Core temperature measurement of an active optical fiber in lasing regime

The output power of a single-mode ytterbium fiber laser radiation (emission wavelength 1.06 mum) achieved 3 kW for the input pump power of about 5 kW at pump wavelength 0.96 mum. The power defect of about 2 kW between pump and laser radiation caused heating of the core of an active fiber. In this case the temperature of active medium increased by hundreds of degrees. Strongly-inhomogeneous heating of the optical fiber modifies not only profile of refractive index, but also spectroscopy properties of active medium, such as absorption and emission cross-sections of Yb³⁺ ions, pump conversion efficiency, emission wavelength etc. To the best of our knowledge no experiments for core temperature measurement in the active optical fiber in lasing regime have been reported up to date.