Optimal thickness of the active layer in a heterojunction laser

Optimization of a symmetrical heterostructure is considered and the minimal value of the threshold current of a heterojunction laser at 300°K is calculated. It is shown that in the case of linear or superlinear (but weaker than quadratic) dependence of the optical gain on the current, the optimal thickness of the active layer d₀ can be deduced from a waveguide model. If the gain-current characteristic is linear, it is shown that d₀ = 0.27 μ when the relative change in the permittivity at the boundary of the active layer is δ ε/ε = 0.04 (a change of this magnitude is observed in GaAs–Al_xGa_1–xAs heterojunctions). If the gain-current characteristic is superlinear, optimization of the dielectric waveguide leads to d₀ → 0. Calculations of this type must make allowance for the upper limit of the rise of the gain. Calculations performed taking all these points into account show that the minimal values of the threshold current density are of the order of 1 kA/cm² for a heterojunction laser operating at 300°K in an unloaded Fabry–Perot resonator.