Power-scaling of blue diode pumped solid-state lasers for the visible wavelength range

Abstract

Lasers as light sources for projection applications provide several advantages: Compact as well as efficient displays can be realized by using lasers, With only 200m W in every primary colour a luminous flux of 100lm on the screen is achievable from a small device [1]. However, the bottleneck for this application is still the lack of integrated green laser sources. Especially blue diode pumped solid-state lasers, where for example a Pr:YLF-crystal is pumped by a GaN-laserdiode, have recently gained a lot of interest for such applications.Blue-emitting GaN-laserdiodes are now commercially available with output powers reaching the Watt-level. The emission wavelength of these devices around 445nm matches nicely the absorption of the Pr 3+-ion . Several recent publications [2][3][4][5] have reported output power levels from GaN-diode pumped Pr-lasers in the order of 30-50m W, which is still too low for the compact projectors mentioned above. In this paper we report the results of our work towards higher output power from these devices .In the experiments we present here, a GaN-laserdiode from Nichia Corp. with an emission wavelength of 443nm and a maximum output power of 500mW was used . The radiation from this diode was collimated by an aspheric lens and the beam was shaped with the help of an anamorphic prism pair. With an achromatic lens the beam was focussed into the P~+-doped YLF-crystal. Different crystals were used : a 12mm long, 0.2% Pr-doped and a 5mm long, 0.5% Prdoped crystal. These crystals were placed in either a plano-concave or a concave-concave resonator with the c-axis of the crystal oriented parallel to the electric field of the pump laser. Using output couplers with different transmission for the laser wavelength, the optimum outcoupling for each setup as well as the round-trip losses were determined.' 20 I • 5mm. 0 5% Pr YLF J -/ roo -" =4~,4%, p~= 155mw / oS 80