Laser Devices and Systems for Industrial and Biomedical Applications

The PhD activity, from which this thesis is taken, has been aimed at the design and development of optical components and subsystems for industrial and biomedical laser applications. Industrial-grade high-power laser market, traditionally dominated by gas and solid state lasers, is currently being revolutionized by two new devices, namely fiber and direct diode lasers, which are partly competing and partly complementary. On the one hand, Fiber Lasers (FLs) make use of rare-earth doped specialty fibers to converted the radiation from semiconductor laser modules (the “pump” source) into a high quality beam at an another wavelength; their strong point is the brightness enhancement, although at the expenses of the conversion efficiency. On the other hand, Direct Diode Lasers (DDLs) rely on the combination of semiconductor laser chips, similar to those used as pump for FLs, to achieve kilowatt-range output power, which is then delivered to the piece under treatment without the need of further conversion or processing; their strong point is the wall-plug efficiency, while the beam quality is typically low. Different applications may privilege one or the other technological approach: for instance, metal sheet cutting and industrial additive manufacturing from metal powders are more suited for FLs, whereas welding and brazing for DDLs. The first part of the PhD activity have been devoted to the design and development of high power laser systems, yielding to the all-fiber realization of a 600W single-mode Yb-doped FL module and 500W to 2000W low-brightness DDLs. First, different architectural solutions have been investigated and optimized, considering feasibility issues, energy consumption and overall ownership costs. Then, key fiber components, such as fused fiber combiners, cladding mode strippers and laser end-caps, which are usually not commercially available being too dependent on the fiber characteristics and other application related constraints, have been designed fabricated and tested. These devices have been later used to assemble demonstrators of fiber and direct diode laser systems for industrial applications. The implemented FL has reached ~600W of nearly single-modeoutput, with Beam Parameter Product (BPP) less than 0.54mm mrad. As for the DDL case, two demonstrators (~500W power with BPP 44.4mm mrad and ~2kW power with laser BPP 66.5mm mrad) have been realized and engineered. In all the cases, the output power has been limited to available diodes only. The second part of the PhD, and thus of the thesis, has been focused on the laser application in the biomedical field, and in particular to the laser ablation of solid tumors in liver and pancreas. Innovative applicators for laser tumor ablation with customized beam irradiation pattern and integrated all-optical temperature sensing capabilities have been designed and manufactured. The prototypes have been validated in liver phantoms (e.g. agar jelly and porcine livers), demonstrating the effectiveness of the proposed solution and encourage further developments toward clinical applications. It is important to highlight that during the PhD the entire chain from design to manufacturing, to applications, has been explored and this has allowed acquiring new skills in different areas, not strictly limited to the main topic of the thesis.