Characteristics of amplification of high-power laser pulses in neodymium silicate and phosphate glasses

An investigation was made of the amplification of microsecond radiation pulses in neodymium-activated glasses. The effective cross sections σ_ef of stimulated transitions were determined before and after the passage of a high-power pulse. The cross section σ_ef, representing the active medium at the maximum of a luminescence pulse, was found to depend on the degree of excitation N₀ of the active element in the amplifier. When the energy density E₀ of the radiation at the amplifier input was increased, the value of σ_ef (measured after the passage of the pulse being amplified) decreased in the case of silicate glasses, but increased for phosphate glasses. A hypothesis was put forward that the equilibrium profile of the luminescence line of neodymium glasses could become deformed under the action of superluminescence during an optical pump pulse, which would lead to the dependence of #sigma_ef# on N₀. This hypothesis was confirmed by computer calculations carried out on the basis of an approximate model of an active medium with a single inhomogeneously broadened laser transition line and with parameters typical of neodymium glasses. The dependence of the efficiency η of energy extraction on E₀ almost reached its maximum value at E₀ ≈ 30 J cm^-2. A further increase in E₀ for the purpose of increasing η would be pointless. The maximum experimental efficiency of extraction of the energy of excited particles was ~95% for phosphate glasses.