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: National Institute of Standards and Technology
frequency combs—extraordinarily precise tools for measuring frequencies
(or colors) of light—have helped propel advances in timekeeping, trace
gas detection and related physics research to new heights in the past
decade. While typical lasers operate at only a single or handful of
frequencies, laser frequency combs operate simultaneously at many
frequencies, approaching a million for some combs. These combs have very
fine, evenly spaced “teeth,” each a specific frequency, which can be
used like hash marks on a ruler to measure the light emitted by lasers,
atoms, stars or other objects. But frequency combs are usually bulky,
delicate lab instruments—about the size of a typical suitcase—and
challenging to operate, which limits their use.
researchers at the National Institute of Standards and Technology
(NIST) have developed a compact laser frequency comb, a step toward
user-friendly and ultimately chip-based combs that could enable new
applications in astronomical searches for Earth-like planets,
high-capacity telecommunications, and—if other components are
miniaturized as well—portable versions of the most advanced atomic
clocks. Large frequency combs are best known as the “gears” in today’s
room-sized versions of these clocks.
prototype micro-comb 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 like those in DVD players might be substituted in the
future to squeeze the whole comb apparatus onto a microchip.
frequency combs have been developed recently by a number of other
research groups, but NIST’s is the first to use a cavity made of fused
silica, or quartz, the most common optical material. This means it could
be integrated easily with other optical and photonic components, lead
author Scott Papp says.
full-size frequency comb uses a high-power, ultrafast laser. By
contrast, the new compact version relies on a low-power laser and the
cavity’s unusual properties. The cavity is designed to limit light
dispersion and 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,
which are difficult to observe in large versions.
its desirable features, NIST’s compact comb has wide spacing between
the teeth—10 to 100 times wider than that found in typical larger combs.
This spacing allows scientists to more easily measure and manipulate
the teeth. Of particular interest to project leader Scott Diddams, the
widely spaced teeth can be individually read by astronomical
instruments. Portable frequency combs can thus be used as ultrastable
frequency references in the search for Earth-like planets orbiting
distant stars. Portable frequency combs can also have many other
important applications. For example, because a frequency comb can
simultaneously generate hundreds of telecommunication channels from a
single low-power source, a micro-comb might eventually replace
individual lasers now used for each channel in fiber-optic
hope this is just the beginning and look forward to bigger and better
developments,” Diddams says. “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.”
The research was supported in part by the Defense Advanced Research Projects Agency.