Temperature measurements in the core of active optical fibers under lasing conditions

A procedure for measuring the temperature in the core of an active fiber at a high-power optical pumping is described. The procedure is based on using a cavity of the optical fiber laser as one of the arms of the Mach-Zehnder interferometer with subsequent spectral filtering of the probe light. The pronounced nonlinearity of the phase dependence on the absorbed pump power has been detected. This nonlinearity can be attributed to different heating regimes of the active fiber before and after the lasing threshold.