The new clock architecture might eventually be used to make portable tools for calibrating frequencies of advanced telecommunications systems or providing microwave signals to boost stability and resolution in radar, navigation and scientific instruments. The technology also has potential to combine good timekeeping precision with very small size. The comb clock might be a component of future "NIST on a chip" technologies offering multiple measurement methods and standards in a portable form.
"The microcomb clock is one way we might get precision frequency metrology tools out of the lab and into real-world settings," NIST physicist Scott Diddams says.
Frequency combs produce precisely defined colors, or frequencies, of light that are evenly spaced throughout the comb's range. (The name comes from the spectrum's resemblance to the teeth of a pocket comb.) The original combs required relatively large lasers that produced rapid, extremely short pulses of light, but more recently NIST and other laboratories have developed much smaller microcombs (www.nist.gov/pml/div688/comb-102511.cfm).
A microcomb generates its set of frequencies from light that gets trapped in the periphery of a tiny silica glass disk, looping around and around the perimeter. These combs can be astonishingly stable. NIST has an ongoing collaboration in this area with Caltech researchers, who made the 2-millimeter-wide silica disk that generates the frequency comb for the new clock.
The microcomb clock uses a laser to excite the Caltech disk to generate a frequency comb, broadens the spectrum using nonlinear fiber, and stabilizes two comb teeth (individual frequencies) to energy transitions in rubidium atoms that "tick" at optical frequencies. (Conventional rubidium atomic clocks operate at much lower microwave frequencies.) The comb converts these optical frequency ticks to the microwave domain.