Dynamic spectra of 1.3-µm InGaAsP lasers under pseudorandom word modulation

The dynamic spectra of 1.3-μm InGaAsP lasers were studied under low-duty-cycle pulse modulation and under 180- and 432-Mbit/s psendorandnm pulse modulation. The average dynamic spectra of these lasers differ from their dc-bias spectra, and also show variation betweeh low-duty-cycle pulse modulation and pseudorandom pulse modulation due to local heating effect. Under pseudorandom word modulation, no distinctive pulse-to-pulse variation in average dynamic spectra was observed. Transient damping times of the individual longitudinal mode of the average dynamic spectra of 2-4 ns were observed in the turn-on stage. Carrier buildup and decay of the side modes are the cause of these spectral transient responses. Essentially, the same average dynamic spectra were observed at 180 and at 432 Mbit/s where the time slot is less than the transient damping time in the turn-on stage. This suggests that the transient damping time of the individual longitudinal mode in the turn-off stage is much faster than that in the turn-on stage when the laser is biased below threshold. This result is supported by simple calculations from the rate equations which show that when the laser is biased below threshold, the transient damping time in the turn-off stage can be an order of magnitude faster than that in the turn-on stage.