Calculation of the velocity and threshold of a thermal absorption wave of laser radiation in an optical fibre

Based on the nonstationary one-dimensional heat conduction equation, taking the efficient heat removal into account, a thermal absorption wave of laser radiation in the core of an optical fibre is studied. The dependence of the wave velocity on the laser radiation intensity is calculated and threshold intensities at which the thermal wave appears are estimated. It is shown that the wave velocity at high intensities is well described by the formula known from the combustion theory and is proportional to the square root of the radiation intensity. Simple expressions are derived which describe the dependence of the threshold intensity on the effective cooling radius and the dependence of the wave velocity on the radiation intensity (including the near-threshold region). The calculated threshold intensities of laser radiation are approximately twice those needed to obtain the equality between the power coupled to the fibre and the effective heat removal power. The calculated dependences of the wave velocity on the radiation intensity and threshold intensities are in agreement with the experimental data available.