Absolute frequency stabilization of the 3.39-μm laser on a CH4 line

Theoretical and experimental studies on a primary standard of wavelength and frequency in the infrared are reported. The 3.39-μm He-Ne laser is tuned to the center of the absorption line of methane which is assigned to the F₁⁽²⁾ component of the P(7) branch of the v₃ band. The absence of Stark and Zeeman effects under the highest fields tested, the strong absorption at 63.2°K as well as at 78°K, and the separation from other lines are excellent features for a high degree of absolute stability and reproducibility. A method of automatic frequency control of the He-Ne laser at the line-center of methane is described.