Development of highly efficient thulium-doped high power fibre lasers

Abstract

Thulium-doped fibers pumped with ~790nm high power diodes have the potential to enable a cross-relaxation process to achieve two excited ions into the 3F4 manifold for one pump photon (two-for-one cross-relaxation) when the fibre core composition including the thulium concentration is optimized. This has the advantage of reaching thulium-doped fibre laser quantum efficiency up to 200% in the two-micron band. Therefore, the development of thulium-doped fibres is a key parameter to exploit laser efficiencies that can exceed the Stokes limit (>40%). The thulium-doped fibres used in this thesis were fabricated in an aluminosilicate host using the well-known modified chemical vapour deposition process in combination with a hybrid gas phase solution doping technique. This novel fabrication method takes advantage of a conventional solution doping technique in which high purity rare-earth precursors that have low vapour pressures at room temperatures up to several hundred degrees Celsius can be used and at the same time by using a gas phase deposition process and high aluminium concentrations can be reached in the fibre core, which is otherwise not possible using the solution doping technique. Furthermore, fibres fabricated using this hybrid approach show more homogeneous and flat-top dopant profiles compared to fibres fabricated using the conventional approach where both aluminium and thulium are incorporated in the core through solution doping in the fibre core which helps to achieve a good laser efficiency in thulium-doped fibre lasers with ~790nm pumping. Contrary to the generally held view of high thulium concentration for efficient fibres, this work demonstrates that a thulium concentration of about 3.5wt% with a more uniform dopant profile is sufficient to maintain superior laser performance with slope efficiencies >70% over a wide wavelength band of 1980nm to 2080nm and >50W output power. In addition, for operation at longer wavelengths >2080nm, thulium-holmium codoped silica fibres were fabricated to investigate the energy transfer mechanism from thulium to holmium, which thereby opens the opportunity for high power operation at longer wavelengths when pumped by ~790nm diodes. The hybrid gas phase-solution doping technique together with the modified chemical vapour deposition process used to fabricate the co-doped fibres allows a more uniform distribution of the interacting rare earth ions across the core region. As a consequence, an efficient two-for-one cross-relaxation process in thulium followed by an efficient energy transfer between thulium and holmium ions was obtained. By optimizing the core composition and the concentration ratio of the dopants in thulium-holmium co-doped silica fibers, laser efficiencies >55% have been reached at an emission wavelengths of ~2100 nm and >37W output power, which has convincingly demonstrated the advantage of this approach and its considerable potential for power scaling.

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Identifiers

ORCID for N. J. Ramírez Martínez: orcid.org/0000-0001-8711-792X

ORCID for Jayanta Sahu: orcid.org/0000-0003-3560-6152

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