Determination of the speed of light by absolute wavelength measurement of the R(14) line of the CO2 9.4-Mum band and the known frequency of this line

Learning from the time and length redefinitions, and the metre demotion

After discussing several issues in a future redefinition of the kilogram, this paper considers the lessons that one might have learned from the analogous redefinitions of the metre and the second. The progress of length metrology was slow and steady, from seven digits reproducibility with the 1889 X-shaped metre prototype, to nine digits with Kr lamps, to 11 digits with the 1983 redefinition of the metre using the speed of light. With laser cooling, the Cs clock improved to 15, now 16, digits (and so also astronomical distance measurements could improve). Laser-cooled ions, and now atoms captured and cooled in an optical lattice, enable accuracy capability of three different optical frequency references to exceed 17 digits, i.e. better than time itself. The optical comb and related techniques vastly simplify frequency comparisons. Such progress stimulates a new satellite experiment, the STAR Mission (Space-Time Asymmetry Research). The goal is to test at the 1E-18 level frequency shifts owing to spatial anisotropy, position, gravitational potential and boost. The onboard optical clock will use stabilization to a molecular transition in I(2) or HCCH or CO(2). The length etalons will be multiply redundant, with stability at the thermo-mechanical mirror motion limit. For a ULE glass etalon spacer (1987), I measure length creep approximately -1.5E-12/d, i.e. below 1E-14 over the 500 s satellite spin period.

Accurate laser wavelength measurement with a precision two-beam scanning Michelson interferometer

This paper gives the details of a precision two-beam scanning Michelson interferometer, designed and perfected for accurate comparison of an unknown laser wavelength and the precisely calibrated wavelength of a reference laser. An iodine Lamb-dip stabilized He-Ne 633-nm laser (calibrated with respect to a Kr standard) is used as the reference. The design incorporates features to minimize instrumental errors and the effect of fringe shifts caused by diffraction (in the IR). It is applied to accurate measurements of a stable CO(2) laser wavelength tuned to the centers of its various transitions. Measurements are done by simultaneous fringe counting and relative fringe-phase comparison at the two wavelengths using on-line data storage and processing with an electronic digital computer. The accuracy in the 10-microm region is several parts in 10(9) and is limited by correction for diffraction fringe shifts. Because of its low-Q and broadband operating characteristics, it can be applied to rapid accurate laser wavelength measurements over the entire wavelength range permitted by its transmitting optics. In the visible range where the diffraction correction is small, the interferometer can be used to perform measurements to within several parts in 10(11). The paper gives theoretical derivation of various diffraction corrections, the design and construction of the interferometer, the alignment procedures, detailed analysis of various error sources, and data processing. It also gives the details of a previously reported accurate measurement of the speed of light using the measured wavelength of the CO(2) R(14) line and its known frequency.