High-power optically pumped green-emitting semiconductor disk lasers using second-harmonic generation

Summary form only given. Optically pumped semiconductor disk lasers (OPSDLs) combine the unique features of high output power, excellent beam quality, and wide wavelength coverage. In the fundamental regime, the wavelengths of these sources can be continuously addressed from 0.65 μm to 2.5 μm by choosing an appropriate material system. This wavelength band is essentially increased by generation of higher harmonics, thus, the shorter visible spectrum as well as the ultra-violet can be accessed. Additionally, the output power is in the watt-level for most of these wavelengths [1] at a rather high brilliance. Frequency down conversion (Raman shifting) can be employed to extend the range of well-established near infra-red semiconductor material systems such as GaAs-InGaAs [2].The presented devices are designed to generate high power both in the fundamental (1040 nm) and the second-harmonic (520 nm) regime. The target wavelength of around 520 nm is advantageous for a large colour gamut in projection displays. The devices are grown in a bottom-emitter fashion where the substrate is eventually removed, leaving a roughly 7 μm thick foil as the actual device. For more efficient heat dissipation, the devices were soldered/bonded to CVD diamond heat spreaders. At the fundamental wavelength of 1045 nm, output power in excess of 21 W was obtained. This optical output was extracted from pump spots of roughly 400 μm in diameter. In general, the excitation of larger areas and subsequent power scaling to the 100 W-level is possible [3]. The differential efficiency of the presented devices at room temperature operation exceeds 47 % as depicted in Fig. 1, and an overall efficiency of 38 % was achieved by sputtered dielectric coatings reducing the surface reflectivity. Moreover, watt-level operation of the devices with power exceeding 3.5 W was possible at heat sink temperatures of 90 C demonstrating the ability for applications in more severe environments.The second-harmonic output was generated in an intra-cavity fashion with a folded resonator setup. The cavity arrangement yields a large reduction of the second beam waist with a ratio of 6:1, thus, the increased intensity at the smaller waist allows a more efficient conversion. We applied a type-I critical phase matching in combination with a lithium triborate (LBO) nonlinear crystal. Slightly below room temperature spectrally narrow outputs of 8.2 W and 9.5 W were generated with an 11 mm long crystal as shown in Fig. 1. The beam quality at maximum optical outputs was approximately 1.2, conversion efficiencies for high pump levels reached values of 22 % and 20 %, respectively. Furthermore, it was possible to cover a wide spectral range of 22 nm (513-535 nm) in the second-harmonic regime by rotating the birefringent filter inside the laser cavity.