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Uncalibrated Helium-Neon Lasers in Length Metrology

Abstract: The vacuum wavelength of a gas laser cannot vary from its central value by more than a few parts in 106. Consequently, an uncertainty of this magnitude can be assigned for the wavelength of the laser even if it has not been calibrated and even if the laser is not frequency stabilized. When this uncertainty is satisfactory for a specific application, calibration of the laser's vacuum wavelength provides no apparent benefit and, from a logical standpoint, should not be required. The wavelength uncertainty of uncalibrated or unstabilized helium neon lasers has been carefully evaluated by an ad hoc subcommittee of the CCL (Consultative Committee for Length) of the International Committee on Weights and Measures (CIPM). The CCL has recommended that unstabilized or uncalibrated helium-neon lasers operating at 633 nm should be included in the new list of standard frequencies, “Recommended values of standard frequencies for applications including the practical realization of the metre and secondary representations of the second.” Among other functions, this new list replaces the “Mise en Pratique for the definition of the metre” as the authoritative document assigning accepted values and uncertainties for laser radiations of interest for length metrology. The adopted value for vacuum wavelength of an uncalibrated helium-neon laser is 632.990 8 nm, and the relative standard uncertainty is 1.5 × 10−6. This paper reviews the work of the CCL subcommittee, discusses the technical basis for assigning a wavelength and uncertainty to the 633 nm radiation, considers the need for documentary standards, and describes what additional steps are required to have confidence in measurements when the traceability of the basic length unit is provided by an uncalibrated laser. An uncalibrated laser can often be employed with full confidence for a measurement, but it is important to realize that the laser does not, by itself, guarantee successful measurement results; there are almost always additional steps required to verify measurement uncertainty, whether the laser is calibrated or not.