Vertical-external-cavity surface-emitting lasers (VECSELs), also called semiconductor disk lasers (SDLs), have developed strongly during the last two decades. Additionally, the range of available wavelengths has been drastically extended during this time, especially when second harmonic generation is taken into account. Nevertheless, these systems run into limits when the refractive indices of the materials used for the necessary distributed Bragg reflectors (DBRs) approach too much. This leads to a much higher number of necessary layer pairs, which increases the structure thickness and makes growth of such DBRs at least extremely difficult. Another limit occurs when the band gap of the gain material used in the VECSEL approaches too close to the band-gap in the used DBR materials. Absorption losses in the DBR are the consequence. Additionally, the performance of VECSELs in general suffers from heat incorporation into the active region caused by the excess energy of the pump photons together with the low thermal conductivity of the substrate and the included DBR.
The recently shown membrane external-cavity surface-emitting laser (MECSEL) concept opens the potential to overcome all the above named challenges as only an isolated active region membrane, sandwiched between intra-cavity heat spreaders is used as gain material. Furthermore, active region membranes in the GaInP/AlGaInP material system aiming on the yellow and red-orange spectral region where direct laser emission has not been realized yet, grown on high-index substrates, open the possibility to deliver sufficient gain realizing a MECSEL.
Optically pumped semiconductor (OPS) vertical external-cavity surface-emitting lasers (VECSELs) are an important category of power scalable lasers with a wide range of applications in bio photonics, medicine technologies and for example spectroscopy. The possibility of band-gap engineering, a laser emission in the fundamental Gaussian mode and the technical simplicity leads to ongoing growth of the area of applications for these tunable laser sources. The open laser resonator allows inserting frequency selective and converting intra-cavity elements as well as absorptive elements to create mode locking. In addition, intra cavity gas cells allow absorption spectroscopy. Aiming on spectroscopic applications for rubidium one important absorption line is located at 780 nm. Nowadays, laser emission in this spectral range has not been shown by VECSELs, neither in direct nor in frequency doubled emission although the available III-V semiconductor materials would provide such a band-gap. A very low charge carrier confinement may be the main challenge here.
We present several strategies to create gain structures based on the AlGaAs- and the AlGaAs/AlGaInP material system. The expected high thermal sensitivity can be counteracted by realizing this VECSEL structure also as a membrane external-cavity surface-emitting laser (MECSEL) to improve the heat transfer out of the active region. Investigations comparing barrier pumping with in-well pumping are also possible. A MECSEL would be in both cases beneficial here as not absorbed pump light is just transmitted instead of being absorbed in the DBR creating unnecessary heat.