Fiber coupled diode laser of high spectral and spatial beam quality with kW class output power

High optical output power in the multi-kW range from a fiber coupled diode laser can reach the beam quality of lamp pumped solid state lasers. Direct diode laser application as deep penetration metal welding becomes feasible. Polarization and wavelength multiplexing are established techniques to scale the optical power of diode lasers at almost constant beam quality. By use of volume diffraction gratings in an external cavity laser it is possible to constrict the spectral bandwidth of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the wavelength multiplexing of diode laser beams at small distance of the center wavelengths can be realized. The development of a fiber coupled diode laser is presented. The set up consists of twelve modules which serve for an average optical power of 1.5 kW. Each module utilizes dense wavelength multiplexing of two diode laser bars with a center wavelength spacing of 3 nm. The diode laser bars are wavelength stabilized at center wavelengths of 908 nm, 911 nm, 975 nm and 978 nm. The spectral bandwidth of all diode laser bars is within 1 nm in the full power range. Stable operation at an average power of 136 W at 908/911 nm and 115 W at 975/978 nm with a wavelength shift less than 0.1 nm is achieved by the modules. Further coarse wavelength and polarization multiplexing and beam transformation enable fiber coupling to a 600 μm fiber with a numerical aperture of 0.175 (95% power inclusion).