Optical-frequency comb promises sophisticated portable instruments

Optical-frequency comb promises sophisticated portable instruments

High-frequency combs have made possible some of the most advanced benchtop instrumentation available today (see for example, “Nonlinear VNA goes to 67 GHz,” www.electronicproducts.com/N-article-tmrc03_nov2011-html.aspx). Now, researchers Scott Diddams, physicist, and Scott Papp, NIST affiliate, at the National Institute of Standards and Technology have developed a compact laser frequency comb that may lead to chip-based combs for use in new applications in astronomy and high-capacity telecommunications, as well as the most advanced portable atomic clocks.

The current prototype microcomb consists of a low-power semiconductor laser roughly half a cubic foot in size and a 2-mm-wide high-quality optical cavity. The researchers suggest that by substituting a miniature laser like those in DVD players for the current one, it would be possible to fabricate the entire comb apparatus on a single IC.

Precise maching creates a stack of 2-mm-wide quartz optical cavities for use in NIST’s compact laser frequency comb. (Only one is actually used.) A low-power laser produces IR waves that travels in a loop inside the cavities. (Photo: S. Papp/NIST)

While a typical optical frequency comb uses a high-power, ultrafast laser, NIST’s compact version relies on a low-power laser and the unusual properties of it’s silica (quartz) optical cavity. The cavity is designed to limit light dispersion and confine the light in a small space, which increases intensity and optical interactions. The IR laser light travels in a loop inside the cavity, generating a train of very short pulses and a spectrum of additional shades of IR light. Further, the static cavity offers insight into basic processes of frequency combs which large versions, with their fast moving components, make difficult to observe in.

Compared to the compact frequency combs developed recently by other research groups, the NIST appratus’s use of a silica cavity makes it easier to integrate with other optical and photonic components, according to NIST’s Scott Papp says. Other beneficial features of the comb include its wide spacing between the teeth — 10 to 100 times wider than that found in typical larger combs. This spacing lets scientists more easily measure and manipulate the teeth.

Project leader Scott Diddams notes that the widely spaced teeth can be individually read by astronomical instruments, so portable frequency combs could thus be used as ultrastable frequency references in the search for Earth-like planets orbiting distant stars. And, 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 telecommunications.

“We 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.” For further information, call Scott Diddams at 303-497-7459 or e-mail s
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Richard Comerford