There is significant interest in developing high-power lasers with excellent beam quality and tunable wavelength in the Short-Wave Infrared (SWIR) to mid-infrared range. Type-II Quantum Well (QW) VECSELs have been demonstrated in the GaAs material system. However, their true potential lies in suppressing Auger recombination at wavelengths beyond 2.3 μm in the GaSb material system where type-I QWs face increasing challenges. Therefore, our research focuses on investigating type-II QW configurations to extend the emission wavelength of VECSELs. Here, we explore VECSEL operation at 2.3 μm using w-like AlSb/InAs/AlGaSb/InAs/AlSb QWs, which offer longer operation wavelength by adjusting their thickness. We aim to compare these novel type-II QW VECSELs with conventional type-I InGaAsSb QWs. Careful optimization of QW number, pump absorption, and overall design is crucial due to reduced wavefunction overlap in the type-II configuration. Precise control of the growth is also essential to achieve accurate bandgap engineering and smooth interfaces for efficient radiative recombination.
SESAMs (Semiconductor Saturable Absorber Mirrors) are crucial in ultrafast laser systems and have been extensively used in the near-infrared region. Extending them to the short-wave infrared (SWIR) regime is essential for many sensing and spectroscopic applications. This investigation examines how wavelength, strain, and barrier material affect SWIR SESAM performance. At 2 μm, SESAMs with InGaSb quantum wells and GaSb barriers demonstrate fast recovery times (⪅30 ps) due to defect states in confined quantum wells. However, using AlAsSb barriers improves rollover parameters but slows down recovery (> 500 ps). Strain-compensated InGaAsSb quantum well SESAMs consistently show slow recovery times, which can be partially explained by delocalized hole states. In contrast strain-compensated SESAMs with AlAsSb barriers have localized hole states with good quantum well confinement, yet still exhibit slow recovery, suggesting an unidentified mechanism related to the AlAsSb barrier. We will give an overview for controlling SESAM characteristics with different quantum well designs and MBE growth parameters over a wavelength range of 2 to 2.4 μm.
Passively modelocked, optically pumped semiconductor disk lasers, commonly referred to as VECSELs or MIXSELs, offer a unique combination of wavelength versatility, wafer scalability, high beam quality, and substantial average output power. While V-shaped cavities are typically used for SESAM-modelocked VECSELs, MIXSELs utilize a simplified straight cavity, integrating the saturable absorber into the VECSEL chip. Here, we demonstrate a dual-comb modelocked MIXSEL in the Short-Wave Infrared (SWIR) regime, employing InGaSb quantum well gain and saturable absorber layers. The free-running dual-comb MIXSEL generates distinct microwave comb lines based on a few interferograms, eliminating the need for stabilization. Two distinct repetition rates enable sampling without aliasing while maintaining rapid acquisition times. Moreover, the phase of the heterodyne beat interferograms can be tracked, allowing for the application of coherent averaging algorithms. This breakthrough lays the foundation for dual-comb spectroscopy in the 2-μm regime providing direct access to CO2 spectroscopy.
We report on a SESAM modelocked GDD balanced VECSEL embedding the active region in quaternary Al15Ga85AsSb. The GDD is flattened by a multilayer semiconductor dielectric top-coating allowing for stable femtosecond operation with a standard SESAM and high-quality YAG Brewster windows. We avoid tradeoffs that would limit the output power. This GDD balanced VECSEL is the next step towards higher level of integration: pump-DBR implementation, demonstration of 1:1 modelocking, and absorber integration will yield a 2-µm MIXSEL, where gain medium and absorber are grown in one monolithic structure.
The key limiting factor for output power scaling of VECSELs is the thermal resistance of the structure owing to the reflector thickness and the heat conductivity of the semiconductor materials. We have successfully fabricated a flip-chip processed VECSEL emitting at 2 µm using a GaSb/AlAs0.08Sb0.92 hybrid Bragg reflector with 10.5 mirror pairs and a 100–nm copper layer. The flip-chip processed VECSEL reaches a record high continuous wave average output power of 3 W. The device thickness is reduced by 2.5 µm (36%) compared to the standard 19.5 layer semiconductor-only Bragg reflector design.
The key limiting factor for output power scaling of VECSELs is the thermal resistance of the structure owing to the DBR thickness and the heat conductivity of the semiconductor materials. We have successfully fabricated a flip-chip processed GaSb/AlAs0.08Sb0.92 hybrid DBR for 2 µm with only 7.5 mirror pairs and a 100-nm gold layer. The hybrid DBR reaches a high reflectivity >99.5% with a reduced total thickness of 2.3 µm. The measured spectral reflectivity of the hybrid DBR reveals a clear gold layer and matches theoretical simulations. High power 2-µm VECSEL development with the presented hybrid-mirror structure is under way.
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