Distributed microwave effects in directly modulated semiconductor lasers

Recently, it has been shown that at high frequency, a directly modulated signal along the length of a semiconductor laser is not uniform in phase and amplitude.1 Thus instead of the conventional lumped RC model in Fig. 1(b), one should adopt the lossy transmission-line model in Fig. 1(c). It has been suggested that this effect can cause a bandwidth degradation in current injections relative to a lumped element case.1 We show, however, that this degradation occurs only if the laser is driven directly by a low-impedance voltage source, and we show that the degradation is minimal if it is driven from a 50-Ω-transmission-line source.