LASER FREQUENCY STANDARDS BASED ON SATURATED-DISPERSION LINES OF METHANE

A review is given of the state-of-the-art and of likely developments i n laser frequency standards based on two-mode gas (He-Ne) and solid-st ate (Li:RbCl) lasers, in which the reference points are ultranarrow sa turated-dispersion resonances of lines in the nu(3) vibrational-rotati onal methane bands (lambda = 3.2-3.4 mu m). The methane standard, trad itionally attracting the attention of investigators, can provide a fre quency reproducibility of similar to 10(15) in compact transportable s ystems. Recent progress in laser techniques (optically pumped solid-st ate lasers, efficient nonlinear crystals) can make this standard the b asis for simplified optical-frequency synthesis systems accessible to many laboratories.