Measurement of the frequency of the 3.39-μm methane-stabilized laser to /spl I.chemo/3 parts in 1011

We have measured the frequency of a methane-stabilized laser at 88 THz (3.39 μm) by a simultaneous beat-counting method which largely eliminates the uncertainties contributed by lower frequency lasers used as transfer oscillators. Our result is primarily of interest for a proposed new definition of the meter in which the speed of light in vacuum (c₀) would be kept constant and reference made to the standard of time and frequency. For this reason our measurement has been referred to the mean frequency of 5 similar portable stabilized lasers normally used as wavelength standards. We estimate the mean frequency of the laser ensemble to have been (88 376 181 616 /spl I.chemo/ 3) kHz. The estimated standard deviation is within about a factor 2 of the reproducibility of the lasers themselves and the result can be extended to the visible iodine-stabilized lasers at 0.63 μm to /spl I.chemo/2 parts in 10¹⁰ by using the wavelength ratio measured by Layer et al.