We investigate the modulation performance of high-speed multiple-quantum-well (MQW) semiconductor lasers emitting in the wavelength of 1.55 µm. The small-signal modulation characteristics are modeled and analytical forms of the relaxation frequency and the modulation bandwidth are derived. The digital modulation characteristics under both 10 and 40 Gbps are investigated using both the return to zero (RZ) and non-return to zero (NRZ) formats of the modulation current. These modulation characteristics include the waveform of the modulated signal and frequency chirp. The results showed that under 10 Gbps, the transient chirps is almost same ( 62.7 GHz) under both the RZ and NRZ bit patterns, whereas it increases to 72.8 and 65.1 GHz under the NRZ and RZ bit patterns, respectively.