Watt-level and sub-100-fs self-starting mode-locked 2.4-µm Cr:ZnS oscillator enabled by GaSb-SESAMs

Femtosecond lasers with high peak power at wavelengths above 2 µm are of high interest for generating mid-infrared (mid-IR) broadband coherent light for spectroscopic applications. Cr²⁺-doped ZnS/ZnSe solid-state lasers are uniquely suited since they provide an ultra-broad bandwidth in combination with watt-level average power. To date, the semiconductor saturable absorber mirror (SESAM) mode-locked Cr:ZnS(e) lasers have been severely limited in power due to the lack of suitable 2.4-µm SESAMs. For the first time, we develop novel high-performance 2.4-µm type-I and type-II SESAMs, and thereby obtain state-of-the-art mode-locking performance. The type-I InGaSb/GaSb SESAM demonstrates a low non-saturable loss (0.8%) and an ultrafast recovery time (1.9 ps). By incorporating this SESAM in a 250-MHz Cr:ZnS laser cavity, we demonstrate fundamental mode-locking at 2.37 µm with 0.8 W average power and 79-fs pulse duration. This corresponds to a peak power of 39 kW, which is the highest so far for any saturable absorber mode-locked Cr:ZnS(e) oscillator. In the same laser cavity, we could also generate 120-fs pulses at a record high average power of 1 W. A comparable laser performance is achieved using type-II InAs/GaSb SESAM as well. These results pave the way towards a new class of high-power femtosecond SESAM mode-locked oscillators operating directly above 2-µm wavelength.