Compact laser frequency comb could go places

Ashley N. Paddock, ashley.paddock@photonics.com

A new compact laser frequency comb is no larger than a shoebox and has a high-quality optical cavity just 2 mm wide. Until now, frequency combs have been delicate lab instruments, bulky – about the size of a suitcase – and challenging to operate. The new microcombs developed at the National Institute of Standards and Technology are a step closer to user-friendly 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. In the past decade, laser frequency combs have helped propel advances in timekeeping, trace gas detection and related physics research.

A stack of 10 optical microresonators made from a solid rod of fused quartz glass 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. A detail of the rod is shown on opposite page. Courtesy of S. Papp, NIST.

The prototype frequency comb consists of a low-power semiconductor laser about the size of a shoebox and a high-quality optical cavity just 2 mm wide. 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: It is designed to limit dispersion and to confine light in a small space to enhance intensity and optical interactions. 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 the new 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 measure and manipulate the teeth more easily. 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.

“So far, our compact comb works in the telecom band near 1550 nm,” Papp said. “But to take advantage of the world’s most precise optical atomic clocks at NIST, we will have to increase the comb’s spectral range by a factor of 10.”

To do this, Papp said his team will have to learn more about the mechanisms that control broad-bandwidth generation of the microcomb spectrum. In addition, they will have to understand and implement appropriate means to interface the device with ultrastable atomic clock signals.

“Both of these steps are critical for microcombs to be used as the ‘gears’ of next-generation optical clocks,” Papp said.