The frequency of the He-Ne laser radiation stabilized by saturated absorption of methane in an internal cell (λ = 3.39 μm) can vary up to several kilohertz when the operating conditions of the amplifying tube are varied. Moreover, a different frequency is observed depending upon whether the photodetector used for servo-control is placed on the absorption-cell side or the amplifying-tube side of the resonator.
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R. J. Felder (1986) studied this question.
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