The simplicity of optical frequency metrology based on optical frequency combs can only be appreciated in comparison to techniques used prior to their development. Briefly, these techniques consisted of frequency multiplication chains, where each link in the chain consisted of an oscillator that had a multiple of the frequency of the previous link. The first link in the chain was a cesium clock, a kind of atomic clock used as the international time standard that defines the second. The cesium clock is based on nine-gigahertz microwaves absorbed by cesium atoms. To reach all the way from nine gigahertz to the frequency of visible light (a factor of at least 40,000) required about a dozen stages. Each stage used a different technology, including lasers for visible light. Running these chains was resource- and personnel-intensive; just a few in the world were built, and measurements were made only intermittently. In addition, in practice the many links in the chain impaired the accuracy of the ultimate optical frequency measurement.
Once stabilized optical frequency combs were invented, it was much easier to precisely measure the frequency of a CW laser. As with a frequency chain, comb-based frequency measurements still must be referenced to a cesium clock. As we will now see, a cesium clock’s ability to measure frequencies up to about nine gigahertz is all that you need to use an optical comb to determine the frequency of a laser line. Several pieces of information involving the comb are needed. First, as we discussed earlier, the comb’s offset frequency and the spacing of its lines must be measured. From those two numbers the frequencies of all the comb’s lines can be calculated. Next, the unknown laser light is combined with the comb’s light to get the beat frequency (that is, the difference in frequency) between it and the nearest comb line.
These three frequencies are all within the microwave range that can be measured extremely accurately using a cesium clock. Recall that the comb’s line spacing is the same as the repetition rate of the pulses producing the comb. Most mode-locked lasers operate at a repetition rate of 10 gigahertz or less, making that quantity easy to measure against the cesium clock. Both the offset frequency and the beat frequency are also within range to be measured by the cesium clock because they must be smaller than the comb spacing.
Two further pieces of data must be determined: to which comb line was the unknown laser light closest and on which side of the line? Commercial wave meters can measure an optical line’s frequency to within less than one gigahertz, which is good enough to answer those two questions. In the absence of such a wave meter, you can systematically vary the repetition rate and the offset frequency to monitor how the beat frequency changes in response. With enough of those data points, you can work out where the line must be.
The simplicity of optical combs has not only increased how often scientists around the world make these extremely precise frequency measurements but also greatly decreased the uncertainty in those measurements. Such benefits may one day lead to an optical time standard replacing the present microwave cesium-based one. With this in mind, groups at NIST led by James C. Bergquist and at JILA led by Ye have been measuring frequencies relative to clocks that use light and a comb to produce the output signal. Already the uncertainties in measurements using the best of these clocks are smaller than those in measurements using the very best cesium standards. It is an exciting time, with many laboratories around the world poised to build optical frequency standards that can surpass what has been the primary frequency standard for many decades. Measurements by Leo Hollberg’s group at NIST, as well as by other groups elsewhere, suggest that the intrinsic limit of the optical comb is still a couple of orders of magnitude better than the uncertainty in current optical frequency measurements.