Absolute gravimeters: status report on the use of iodine-stabilized He-Ne lasers at λ ≈ 633 nm

Abstract

In the error budget for absolute measurements of g, one of the most important contributions is the uncertainty of the frequency of the stabilized laser used as the reference wavelength. This was confirmed during the third International Comparison of Absolute Gravimeters held at the BIPM in 1989, at which the frequencies of all the lasers used in the gravimeters were checked by beat frequency against a BIPM reference laser. This problem was solved by the replacement of a He-Ne laser at λ ≈ 633 nm stabilized by the requirement that the intensity of two orthogonal modes be equal by a laser stabilized with respect to the saturated absorption of iodine at the same wavelength. A new design of portable laser, realized at the BIPM and developed by commercial companies, is described and the capabilities of this laser for use as a primary standard of wavelength or reference wavelength in a new generation of absolute gravimeters are presented. The results of the frequency calibration of the lasers used in the absolute gravimeters which participated in the fourth International Comparison of Absolute Gravimeters held at the BIPM in May and June 1994 are also presented.