The (AlGaIn)(AsSb) material system has been shown to be ideally suited to realize VECSELs for the 2-3 μm wavelength range. In this report we will present results on increasing the output power of the SDL chips with special emphasis on the 2.8 μm emission wavelength by means of low quantum defect pumping. Further on we have investigated concepts for a VECSEL-pumped Q-switched Ho:YAG laser in order to convert the high cw-power of the VECSEL into pulses with a high peak power. Up to 3.3 mJ of pulse energy were achieved with a compact setup (corresponding to a peak power of 30 kW at 110 ns pulse length) combined with stable pulsing behavior.
Using the (AlGaIn)(AsSb) material system, VECSELs covering the 2 – 3 μm wavelength range can be realized. The best laser performance of GaSb-based VECSELs was achieved so far at emission wavelengths around 2.0 μm with a slope efficiency of more than 30 %, a low threshold pump power density of 1.1 kW/cm2 at 20°C heatsink temperature and concomitant a high output power exceeding 7 W in CW operation (depending on the mounting technology). These parameters were degrading significantly for longer wavelength devices emitting around 2.5 μm and 2.8 μm. But for applications like the generation of MWIR light (3-8 μm) by pumping ZGP-OPOs, high-power VECSELs around 2.5 μm are required to suppress absorption losses, while for medical laser treatment, high-power operation near the water absorption peak at around 2.9 μm is desirable. We will present results of our ongoing research strand for further optimization of the semiconductor heterostructure design of ≥ 2.5 μm emitting GaSb-based VECSELs. By using a low quantum deficit design (i.e. optical pumping at around 1.5 μm) in combination with highly strained QWs (compressive strain 2.1 %) we were able to realize a 2.5 μm emitting VECSEL with a slope efficiency above 30 %, corresponding to an external quantum efficiency exceeding 50 %, and a low threshold pump power density of 0.8 kW/cm2. These values are as good as those for the best performing 2.0 μm VECSELs. With a frontside SiC heatspreader and operated in a standard linear cavity, over 7 W of CW output power were achieved for this 2.5 μm emitting VECSEL structure when operated at 20°C. Furthermore, we will compare laser structures with different emission wavelengths and discuss the role of the QW strain, band-offset and active region composition on laser performance.
This paper presents recent advances of 2-μm GaSb-based vertical external cavity surface emitting laser (VECSEL) with special emphasis on quantum deficit reduction and miniaturization. Operating the VECSEL in a 5-cm long cavity, we could demonstrate an increase in maximum cw output power from 4.2 W to 7.2 W at room temperature when barrier pumping a 2.0-μm emitting VECSEL at a pump wavelength of 1.5 μm instead of 980 nm. Furthermore, miniaturized VECSELs were realized by depositing a high-reflectivity (~97 %) coating on top of a 375-μm thick SiC heat spreader, which acts as output coupler of the micro cavity (μC) formed. This planar cavity is rendered stable by thermal lensing induced by the absorption of pump light. At the same time, thermal lensing influences the beam quality. We will report a detailed study of the influence of the thermal lens on the stability and beam diameter of the μC-VECSEL by using two different VECSEL structures optimized for 980 nm and 1.5 μm barrier pumping, respectively. Using different pump photon energies results in different amounts of heat generated at a given pump photon flux, and thus thermal lenses with different focal lengths. Using the low-quantum deficit pumping scheme we could achieve a factor-7 increase in output power in TEM00 emission from the μC-VECSEL compared to the 980 nm pumped device, as well as a maximum output power of 2.2 W. This 2-μm μC-VECSEL exhibits 110-nm tunable single-frequency emission at a 7-MHz linewidth at an output power of up to 90 mW. The linewidth of the μC-VECSEL is comparable to that of VCSELs, which typically emit output powers in the milli-Watt range.