We propose a new way for temperature measurement in the core of active optical fiber under high-power laser generation. The technique is based on quadrature fiber Michelson interferometer assisted with lock-in amplification for separated amplitude and phase modulations of the probe light, which allows to perform measurements in arbitrary heat transfer conditions. The technique is demonstrated on the Yb-doped fiber laser. The pump power up to 25 W under natural convection was applied. The value of convective heat transfer coefficient and its dependence on temperature difference between fiber and environment are determined from these measurements.
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