Frequency measurements on HI and the Kr line at 1.548 μm

Spectral lines in the 1.55 μm region are of interest as frequency standards for optical communication because they are in the gain region of the erbium-fibre amplifier. Using diode lasers locked to the respective spectral lines, heterodyne frequency measurements have been made between the CO R(20) line at 1.56 μm and the HI P(3) line, for which f_CO-f_HI was 9277.0(3.6) MHz, and between the Kr optogalvanic line at 1.548 μm and the HI R(0) 5/2-5/2 component, for which f_Kr-f_HI was 25014.5(6.4) MHz. The HI R(0) line was split by nuclear quadrupole effects into two components, the lower frequency one containing only the 5/2-5/2 transition. The latter was used for measurement, and the splitting between the two components was 438(19) MHz. A result for the Kr line frequency obtained via lambda-meter ratios against the CO R(20) line was 193 686 109 (45) MHz. The value used here for the CO R(20) locked-laser frequency was 192 068 038(31) MHz, and was derived from earlier measurements against the CO R(21) line² and from additional lambda-meter measurements