Laser frequency standards based on saturated-dispersion lines of methane

A review is given of the state-of-the-art and of likely developments in laser frequency standards based on two-mode gas (He — Ne) and solid-state (Li:RbCl) lasers, in which the reference points are ultranarrow saturated-dispersion resonances of lines in ν₃ vibrational — rotational methane bands (λ = 3.2 — 3.4 μm). The methane standard, now traditionally attracting the attention of investigators, can provide a frequency reproducibility of ~10¹⁵ in compact transportable systems. Recent progress in laser techniques (optically pumped solid-state lasers, efficient nonlinear crystals) can make this standard the basis for simplified optical-frequency synthesis systems accessible to many laboratories.