Operating characteristics of a frequency-stabilized ring laser with a methane cell

An experimental and theoretical investigation is reported of the effect of the modulation frequency of the resonator perimeter on the operation of an He–Ne/CH₄, ring laser at λ = 3.39 μ. It is shown that at high modulation frequencies (F>5–7 kHz), the amplitude of the first intensity harmonic I_Ω at the center of the methane "reflecting" power peak does not vanish in the general case and depends strongly on the pressure in the active medium. At a pressure corresponding to coincidence of the centers of the amplification and absorption lines, I_Ω has a minimum at the peak extremum. This effect yields a simple and accurate method of matching the line centers of the active and absorbing media. A dependence of the stabilized laser frequency on the phase of the wave back reflections is also reported.