider in energy comparing to the single energy state in a single quantum well.