In order to realise the metre in a laboratory environment, NPL maintains and develops a number of frequency-stabilised lasers in the visible and near-infrared part of the spectrum. The frequency (vacuum wavelength) of these lasers has been measured by a number of standards laboratories worldwide, using femtosecond laser combs. These measurements relate the optical frequency directly to the realisation of the second, via a caesium clock. The mean frequency value of these lasers has been internationally agreed in the case of a limited number of commonly used laser systems. This list is the ‘mise en pratique’ of the Comité International des Poids et Mesures (CIPM) for the realisation of the metre.

Whilst cold trapped ions or atoms provide the most reproducible frequency references, the more mature technology relies on Doppler-free spectroscopic frequency references in gases or vapours contained in short cells. These are still the most convenient frequency references in cases where the required accuracy is no more than around 2 parts in 10¹¹ of the optical frequency. Gas (or vapour) cell based systems on the list include a number of HeNe laser lines and a neodymium-YAG laser referenced to iodine absorptions, a 778 nm laser referenced to a two-photon transition in rubidium and an absorption in acetylene at around 1542 nm.

NPL currently maintains a number of HeNe lasers iodine stabilised lasers at 633 nm and 543 nm. We have also developed diode laser standards at around 1542 nm, based on absorptions in acetylene.