Impact of energy-transfer-upconversion on performance in quasi-three-level solid-state lasers

Energy-transfer-upconversion (ETU) has a detrimental impact on performance in a number of different solid- state lasers. ETU manifests itself as a reduction in the effective upper laser level lifetime leading reduced energy storage and increased thermal loading, and hence can be particularly problematic when attempting to scale to high average power or operate in Q-switched mode. In some situations ETU can be so severe that a very large fraction of the absorbed pump light is converted to heat and thermally-induced damage results, or it may prevent the build-up of a large enough population inversion for threshold to be reached. The requirement for a larger fraction of the active ions to be excited in a quasi-three-level laser can further exacerbate these issues. Thus, an accurate knowledge of the upconversion parameter and the impact of ETU on laser performance are crucial for power scaling of solid-state lasers and for efficient operation in Q-switched mode. This paper investigates a simplified analytical approach and derive an approximate expression for threshold in an end-pumped quasi-three-level laser taking into account ETU. The predictions of the model have been compared with experimental results for an in-band pumped quasi-three-level Er:YAG laser at 1645 nm confirming its validity.