Laser Pulses Wirelessly Link Atomic Clocks

Laser pulses were transferred through open air with an unprecedented precision equivalent to the “ticking” of the world’s best next-generation atomic clocks. The demonstration showed how such devices at different locations could be linked wirelessly to improve satellite navigation, radar arrays, geodesy (altitude mapping) and the distribution of time and frequency information.

Operating frequency is one of the most important factors in the precision of optical atomic clocks, which have the potential to provide a hundredfold improvement in the accuracy of future time standards. To do this, the signals must be distributed with minimal loss of precision and accuracy.

Previously, clock signals of this type were transferred by fiber optic cables, but a wireless channel like the infrared laser device demonstrated by researchers at the National Institute of Standards and Technology (NIST) offers greater flexibility and the eventual possibility of transfer to and from satellites. Infrared light is very close to the optical frequencies used by these clocks, and both are much higher than the microwave frequencies in conventional atomic clocks that are currently used as national time standards.

NIST’s two-way wireless link demonstration was performed outside using two eye-safe, low-power laser frequency combs, which generated a steady stream of ultrashort optical pulses with a spacing that can be synchronized with the “ticks” of optical atomic clocks. In the experiment, the two combs were synchronized to the same stable optical cavity, which served as a stand-in for the clock. Each comb pulse was sent from one of two locations on NIST’s Boulder campus, reflected off a mirror on the nearby Kohler Mesa, and returned to the other site, traveling a total distance of two kilometers.

NIST researchers transferred ultraprecise time signals over the air between a laboratory on NIST’s campus in Boulder, Colo., and nearby Kohler Mesa. Signals were sent in both directions, reflected off a mirror on the mesa, and returned to the lab, a total distance of approximately 2 km. The two-way technique overcomes timing distortions on the signals from turbulence in the atmosphere, and shows how next-generation atomic clocks at different locations could be linked wirelessly to improve distribution of time and frequency information and other applications. Courtesy of Kelly Talbott, NIST.

Travel times for the transmitted pulses were measured. The cumulative timing differences and frequency instabilities were infinitesimal, just one-million-billionths of a second per hour, a performance level sufficient for transferring optical clock signals, the investigators say.

The transfer technique overcomes typical wireless signal problems such as turbulence in the atmosphere — the phenomenon that makes images shimmer when it’s very hot outside. Because turbulence affects both directions equally, it can be cancelled out. The method also can withstand signal losses because of the temporary obstruction of the light path.

The method should be able to operate at much longer distances, possibly even over future ground-to-satellite optical communication links as an added timing channel, researchers say.

The work, funded in part by DARPA, was described in Nature Photonics (doi: 10.1038/nphoton.2013.69).

For more information, visit: www.nist.gov