Fiber-coupled high brightness, high power diode laser for solid-state laser pumping and material processing

Bodo Ehlers, Stefan Heinemann, Falk Doerfel

Fraunhofer USA, Center for Laser Technology, 46025 Port Street Plymouth, MI 48170-6080, USA  

Abstract:

Two applications emerge as drivers for higher brightness fiber-coupled diode lasers: advanced solid-state pumping schemes and materials processing. In contrast to the well-established side-pumping schemes of laser rods, advanced pumping schemes for today's solid-state lasers make use of the high brightness of the pump sources to increase the performance and efficiency of the solid-state laser. Materials processing applications such as metal welding and cutting are commonly served with solid-state lasers or CO2 lasers. Lately, the increased lifespan, reduced systems costs and increased brightness of fiber-coupled diode laser systems make them a new alternative. [1]-[3]  In this work, a diode laser system is described that yields 250 watts in a 600 micrometer spot with a numerical aperture 0.2 of the focused beam, corresponding to a F/# of 2.4. The system is based on a single 15 bar stack that operates in cw-mode. For brightness enhancement, it incorporates a measure to increase the fill factor of the emitting aperture and polarization multiplexing. The brightness in the focus spot is 105 W/cm2 with a F/# of 2.4 focusing optic and 3·105 W/cm2 with a high speed F/# of 1.4.  To achieve the required symmetry for fiber coupling, the system incorporates a beam transformer that assimilates the beam quality along the two main axes of the beam profile. A monolithic design is chosen to reduce alignment tolerances and to increase ruggedness.