Saturation and Gain of Gas Lasers from Modulation Experiments

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Abstract
It is shown theoretically that an analysis of single mode output power modulation, obtained via excitation density or via resonator Q modulation is a very sensitive method for studying both the unsaturated net gain and the saturation behavior of gas laser transitions. This theoretical description starts from the theory of Lamb. Experiments with excitation density modulation in a 1.15 μ He–Ne laser are reported. The output power modulation as a function of the detuning of the resonator, which yields information on the saturation behavior, is found to agree partly with the theory if collision effects are taken into account. Deviations from the theory may be due to higher‐order saturation terms, which is shown qualitatively by starting from the extended theory of Lamb, derived by Uehara and Shimoda. A detailed quantitative explanation of the observations at higher excitation levels awaits incorporation of collision effects in the higher‐order saturation terms. The output power modulation as a function of modulation frequency may give information on the unsaturated net gain. In our case, however, the frequency response is determined by the long metastable lifetimes of He.