Active Multipass Geometry Boosts Pulse Energy

Scientists from the University of Konstanz and from Trumpf Laser GmbH + Co. KG in Schramberg, both in Germany, have achieved pulse energies of more than 13 μJ from a thin-disc laser operating in air. These ultrashort pulses have applications in micromachining and in experimental physics as well as in the direct pumping of parametric devices.

This 3-D perspective of a thin disc demonstrates the multiple passes of the laser mode. Courtesy of Trumpf Laser GmbH & Co. KG.

The Yb:YAG disc is 60 μm thick and 10 mm in diameter and has good thermal management and optical linearity. Previous high-energy pulse devices used helium to avoid the nonlinear effects of air. This laser uses a novel active multipass geometry — a pair of spherical mirrors is used to pass the light 44 times through the gain medium, producing better than 50 per cent output coupling and avoiding the need for helium. A semiconductor saturable absorber mirror provides passive mode-locking. The design yields a cavity that measures 39.93 m long in a footprint of only 0.3 × 1.0 m. The laser provides 55-W average power in 1.36-ps pulses at a 1030.3-nm centre wavelength, with a beam quality of M² <1.2.

(Optics Letters, in press)