Project Members:
Nathan Newbury , Project Leader
Esther Baumann
Ian Coddington
Fabrizio Giorgetta
Bill Swann
Paul Williams

Summary
Optical frequency combs convert a laser source containing a single frequency of light into pulses that include thousands of frequencies. This project aims to develop frequency comb technology for a growing list of applications. Using a comb working at optical telecommunications wavelengths, project physicists have transmitted signals from next-generation optical atomic clocks across hundreds of kilometers. They have demonstrated how pairs of combs working together can increase by a factor of one hundred the speed of trace chemical analysis with significant enhancements in sensitivity. Frequency combs also promise to enable extremely accurate distance measurements and to assess the quality of high-speed telecommunications signals with unprecedented precision.

Description
In 2005, NIST scientist John L. Hall shared the 2005 Nobel Prize in physics for his part in the invention of the optical frequency comb. In the frequency domain, the output of a frequency comb is literally a comb of narrow, sub-Hz, optical lines while in the time domain, the output is a highly coherent pulse train with sub-femtosecond timing jitter. The original comb relied on a titanium-sapphire laser, but in the last five years, scientists in this project and elsewhere have built combs using the much less expensive commercial fiber-optic telecommunications components, and are exploring its promise to improve timekeeping, chemical analysis, precision ranging, and telecommunication diagnostics.

Working with the NIST Time and Frequency division, project scientists have demonstrated that fiber-laser based frequency combs can support measurements of the highest performance optical clocks at fractional frequency uncertainties below one part in 1017. At these uncertainties, current satellite transmission methods are inadequate; however, project scientists have demonstrated that it is possible to transmit these highly stable signals over 250 km of standard optical fiber, as a first step to a nationwide fiber-optic distribution of clock signals.

The project scientists have begun to explore other applications that could benefit from the uniquely coherent and broadband output of these comb sources. In the area of spectroscopy, they have measured the coherent optical response of a molecular gas over 15 THz of optical bandwidth. In laser ranging, they have demonstrated micron-level ranging at kilometer distances in millisecond timescales. This rapid absolute ranging could support large scale manufacturing or the future formation flying of satellite arrays. Finally, in the area of telecommunications, project scientists have demonstrated high-resolution measurements of high-speed phase-modulated waveforms. Such measurements could provide novel diagnostic capabilities in next-generation high-speed telecommunications.