Using developed theoretical model of mode synchronization in semiconductor passively mode locked laser (PMLL) we have calculated influence of length and relaxation rate of saturable absorber (SA) on operation regimes of PMLL. We included in consideration the influence of emission rate of free carriers from the levels of dimensional quantization in the SA and found out the optimum operating voltage for PMLL in the fundamental synchronization mode (one harmonic component) as -1.5 - -2 V with optimal length of the SA as 0.637 mm.