Optical frequency combs—which combine the metrological virtues of white light and single-wavelength lasers by generating hundreds or thousands of tight, sharp, evenly spaced frequency peaks covering a wide swath of the spectrum—are increasingly used to calibrate electromagnetic radiation, refine time measurements, identify spectroscopic signatures, find exoplanets, and even define mass in quantum-mechanical terms.
Conventional optical frequency comb devices are generally table-top size, driven by high-powered femtosecond lasers, and take hours or even weeks to build around optical cavities. What's more, they must be fabricated in clean-room facilities using equipment that costs anywhere from US $1 million to $10 million.
In two recent papers, though, researchers at the National Institute of Standards and Technology in Boulder, Colo., report on the construction and performance of an optical frequency comb apparatus built around optical cavities machined from fused quartz using $10 000 worth of equipment, most of it for a carbon-dioxide laser. Making the cavity takes less than a minute. And in another minute, the researchers say, the cavity resonator can be teamed with a small, low-powered infrared laser to produce a usable optical frequency comb powered by a small, low-powered infrared laser.
In Physical Review X (posted on ArXiv) and Applied Physics Letters (also posted on ArXiv), NIST researchers Pascal Del'Haye, Scott Diddams, and Scott Papp report that the one-minute optical cavities have “ultra-high” quality factors (Q=109), which equal those of more laboriously constructed resonators. The lenticular quartz-glass “whispering galleries” have been made with diameters ranging from 0.17 mm to 8 mm (see image for four cavities, indicated by arrows, machined from a single rod), producing free spectral ranges between 390 GHz and 8 GHz. In one set of tests, the spacing between the frequency peaks was uniform to within 5 x 10-15 over periods as long as 1 second—consistent, that is, to within five parts per quadrillion, which is comparable to measuring the 4.37 light years between us and Alpha Centauri with an error of less than 207 meters.
The prospect of compact, portable optical frequency combs opens the door to more precise, more convenient, and more widely used calibration and measurement in the field. To see a laser lathe turn a glass rod into an ultra-high Q resonator in 60 seconds, look at Scott Papp’s video.