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Timestamp: 2019-04-21 11:11:15+00:00

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We utilize an asynchronous optical sampling technique to study the gain dynamics of vertical-external-cavity-surface-emitting lasers (VECSELs) under mode-locked operation. This allows for an in situ characterization of the gain depletion and recovery over nanoseconds with femtosecond-scale resolution. Our method allows for a more direct study of intracavity gain dynamics than traditional pump/probe measurements. We observe a rapid depletion of the gain on the timescale of the intracavity pulse. Afterward, a rapid recovery over a few picoseconds due to intraband scattering and carrier heating takes place, followed by a long recovery attributed to the continuous supply of carriers by the pump laser.
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Fig. 1. (a) Time resolved differential reflectivity of a SESAM used in the experiments. (b) Resulting pulse duration of the mode-locked VECSELs for the case of the two different gain chips.
Fig. 2. Spectrum of the probe laser after spectrally broadening and the spectra of the two investigated VECSEL structures.
Fig. 3. Illustration of the (a) frequency control scheme and (b) in situ pump and probe setup. The probe laser is locked to a multiple of the repetition rate of the VECSEL cavity and focused on the VECSEL chip. A differential detector is used to enhance the signal to noise ratio.
Fig. 4. (a) Scan signal showing the phase between VECSEL and probe laser. (b) Scan of the reflectivity change of the gain medium as a function of the time. In each round trip, the intracavity pulse hits the gain medium in two different conditions due to the different length of the two cavity arms. Thus, during a round trip, two different carrier distributions are present.
Fig. 5. (a) Scan of the reflectivity change of the gain medium at 1040 nm for a 1.5 ns time window. (b) Comparison of the normalized signals obtained for the 980 and 1040 nm VECSEL chips. Finer time resolution provides a more visible display of faster carrier restoration timescales.

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