Until recently, the absolute connection between microwave and optical frequencies, a range of four to five decades, required chains of phase-locked oscillators, many of them complex lasers. The equipment often filled rooms and would not function for very long periods. Very skilled people were needed to build and run them. Also, only a very small range of optical frequencies could be covered without having to redesign the whole chain.
The technique of using regularly pulsed lasers to generate a comb of optical frequencies has been known for a long time. It follows from Fourier analysis that a uniform series of pulses in time spaced by τ leads to a uniform series of spectral lines in the frequency domain spaced by frequency 1/τ. However, unless there is something that constrains the optical phase to be the same from pulse to pulse, the origin of the spectral lines is offset from zero by a frequency Δɸ/2πτ , where Δɸ is the change in phase from pulse to pulse in radians. Therefore, all the comb frequencies are offset by the same amount, which is in general unknown.
What is new is that a technique has been invented that allows the measurement and control of this offset frequency so that the absolute frequencies of the comb can be determined in terms of the pulse frequency, 1/τ, and the comb offset frequency. If these are measured or determined by, for example, a microwave frequency standard, the optical frequencies are known with the same accuracy (already tested to about ±1 × 10−17). By locking one of the comb lines to an optical frequency standard, a microwave frequency is generated that has the same accuracy as the optical standard.
The technique for measuring and controlling the offset frequency involves broadening the comb spectrum with a nonlinear optical fiber so that it extends over a two-to-one frequency range, an octave. A group of lines from the low-frequency end of the comb is frequency doubled and frequency subtracted from a group of lines at twice the frequency of the first group. The result can be shown to contain a signal at the comb-offset frequency, thus allowing its frequency to be measured or controlled.
The system can easily handle any optical frequency in the range of the comb. It is a simple and accurate way of connecting the microwave and optical frequency ranges. A number of laboratories in the United States, including NIST, are already building such combs, which are critical to realizing the performance capabilities of the new optical frequency standards as well as to measuring their frequencies.