Comparison of mode-locking performance in monolithic two-section InAs/InP quantum dot and InGaAsP/InP quantum well lasers

A common method to generate ultrashort pulses in a two-section laser diode is to pump the large section with gain current and apply a reverse absorber voltage to the small section, which acts as a saturable absorber. Ultrashort pulses detected in such a configuration are typically attributed to passive mode-locking. However, our results indicate that self-mode-locking is the dominant process for the generation of ultrashort pulses within the examined InAs/InP quantum dot laser and InGaAsP/InP quantum well laser, even in a passive mode-locking setup. To verify this, we controlled both laser sections with gain current only. The pulses detected in this setup were shorter than those observed in the passive mode-locking configuration. While the quantum dot laser was largely unaffected by the applied reverse voltage, the quantum well laser exhibited changes in behavior at medium and high reverse voltages, eventually preventing pulse generation. Additionally, we investigated repetition frequency tunability in pure self-mode-locking operation. Repetition frequency tunability was observed for both lasers: 47 MHz for the quantum dot laser and 86 MHz for the quantum well laser.