Optical frequency standards based on laser-cooled trapped ions or atoms have the potential for improved stability and reproducibility beyond those of the current generation of microwave standards. A femtosecond optical frequency comb provides the 'clockwork' required to operate such standards as optical atomic clocks, producing a highly stable microwave output at a readily countable frequency.

To measure an optical frequency using a femtosecond comb, the spacing of the comb modes is locked to a microwave standard and the offset of the comb is measured and stabilised using the self-referencing technique. In this way the frequency of every comb mode can be stabilised to a level which is limited by the stability of the microwave reference used for phase-locking the mode spacing. The frequency of an optical standard can then be measured by observing a beat with the nearest comb mode.

Frequency measurement mode of operation

For optical clock operation this procedure is reversed. The offset frequency is stabilised as before, but now a particular mode of the comb is locked to the optical standard. Since the offset is fixed, this has the effect of stabilising the comb mode spacing, which can be detected as an intermode beat.

Optical clock mode of operation

This second mode of operation produces two distinct (but phase-coherent) outputs:

1. A highly stable microwave frequency generated directly from a laser stabilised to an atomic transition, i.e. an 'optical clock'
2. A comb of optical frequency markers spanning the visible and near infra-red regions of the spectrum, in principle all with the same level of stability as the optical standard, i.e. an 'optical synthesizer'