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From the early days of stabilized lasers, the BIPM has offered Member States of the Metre Convention the opportunity to compare their laser standards against reference systems maintained at the BIPM. This long-standing service based on heterodyne beat-frequency measurements largely concentrated on two types of stabilized lasers:
- iodine-stabilized He-Ne systems operating at wavelengths of 515 nm, 532 nm, 543 nm, 612 nm, or (most commonly) 633 nm;
- a methane-stabilized He-Ne laser operating at 3.39 µm.
Recommended wavelength values for these systems are contained in the Mise-en-Pratique adopted, and regularly updated, by the CIPM. This document contains the internationally agreed values to be used for a wide range of light sources as well as optimum conditions for their operation.
In the early days of stabilized laser systems, it was almost always necessary for lasers to be brought to the BIPM for the measurements to be made. This was not always convenient, so the BIPM developed small, easily transported but highly stable and accurate laser systems. As a result, the reference values maintained by the BIPM could be realized away from Sèvres. In these "remote" comparisons, it became relatively easy for a number of "regional" laboratories to bring their lasers for a joint comparison. This had the additional benefit of enabling a number of smaller national laboratories that may not have had the resources to bring their equipment to Sèvres, to obtain direct access to the metre as maintained at the BIPM.
In addition to its substantial programme related to the He-Ne stabilized lasers, the BIPM also carried out a small research programme into the performance and metrological qualities of the frequency-doubled Nd-YAG laser at 532 nm. This relatively high-power system turned out to have excellent short-term stability and it is often used in a number of applications. The BIPM's comparison programme therefore included Nd-YAG systems by heterodyne and, more recently, by absolute frequency measurements.
The 3.39 µm laser programme dealt with a well-characterized system that was a critical element in the frequency chains used in the earlier measurements of the speed of light. They also have applications in infrared spectroscopy. The BIPM has, therefore, maintained a high performance system and participated in a number of comparisons with several NMIs. A similar facility was provided for 778 nm Rb-stabilized systems which were of interest to the telecommunications industry. Both programs are now drawing to a close in the light of frequency comb technique.
With the introduction of the new comb techniques allowing direct frequency measurements of optical laser frequencies, the activity of heterodyne frequency comparisons between laser standards has been reduced. As a natural continuation of the long-standing comparison programme at the BIPM a service of laser standard calibrations is now operational, organized in two measurement campaigns per year for the Member States of the Metre Convention.
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Summary
