Rare-earth-doped gallium lanthanum sulphide glasses for mid-infrared fibre lasers

Gallium lanthanum sulphide (GLS) glass was investigated as a potential host material for rare-earth (RE) doped mid-infrared fibre lasers. <br/><br/>Glasses were fabricated from gallium sulphide and lanthanum sulphide powders by melt quenching and drawn into fibres using the rod-in-tube technique. Typical values for the attenuation in the loss minimum around 4 µm are a few dB/m. The quality of the fibre is currently limited by impurities such as transitions metals, silicon, and aluminium, imperfections at the core/clad interface, and the tendency to crystallisation. <br/><br/>Absorption, fluorescence, and lifetime measurements were performed for eleven RE ions for wavelengths ranging from the visible to the mid-infrared with a focus on mid-infrared transitions. The results of the measurements were used to study the multiphonon decay in GLS glass and to obtain important laser parameters such as the absorption and emission cross sections, branching ratios, and quantum efficiencies from the Judd-Ofelt theory, the Füchtbauer-Ladenburg equation, and the McCumber theory. Twenty-one transitions with peak emission wavelengths longer than 2 µm were identified, seven of which have not been reported in a glass before. Some transitions overlap with the fundamental absorption bands of environmentally important gases such as the 3.4 µm (methane and other hydrocarbons) and 4.7 µm (carbon monoxide) praseodymium transitions and the 4.3 µm (carbon dioxide) dysprosium transitions, whereas the 3.8 µm thulium and the 3.9 µm holmium transitions coincide with an atmospheric transmission window. <br/><br/>Co-doping schemes such as thulium/terbium, praseodymium/ytterbium, praseodymium/erbium, and dysprosium/erbium which offer more favourable absorption bands for diode laser pumping were investigated with respect to ion-ion energy transfer. <br/><br/>Excited-state absorption (ESA) measurements in erbium and thulium doped GLS glasses revealed transitions from excited RE energy levels to levels at energies relative to the ground state which are larger than the energy bandgap of the glass (~ 2.6 eV). Transitions from the RE ions to the conduction band of the glass, as they have been reported in RE doped crystals, could not be found. The results support the model of isolated ions with shielded 4f levels and rule out ESA from RE ions to the conduction band of the glass as a potential loss mechanism for RE lasers in GLS glass. <br/><br/>Lasing was demonstrated in neodymium doped GLS glass and glass fibre at 1.08 µm, representing the first reported laser action in a RE doped chalcogenide bulk glass and glass fibre, respectively. The strong thermal lensing in the bulk glass rules out the use of GLS glass as a bulk laser material. The effect was eliminated by using a fibre geometry showing the advantages and necessity of GLS glass fibre.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

Universität Hamburg

Year dc:date.issued

2000

Author dc:creator

• Schweizer, T.