High brightness diode lasers and single-mode large-core fiber lasers open door for power scaling  

Bodo Ehlers1, Falk Doerfel1, Stefan Heinemann1, Chi-Hung Liu2, Almantas Galvanauskas2  

1 Fraunhofer USA, Center for Laser Technology, Plymouth, MI

2 University of Michigan, Center for Ultra Short Optical Sciences, Ann Arbor, MI

Abstract:

Keywords (OCIS): diode lasers, semiconductor lasers, laser beam shaping, diode pumped lasers, pumping, fiber lasers  

High power fiber lasers and amplifiers as well as novel pump schemes for solid-state lasers demand higher brightness pump sources in packages with moderate optical output power. In this paper, a fiber-coupled diode laser system is presented that fulfills this requirement and offers robustness for military and industrial applications.  A monolithic beam transformer replaces a large number of micro-optical components compared to prior designs. This decreases the costs, assembly time and optical losses. The system yields 300 Watts of pump power through a 600 micrometer fiber with a NA of 0.2.  These diode lasers are ideal for cladding pumping large-core Yb-fiber lasers and amplifiers. Large-core fibers with low core perturbations and adiabatic tapers are usually used for preventing stimulation of higher order modes within the large multimode core and to maintain single mode signal output. Diode laser of the above architecture with output power of approximately 120 W was used to pump 300-um cladding and 25-um core Yb-fiber laser. Due to this particular fiber geometry, nearly complete absorption of the coupled pump power have been achieved within a short fiber length of 3 meters, thus enabling to significantly reduce nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering at high power operation. Currently output power of up to 62 W has been obtained with a slope efficiency of more than 65%. Further power scaling for cw laser operation and pulse amplification are in progress. First applications are investigated.