Comb technology enables metrologists to throw away the complex frequency "chains" and to relate microwave frequencies to optical frequencies in one step. Combs are actively being investigated in a number of national metrology institutes and at the BIPM, for stability, accuracy and any unsuspected systematic errors. In the meantime they enable frequency measurements that are traceable to the SI second to be made quite quickly on optical and infrared laser sources. There is also the interesting possibility that a combination of super-stable optical frequency sources and comb generators may well enable us to redefine the second in terms of an optical rather than a microwave radiation. Whilst we could still then live with the existing definition of the metre, the future for the next generation of time standards, with applications to navigation and to the ever-growing need for precise time measurement, is an intriguing and exciting one.

Very early on, the BIPM realized the potential that femtosecond comb techniques offer to metrology and it was among the first laboratories to construct a comb system. In these, pulsed laser sources have their pulse repetition frequency locked to that of an atomic clock frequency standard. The output pulse of the femtosecond laser is launched into a non-linear optical fibre and, as a result, a single frequency pulsed source is transformed into a "comb" of equally spread, very narrow light sources that cover almost all the visible spectrum.

Standard spectroscopic and interferometric techniques then enable other, conventionally stabilized, laser sources to be compared against a selected "tooth" of the comb that has an absolute frequency traceable to the SI definition of the second.

In a relatively straightforward measurement it is now possible to monitor the accuracy of the BIPM's 633 nm-I₂ He-Ne systems used to realize the metre and to provide international reference systems as well as to direct, absolute frequency measurements on a range of optical and near-infrared systems.

The BIPM's current programme therefore concentrates on verifying the performance and characteristics of comb systems. Already we have constructed two combs, one of which is designed to be portable, and which can be used simultaneously to measure the absolute frequency of a stabilized laser. The immediate aim of this project is to investigate any systematic differences between the two comb systems and so to set limits on their capabilities for accurate absolute frequency measurement. Our aim is to construct a second portable comb and to offer a comb comparison service to NMIs.