High-power, highly-efficient thulium-doped potassium double tungstate channel waveguide lasers

The subject of this thesis is the development of 2-μm rare-earth lasers in thuliumdoped yttrium-gadolinium-lutetium-co-doped potassium double tungstate film layers. These thulium-doped layers were grown onto undoped potassium yttrium double tungstates by liquid-phase epitaxy and were lapped and polished afterwards, prior to a photo-lithographic process to define channel waveguides. Channels were subsequently obtained by argon-beam milling of the samples, resulting in ridge-type channel waveguides. During another liquid-phase epitaxy growth these channels were overgrown with a double tungstate cladding to obtain buried channel waveguides. The concentration of the co-dopants and the dimensions
of the buried channel waveguide are chosen such that the overlap between pump and laser optical modes is maximised, whilst preventing lattice stress and cracking of the layers and ensuring single-transverse-mode operation at both the pump and laser frequency. The fabricated channels on multiple samples have a width of 7.5 − 25 μm and a height of 6.6 − 14.3 μm, and have thulium dopant
concentrations of 1.5 − 20at.%.

Laser experiments on the channel waveguides were performed by using a Ti:-sapphire laser near 800 nm as the pumping source. The channel waveguides were tested with different out-coupling transmission of up to 89%, provided by various combinations of butt-coupled dielectric mirrors, or an out-coupling transmission of up to 99% in case no mirrors were used. For a 1.5% thulium-doped channel waveguide, a threshold of 7 mW, a slope efficiency of 31.5%, and an output power of 149 mW were measured and a value for the propagation loss of 0.1 ± 0.03 dB/cm at the lasing wavelength of 2 μm were derived from relaxation-oscillation measurements. Laser experiments on channel waveguides with a higher thulium
dopant concentration of 5at.% yielded a maximum slope efficiency of 53%. The optimum thulium dopant concentration was 8at.% which yielded a maximum slope efficiency of 81 ± 3%, which is close to the theoretical maximum for this laser of 83%. An output power of 1.6 W was obtained from this laser for 2.3
W of absorbed pump power. The high efficiency is a result of cross-relaxation which increases the maximum quantum efficiency for this laser to ⌘q = 1.94. For higher thulium concentrations of 12at.% and 20at.%, the maximum obtained slope efficiency was 60%.

Depending on the out-coupling transmission selectable by the dielectric mirrors, the laser output wavelength was found to shift between 1840 nm and 2037 nm, as a result of the varied threshold inversion. By using a blazed diffraction grating in Littrow configuration, tuning of the laser output wavelength between 1810 – 1950 nm has been achieved.