In this work, we calculate optimum thickness of bulk active layer for Ga As/〖Al〗_x 〖Ga〗_(1-x) As laser diodes. We have done these calculations for fundamental oscillation mode of laser with different aluminium contents (fractional percents) in confinement layers. Our calculations were based on the analytical solution of Maxwell equations. The results indicate that the optimum thickness for fundamental mode is dependent on difference of refractive indices of active and confinement layers. The results reveal that the best active layer thicknesses for fundamental mode of laser are d_0=0.63,0.44,0.36 and 0.32 μm for x=0.1,0.2,0.3 and 0.4 aluminium percents in separate confinement heterostructure (SCH) layers respectively. تفاصيل المقالة