Approaching the thermodynamical limit of optical pumping — Intra-cavity pumped thin disk laser with very low quantum defect

Optically pumped lasers are basically brightness converters which convert pump radiation with low brightness into laser radiation with higher brightness. The second law of thermodynamics dictates that thermal energy or incoherent radiation as fluorescence must be expelled to the environment[1]. Furthermore, it determines the maximum efficiency under which the conversion is allowed to take place. When stimulated emission is the dominating process, the main source of heat is the quantum defect. This thermal energy is needed to comply with the second law of thermodynamics.