Semiconductor disk lasers, also called vertical external-cavity surface-emitting lasers (VECSELs) have advantageous properties such as high output power, wavelength flexibility due to bandgap engineering and near-diffraction limited beam quality. The possibility to insert intra-cavity elements – filters, frequency doubling crystals or semiconductor saturable absorber mirrors (SESAMs) – enables wavelength tuning, second harmonic generation or mode locking with ultra-short pulses. A major challenge for these laser sources is the removal of heat which is introduced by optical pumping. The thermal management can improved by placing only the active region directly between two heat spreaders. This membrane external-cavity surface-emitting laser (MECSEL) allows emission in an even larger wavelength range, since the growth is not restricted by a distributed Bragg reflector.
We present the fabrication, processing and characterization of MECSELs using different material systems for laser emission at various wavelengths in the visible and in the infrared spectral range. Our semiconductor structures are grown by metal-organic vapor-phase epitaxy and contain quantum wells (QWs) or quantum dots (QDs) in the active regions. We discuss our latest results of the membrane laser concept with investigations of strain effects on the photoluminescence and the laser emission and different pumping schemes.
In particular, we show results of a MECSEL placed into a linear cavity and pumped by a 532 nm laser. The system was operated at a heat sink temperature of 10°C and achieved nearly 600 mW at 3.7 W pump power. The slope efficiency achieved here was 22.3 % with a threshold pump power of 1 W. This slope efficiency exceeds any slope efficiency published before with green pumped conventional VECSEL in this emission range at these elevated heatsink temperatures. Including a birefringent filter into the laser cavity allows for a tuning of the emission laser wavelength. The group could demonstrate a tuning range of 24 nm (650 nm – 674 nm), which is the highest value achieved in this spectral range by semiconductor lasers to date. All these achievements come with the expected Gaussian TEM00 mode with a beam quality factor of M2< 1.06[Optica 3(12), 1506-1512 (2016)].
To demonstrate the flexibility and usability of the MECSEL approach, we have created an optically pumped laser with a GaInAsP membrane for emission around 1000 nm, together with a group in Dundee. The set-up of the laser was similar to that of the red spectral range but instead of diamond, SiC was used as heat spreader material. Nevertheless, the achieved output powers could exceed 10 W with a slope efficiency of 27.5% and this with the heat spreader on only one side. This work was as well recently published in Electronic Letters 2017 [DOI:10.1049/el.2017.2689]. Actually, we realized a running membrane laser at a wavelength of 608 nm, a wavelength not accessible so far. The characterization measurements are now ongoing, first results will be presented at the conference.