Can one probe a laser cavity to examine its sub-picosecond gain dynamics? This paper demonstrates a technique for doing exactly that using a femtosecond fiber laser as the probe. The large gain bandwidth of semiconductor lasers should, in principle, allow the generation of mode-locked pulses of a few tens of femtoseconds. However, this is within the same order of magnitude as the intraband carrier scattering rates, so a better understanding of the gain dynamics is essential for exploiting the full gain bandwidth of the material. In this paper, a femtosecond fiber laser is spectrally broadened to more than 100nm wide, and phase locked to the repetition rate of the mode locked semiconductor laser. The latter is an optically-pumped VECSEL (external cavity variant of the conventional VCSEL) consisting of an InGaAs MQW gain region with a saturable absorber mirror at one end, and an output mirror at the other end of a V-shaped cavity. Their technique gives us a glimpse into the gain dynamics of the laser, including sub-picosecond phenomena such as carrier heating, changes in Fermi distribution and temporal hole burning. Measurements are demonstrated on two VECSEL chips that were optimized for 980nm and 1030nm and grown by MOVPE. Compared to traditional pump-probe techniques, this technique is significantly faster and does not rely on optical delay lines that could introduce artifacts due to misalignments. It is an interesting technique for producing sensitive, high-resolution measurements of gain depletion and recovery during a full round trip of the laser cavity.

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