Amplified spontaneous emission in a Ti:Al2O3 amplifier

Recently the time dependence of gain in a Ti:Al2O3 amplifier was measured.1 It was shown that the decay of gain was much more rapid than expected if only spontaneous decay was responsible for depletion of the upper state. Although it was believed that amplified spontaneous emission (ASE) might be responsible for this difference, the apparent difficulty of calculating the effect of ASE inhibited pursuit of this conjecture. A rate equation has been used to demonstrate that ASE can quantitatively explain the fast decay. In this model, the gain of the amplifier is e γ , where γ obeys the time-dependent spatially integrated equation The three terms on the right-hand side of this equation describe the spontaneous emission rate, pump rate, and stimulated emission rate caused by the input to the amplifier and by ASE, respectively. Excellent fits of the experimental data have been obtained using only one parameter Ω, the effective solid angle for ASE. The fitted parameter is in good agreement with geometrical estimates.