High Power Fiber Lasers for Industrial Applications

Fiber lasers can be considered a revolutionary technology in the laser field thanks to their unique properties, such as high efficiency, simplicity, compactness and robustness. These features have allowed in the last ten years their outstanding growth both in scientific and industrial applications, eroding the market share of traditional laser sources like solid-state and gas lasers. Fiber lasers power scaling to the kilowatt range is now well established and, thanks to the fiber confinement, excellent output beam quality can be obtained, with a remarkable benefit for applications. Today, high power laser sources are based on ytterbium doped, large mode area fibers because ytterbium can be efficiently pumped in the range 915nm − 975nm (where pump sources are widely available), generating laser action at 1060nm − 1090nm. With this configuration, impressive power scaling has been demonstrated in the last few years. This Ph.D. thesis has been focused on the design and development of high power fiber lasers for a wide range of industrial applications, like cutting, wending, drilling and micro-machining. Both continuous and pulse wave fiber lasers have been demonstrated and particular attention has been devoted to the development of critical technological de-vices like fused fibers combiner, strategic components either for pump light coupling into the laser active fiber (pump combiner) and for power scaling through the beam combining of several fibers lasers (signal combiner). Ytterbium doped fiber lasers have been developed during the Ph.D activity and, in particular, after a theoretical analysis devoted to the modeling of fiber laser cavities and amplifiers, a continuous wave fiber laser and two pulsed laser systems have been demonstrated. The CW fiber laser has been developed with a modular approach: 7 laser modules, capable of emitting hundreds of Watts each, have been coupled together thanks to a fused fiber combiner. A multi-kilowatts output power has been demonstrated. The photo-darkening effect in the active fiber of the laser modules has also been exper-imentally investigated. The pulsed architectures are instead a Q-switched MOPA and a Seed MOPA fiber lasers. The first system is based on a fiber laser oscillator operating in the Q-switching regime, followed by a power amplifier. This laser is capable of delivering 100ns pulses with 10W average power (2kW maximum peak power). The Seed MOPA consists instead of a current modulated laser diode followed by two amplification stages; 2W output average power with adjustable pulse widths from 10 to 100ns has been demonstrated. In the last part of the activity, a preliminary version of a thulium doped fiber laser emitting at about 2000nm (i.e. in the so-called eye-safe region) has been developed. The laser is a Seed MOPA system that has been tested in cw regime but in the near future the pulsed behavior will be investigated.