Upconversion laser produces 0.47 W of green light

Based on photon avalanching and other processes, frequency upconversion is a method for creating short-wavelength lasers that does not involve nonlinear frequency conversion and its associated intensity-fluctuation problems. Aiming for high-power upconversion at visible wavelengths, scientists at the Universität Hamburg (Hamburg, Germany) and Coherent Lambda Physic (Lübeck, Germany) have developed an upconversion laser based on erbium-doped lithium lutetium tetrafluoride (LiLuF4) that emits 0.47 W of continuous-wave power at a 552-nm wavelength, or a 0.76-W peak power at a 50% duty cycle. The room-temperature laser is pumped with 970-nm light from an optically pumped semiconductor (OPS) laser.

The 1.6-mm LiLuF4 crystal is coated on one side to reflect highly at the pump wavelength and on the other to reflect highly at the upconversion wavelength; curved mirrors external to the crystal and a small angular offset of the pump beam result in a multipass pumping scheme. (The OPS pump laser emits up to 9 W with a beam parameter M2 between 2 and 2.5.) The upconversion laser has a threshold of 70 mW of absorbed pump power and a slope efficiency of 32%. Contact Ernst Heumann at heumann@physnet.uni-hamburg.de.