Researchers from the National Institute of Standards and Technology (NIST; Boulder, CO) and California Institute of Technology (Caltech; Pasadena, CA) have reported the first all-optical control of a silicon-chip-based microresonator comb optical clock in which an optical frequency reference is converted to a microwave signal.1 (Optical frequencies are too high to count; microwave frequencies can be counted with electronics.)
A 2-mm-diameter silica whispering-gallery disk produces a frequency comb that is broadened in a nonlinear fiber, resulting in a 25 THz span. Two "teeth" 108 modes apart in the comb are stabilized to rubidium frequency references separated by 3.5 THz. The center of the comb spectrum is locked to an infrared laser operating at 1560 nm.
The comb-on-a-chip technology was developed at NIST; the microresonator disk was created at Caltech.
The output is 100 times more stable than the intrinsic timekeeping of the rubidium atoms. "A simple analogy is that of a mechanical clock: The rubidium atoms provide stable oscillations—a pendulum—and the microcomb is like a set of gears that synthesizes optical and microwave frequencies," says NIST physicist Scott Diddams.
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.
Future 'NIST on a chip'
NIST researchers have not yet systematically analyzed the microcomb clock's precision. The prototype uses a tabletop-sized rubidium reference. The scientists expect to reduce the instrument size by switching to a miniature container of atoms like that used in NIST's original chip-scale atomic clock. Scientists also hope to find a more stable atomic reference.
The microcomb chip was made by use of conventional semiconductor fabrication techniques and, therefore, could be mass produced and integrated with other chip-scale components such as lasers and atomic references.
1. S. B. Papp et al., Optica 1, 10-14, July 22, 2014; http://dx.doi.org/10.1364/OPTICA.1.000010