Ultrafast optically pumped semiconductor disk lasers (SDLs) provide an enabling combination of gigahertz pulse repetition rates, short pulse durations, high peak power and good beam quality. However, the successful demonstration of shorter pulse durations with pulses as short as 100 fs has come at the expense of lower optical-to-optical pump efficiency for ultrafast SDLs based on active InGaAs quantum wells (QWs). The optical pump efficiency, which also depends on the pulse repetition rate, decreases with the pulse duration to values typically below 1% in the sub-300-fs regime. For a better understanding of this trade-off between shorter pulse durations and optical efficiency we present an empirical model based on three time constants: the carrier lifetime in the conduction band, the time required for a pulse to empty the carrier reservoir of a QW and the time needed to refill the QW through the continuously pumped GaAs barriers. With the time constants used as fitting parameters we obtain a reasonably good agreement for the measured efficiency of all previously published MIXSEL results. Furthermore we have investigated the SDLs gain dynamics and our measurements have confirmed that shorter pulses significantly reduce the gain saturation fluence. A simplified description of the QWs as a 3-level system could accurately reproduce the gain saturation curves and let us identify spectral hole burning as the main cause of the reduced gain and efficiency. This will be a challenge for further improvements of the ultrafast performance of ultrafast SDLs based on InGaAs QWs.
Cesare G. E. Alfieri, Dominik Waldburger, Sandro M. Link, Matthias Golling, and Ursula Keller, "Optical efficiency and gain dynamics of ultrafast semiconductor disk lasers (Conference Presentation)," Proc. SPIE 10087, Vertical External Cavity Surface Emitting Lasers (VECSELs) VII, 1008708 (Presented at SPIE LASE: January 30, 2017; Published: 21 April 2017); https://doi.org/10.1117/12.2250138.5387835718001.
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