An in situ spatially-resolved diagnosis of thermal and Brillouin characteristics of a double-clad ytterbium-doped fibre (YDF) laser operating at 1.09 µm is demonstrated. For this a Brillouin optical time domain analysis technique based on 1.55 µm Brillouin pump and probe beams is utilised. A 2.4 K temperature difference across the YDF laser when it was running with 6.5 W pump power has been measured and resolved. Based on the measured thermal and Brillouin characteristics of it is expected the YDF, that the effective Brillouin gain coefficient would decrease by 20% for 1.09 µm radiation if an 80 K temperature variation is built up across it, as a result of the quantum heating by the pump powe
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