RESULTS AND CONCLUSIONSTwo models have been presented for the valence bands: the parabolic and the LK models. It was found during the calculations, that for modeling the electrostatic effects it was not necessary to use the LK model; there is only a small difference in the carrier densities if the parabolic model is used, and this difference becomes negligible in the electrostatic potential. Because the parabolic method is much faster to solve, it makes sense to choose it for this part. For the modal gain, however, it was found that band mixing is a significant effect and therefore it is needed in the slower LK model. The structure to be simulated is two 5-nm InGaAsP quantum wells with a variable barrier width separating them and 500-nm barriers on either side; the barriers are also InGaAsP. The cladding and substrate surrounding the structure are InP. The wells and barriers are lattice matched to InP so there is no strain in the system. The barriers have a bandgap separation of 1.1 m. The composition of the wells is chosen such that maximum gain occurs around 1.3 m (this is an approximate relation because the energy at maximum gain will change depending on carrier density and separation barrier width). The parameters chosen and band-gap calculations are as in [9]. A GaAs system was not simulated because it has been shown that self-consistency does not have much effect because of the deeper wells in these systems [4,6].Equations (2) and (6) were solved numerically with the use of a finite difference. Equation (8) was solved with the use of a transfer-matrix method. Figure 1 shows how self-consistency modifies the original heterostructure potential. This modification causes more conduction electrons to be confined within the well and fewer holes to be confined. Because conduction electrons are normally the limiting factor to gain, higher confinement of electrons in the well results in larger gain ( Figure 2). As has been noted in [1], different separation barrier widths will cause different subbands to be the dominant transitions. This is the effect of well coupling, and self-consistency causes qualitative changes in which transitions are dominant (Figures 2 and 3), which implies that self-consistency modifies the well-coupling properties, as should be expected because of the change in the potential profiles.In conclusion, electrostatic effects can change the gain in a coupled QW laser significantly because of changes in the potential profile and changes in the charge distribution. An accurate model for coupled wells must therefore take this into account. Snowden, and T. Boettcher, Int. J Numerical Modelling: Electronic Networks, Devices and Fields, 10 (1997) GaAs/AlGaAs multiple quantum well laser diodes, IEEE J Quantum Electron 29 (1993), 2607. 7. E.A.B. Cole, C.M.