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

Highly efficient channel waveguide lasers at 1 µm and 2 µm in refractive-index-engineered potassium double tungstates

Epitaxial growth of rare-earth-ion-activated KY(1-x-y)Gd(x)Lu(y)(WO4)2 co-doped thin layers onto KY(WO4)2 substrates has enabled lattice-matched waveguides with high refractive-index contract and large variation of the active rare-earth-ion concentration. In Yb3+-activated micro-structured channel waveguides, we demonstrated lasers with 418 mW of continuous-wave output power at 1023 nm and a slope efficiency of 71% versus launched pump power at 981 nm. Channel waveguide lasers operating on the 981-nm zero-phonon line were demonstrated under pumping at 934 nm with an output power of 650 mW and a slope efficiency of 76% versus absorbed pump power. Lasing with a record-low quantum defect of 0.7% was achieved. In a feasibility study, a device comprising a tapered active channel waveguide and a passive planar pump waveguide, fabricated by multi-layer growth of lattice-matched layers, was demonstrated as a laser by diode-side pumping with a high-power, multi-mode diode bar. This approach offers the potential for significantly increased output powers from channel waveguide lasers. Tm3+-activated channel waveguide lasers demonstrated a maximum output power of 300 mW and slope efficiency of 70%, when pumping near 800 nm. Lasing was obtained at various wavelengths between 1810 nm and 2037 nm. These lasers were operated with resonators exploiting either butt-coupled mirrors, providing only a non-permanent solution, or based on Fresnel reflection at the waveguide end-facets, resulting in laser emission from both waveguide ends and without control of the laser wavelength. Currently we are inscribing Bragg gratings into the top cladding to provide a stable resonator configuration that allows for effective wavelength selection

Highly efficient lasers and amplifiers in double tungstates

The double tungstates KY(WO4)2, KGd(WO4)2, and KLu(WO4)2 are excellent candidates for solid-state lasers because of the large transition cross-sections of optically active rare-earth ions doped into these hosts. We grow actively doped KY1-x-yGdxLuy(WO4)2 layers onto KY(WO4)2 substrates by liquid-phase epitaxy. Co-doping the layers with optically inert Gd3+ and Lu3+ ions simultaneously allows for lattice matching and enhanced refractive index contrast with respect to the substrate. Low-loss channel waveguides are microstructured into the layers by Ar+-beam etching, resulting in strong pump- and signal-mode confinement. Yb-doped channel waveguide lasers deliver 650 mW output power at 1 µm. Record-high slope efficiency (85%) and record-low quantum defect (0.7%) for dielectric lasers are achieved. In pump-signal experiments, exploiting highly doped KGd0.447Lu0.078Yb0.475(WO4)2 channel waveguides, we demonstrate a giant optical gain of 950 dB/cm, exceeding the gain previously reported in rare-earth-ion-doped materials by two orders of magnitude and comparable to the gain obtained in semiconductor optical amplifiers

Highly efficient solid-state waveguide lasers

This paper reviews our recent results on highly efficient rare-earth-ion-doped planar and channel waveguide lasers in crystalline potassium double tungstates and amorphous aluminum oxide on silicon chips