Active Photonic crystal fibers for high power applications

The photonic crystal ber technology provides means to realize bers optimized for high power operation, due to the large single-mode cores and the unique design exibility of the microstructure. The work presented in this thesis focuses on improving the properties of active photonic crystal bers for high power ber lasers and ampliers, and on adding new functionality to the fibers - all with the purpose of pushing the technology towards high powers.

The first part of the work has been to investigate photo darkening, the mitigation of which is crucial in the quest for higher powers. The work has contributed to the compounding of new and improved material compositions. The second part is an investigation of pump absorption in photonic crystal bers, demonstrating that the microstructure in photonic crystal bers improves the pump absorption by up to a factor of two compared to step-index bers.

This plays an important role in high power lasers and ampliers with respect to efficiency, packaging, and thermal handling. The third part of the work has involved developing tools for characterizing the mode quality and stability of large core bers. Stable, single-mode bers with larger cores are essential to the pursuit for higher powers and the tools have enabled the development of one of the largest and most stable exible large core bers on the market.

The forth, and largest part of the work, has focused on developing a ber amplier with gain shaping for high power amplication at 1178 nm. Several high power records have been set using this ber. An output power of 167 W has been achieved, which, at the time of writing, is the highest output power generated from ytterbium bers in this wavelength region and from photonic bandgap bers in general.

The 1178 nm light has subsequently been frequency doubled to 589 nm with high efficiency. Finally, the last part of the work has been the development of the rst ever tunable ber laser based on a liquid crystal inltrated photonic crystal ber for operation at 1040{1065 nm.