Considering the diffusion of heat generated by semiconductor lasers in the heat sink, the thermal interaction among single devices in the array have been analyzed. The idea of the thermal interaction caused from the diffusion of heat current is proposed. From analyses, how to determine the separation between upper and lower devices in the 2D array and how to select the duty cycle in the pulsed operation have been studied. The result of analyses has been applied to design the 1.55 μm wavelength semiconductor laser array. Two kinds of 2D arrays of 2×2, 2×4 consisted of single devices with leaky waveguide structure with their pulsed peak power output of 7 W and 11 W, respectively, have been completed.