The absolute measurement of any frequency must be based on the SI unit of time, the second. The frequency of an optical frequency standard might typically be around 500 THz. To measure the frequency of such a standard relative to the caesium primary frequency standard it is therefore necessary to compare frequencies which differ by nearly five orders of magnitude.

Spectrum from a femtosecond optical frequency comb

Traditionally this was a complex task using harmonic frequency multiplication chains designed specifically to link one particular optical frequency to the caesium primary standard. However, the development of octave-spanning optical frequency combs based on Kerr-lens mode-locked femtosecond lasers has led to a huge simplification in optical frequency metrology, and it is now possible for the link between the optical and the microwave to be made in a single step.

At NPL we have three femtosecond combs. Two of these are based on mode-locked Ti:sapphire lasers operating at repetition rates of 90 MHz and 800 MHz. The third is a transportable fibre-based optical frequency comb. These combs can be used to measure the absolute frequency of optical standards relative to the caesium primary standard with accuracy limited only by the performance of the standards themselves. The absolute frequency measurements we have carried out to date include measurements of our strontium and ytterbium ion optical frequency standards, iodine-stabilised helium-neon lasers and our acetylene optical frequency standards.

Optical frequency standards have the potential for improved stability and reproducibility beyond those of the current generation of microwave standards. The femtosecond combs can also be used to operate these standards as optical clocks, providing a microwave output for direct comparison with the caesium primary microwave standard.

In addition to our work on femtosecond optical frequency combs, we have also developed passive optical frequency combs based on electro-optic modulators. These can be used to measure frequency intervals of up to several THz, for example in the 1.5 micron optical communications band.

Recently, NPL led a study commissioned by the European Space Agency to look at the potential uses of femtosecond optical frequency combs for space-borne optical frequency metrology.