Measurement of the cross section for stimulated emission in neodymium-doped glass from the output of a free-running laser oscillator

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Abstract
The cross section for stimulated emission for a pulsed four-level laser is deduced from the known gain of a laser rod at the threshold of oscillation and from the inverted population at threshold which is obtained from the output at a fixed amount above threshold. The theory takes account of finite pump pulse length and the various forms of resonator loss. The residual absorption of the glass does not need to be known, so that comparison of several samples is easy. Values found are 122 × 10−20 cm2 for Schott LG 55, 235 × 10−20 cm2 for Kodak Nd11 and 13 × 10−20 cm2 for Chance-Pilkington LN6 with a relative accuracy of 3% and an absolute accuracy of 7%. Values for the fluorescence efficiency of the upper laser level are deduced and the implications for laser efficiency discussed. Evidence of slow cross relaxation across the fluorescence line is found in Chance-Pilkington LN6.