Coherent combination of mismatched fibre lasers - steps towards real world applications

Large arrays of coherently-combined Quasi-Continuous-Wave (QCW) fibre lasers have been identified as a plausible solution to creating high-repetition rate (~1 kHz), high-wall-plug efficiency lasers capable of driving a wakefield for accelerator applications. The coherent combination of four near-identical Photonic Crystal Fibre (PCF) lasers to produce laser pulses with pulse energies of 5.7 mJ and peak powers of 22 GW has been successfully demonstrated by other groups. Extensions of the four PCF experiment also demonstrated the coherent combination of 8 photonic crystal fibres to attain pulse energies of 12 mJ and peak powers of 35 GW. However, these PCFs were near-identical in length, core size, fibre structure and dopant concentration. Work done in Oxford focused on the coherent combination of fibre lasers which were unmatched in length, core size, internal structure and dopant concentration. The aim was to identify the point at which the fibre differences caused the combining efficiency to degrade. Potentially, this information would be used to identify the tolerance levels required for the large-scale manufacture of the fibres needed to construct a facility-scale laser system for laser-wakefield acceleration. This thesis covers the coherent combination of mismatched PCFs, a scenario which could occur during the construction of a facility-scale terawatt laser system. Here, hundreds or potentially thousands of PCFs could be purchased from different manufacturers, each of whom use their own methods of manufacture and tolerances. An investigation into the challenges associated with combining two highly-mismatched PCFs was performed, with the PCFs chosen to be very different to exaggerate any effects which could degrade their coherent combination. This work resulted in the successful coherent combination of these highly-mismatched PCFs, with a measured degree of linear polarisation of 85.9 ± 0.5 % and a fraction of available beam power of 93.0 ± 0.5 %. Also, supporting work investigated the difference between simulation and measurement in explaining the observed degree of linear-polarisation of the combined-beam.