On the repetition frequency tunability and stability of BH InAs/InP QD and InGaAsP/InP QW for ultrafast THz-TDS

Two monolithically integrated MLLDs operating at telecom wavelength are compared. A quantum dot (QD) MLLD with an RF of about 50.1 GHz and a quantum well (QW) MLLD with an RF of about 51.4 GHz. The RF tunability is characterized by sweeping the pump current of the laser, the temperature, and the reverse voltage of the saturable absorber (SA). The QW-MLLD has a tuning range of 31 MHz with an average repetition frequency (RF) linewidth of 53 kHz, while the QD-MLLD has a smaller tuning range of 26 MHz with a higher average RF linewidth of 172 kHz. Both MLLDs are used in a terahertz time-domain spectroscopy (THz-TDS) setup with asynchronous optical sampling (ASOPS) using two lasers with slightly different RFs. One laser serves as a pump laser and the other as a probe laser. The measurement speed can be set directly by the RF difference and is limited only by data acquisition and stability, allowing ultra-fast measurement speeds with a very compact system.