Optically pumped GaSb-based thin-disk laser design considerations for cw and dual-comb operation at a center wavelength around 2 μm

Vertical emitting optically pumped semiconductor laser technology in the GaSb material system, operating in the short-wave infrared (SWIR) regime, has made significant advancements recently. This paper reviews the key achievements leading to the first demonstration of a passively modelocked optically pumped thin-disk semiconductor laser, where both the saturable absorber and the gain quantum wells are integrated into a single semiconductor chip, known as the Modelocked Integrated eXternal-cavity Surface Emitting Laser (MIXSEL). This GaSb-based MIXSEL operates at a center wavelength of 2 μm, supporting both single and dual-comb operations, with an average output power of 30 to 50 mW, pulse repetition rates of approximately 4 GHz, and picosecond pulse durations. It enables initial proof-of-principle dual-comb spectroscopy measurements. For this, we optimized continuous wave (cw) Vertical External Cavity Surface Emitting Laser (VECSEL) operation at 2 μm without an intracavity heatspreader, enhanced group delay dispersion (GDD) compensation, and introduced an additional pump mirror integration. Compared to previous results, we achieved a significant performance increase with pump-DBR 2-μm VECSEL with an average output power of 6 W, an optical pump efficiency of 30% and a reduced thermal resistance of 1.9 K/W. Additionally, the better GDD compensation improved modelocking at 2 μm with a SESAM (Semiconductor Saturable Absorber Mirror), producing near-transform-limited femtosecond pulses with a duration of 331 fs, an average power of 30 mW at a pulse repetition rate of 2.77 GHz. Successful integration of the saturable absorber within the MIXSEL chip required matching of the cavity mode sizes on both the SESAM and the VECSEL chip. This paper details the optimization processes and resulting performance enhancements that mark a significant milestone in the development of GaSb-based thin disk laser technology.