The position of the quantum wells is crucial if one is to maximise the gain of the device. The light within the cavity must be reflected back into the cavity many more times than with a Fabry Perot laser. The average time the photons spend within the cavity is known as the photon lifetime. The reflectivity of the mirrors must be very high to increase the photon lifetime and thus the time of interaction with the excited electron states. As an example, we obtain an estimate of the reflectivity that is required for these mirrors, we can apply some optimistic values to the gain threshold condition for a single quantum well device.
Threshold gain occurs where the optical gain equals the optical loss.
R1 and R2 are the mirror reflectivities. Assuming an ideal device with no absorption. Dielectric materials and semiconductors have a very low absorption coefficient for photons with energies below the bandgap energy of the material. If two dielectric materials, with a differing refractive index are placed together to form a junction, light will be reflected at the discontinuity. The amount of light reflected off one such boundary is small.