A compact laser frequency comb has been developed that is no bigger than a shoebox. Until now, frequency combs usually have been bulky, delicate lab instruments — about the size of a suitcase — that are challenging to operate. The new technology is a step closer to user-friendly and, ultimately, chip-based combs that could enable new applications in astronomy, high-capacity telecommunications and — if other components are miniaturized as well — portable versions of the most advanced atomic clocks. The work was conducted at the National Institute of Standards and Technology (NIST). A stack of quartz optical “cavities” — precisely machined disks of solid quartz crystal — for use in NIST’s compact laser frequency comb. (Only one is actually used.) A low-power infrared laser produces light that travels in a loop inside one of the cavities. Each cavity is 2 mm wide and shaped like a flat ellipse. (Image: S. Papp/NIST) In the past decade, laser frequency combs — precision tools for measuring frequencies (or colors) of light — have helped propel advances in timekeeping, trace gas detection and related physics research. NIST’s prototype microcomb consists of a low-power semiconductor laser about the size of a shoebox and a high-quality optical cavity just 2 mm wide. A miniature laser such as those used in DVD players might be substituted in the future to squeeze the whole comb apparatus onto a microchip. NIST says it is the first to use a cavity made of fused silica, or quartz, the most common optical material. This means that the device could be integrated easily with other optical and photonic components, said researcher Scott Papp. The new compact version relies on a low-power laser and the cavity’s unusual properties. The cavity is designed to limit dispersion and to confine the light in a small space to enhance intensity and optical interactions. The infrared laser light travels in a loop inside the cavity, generating a train of very short pulses and a spectrum of additional shades of infrared light. The small cavity, with no moving parts, offers insight into basic processes of frequency combs, the large versions of which are difficult to observe. Among the features of NIST’s compact comb is the wide spacing between the teeth — 10 to 100 times wider than the gaps found in typical larger combs. This spacing allows scientists to more easily measure and manipulate the teeth. The widely spaced teeth can be individually read by astronomical instruments. The combs could thus be used as ultrastable frequency references in the search for Earthlike planets orbiting distant stars. Portable frequency combs could have other applications also, the researchers said. For example, because a frequency comb can simultaneously generate hundreds of telecommunication channels from a single low-power source, a microcomb eventually might replace the individual lasers now used for each channel in fiber optic telecommunications. “In the short term, we want to learn if this new type of comb can one day replace ultrafast laser-based combs used with NIST’s best atomic clocks. And if not, its small size will likely lead to other opportunities,” said project leader Scott Diddams. The work is described in the paper “Spectral and temporal characterization of a fused-quartz microresonator optical frequency comb,” which will be published in an upcoming issue of Physical Review A. For more information, visit: www.nist.gov