Using self-consistent two-dimensional numerical simulation (LASTIP), the layer structure of the laser diodes is optimized. A ridge waveguide structure with GaInAs/GaAs and GaInAs/GaInAsP active regions has been simulated. The influence of the well numbers and waveguide thicknesses on the threshold current is studied. Compared to GaInAs/ GaAs, GaInAs/GaInAsP active region suffers from larger spread in the threshold current due to non-uniformities in the carrier density with increasing quantum wells. Simulations have been performed for different temperatures and at different cavity lengths. (C) 2002 Published by Elsevier Science Ltd.