The design of interface between the laser diode and its driver is important issue for high-speed fiber optic communications design. But the high speed interfacing between laser and the laser driver has few studies. The interface problem between the laser and the laser driver is reviewed. And then, the equivalent lumped parameter circuit models of the laser driver and the laser diode is demonstrated. Based on the theory above, the stability condition of the bias current and the modulation current is presented. At last, the directly modulated laser of 2.5 Gb/s is demonstrated in experiment.