Steady-state heating of active fibres under optical pumping

We have measured the temperature in the core of rare-earth-doped optical fibres under lasing conditions at high optical pump powers using a fibre Mach — Zehnder interferometer and probe light of wavelength far away from the absorption bands of the active ions. From the observed heating kinetics of the active medium, the heat transfer coefficient on the polymer cladding — air interface has been estimated. The temperature of the active medium is shown to depend on the thermal and optical properties of the polymer cladding.