Electronic Components Speed Spectroscopy

A new solution employing electronic components could make atmospheric-gas measurement more even and 1000 times faster than with conventional techniques and could advance greenhouse-gas measurement.

Searching for unusual gas mixed in the air is called trace gas sensing; the most common method for detecting trace gases is spectroscopy, which can identify gas molecules by the patterns of dark lines in a spectrum.

Accurately measuring these low-concentration gases requires a lot of light, generated by a laser that can be tuned to different colors. Until now, tuning a laser to shine in a wide enough range of colors typically required a mechanical device to change the frequency, but all of the methods available adjust the laser too slowly to obtain meaningful snapshots of the turbulent atmosphere.

Now, scientists at the National Institute of Standards and Technology (NIST) have overcome an issue preventing the effective use of lasers to rapidly scan samples. They combined an electro-optic modulator and an optical resonator to alter the laser so that its light shines in a number of different frequencies, and then to filter these frequencies so that the laser shines in only one color at any given instant.

Fast, accurate spectroscopy scans of the atmosphere across a wide range of light frequencies could improve the performance of greenhouse-gas measurement devices. This artist’s conception compares current technology, which functions slowly and unevenly, with the NIST team’s improvement, which changes the scanning laser’s frequency evenly and more than 1000 times faster, permitting full-spectrum scans within a few milliseconds. Courtesy of Talbott, Gerskovic/NIST.

The method permits a full spectrum of frequencies to pass through a gas sample in a few milliseconds or less, providing a clearer and more accurate resulting spectrum than the previous “slow scan” methods could.

“One of the major goals in climate science is to combine a wide variety of high-accuracy atmospheric measurements, including ground-based, aircraft and satellite missions, in order to fully understand the carbon cycle,” said David Long, a scientist in NIST’s Chemical Sciences Division. “The technology we've developed is general enough to be applicable for each of these platforms. The high speed of the technique allows for very accurate measurements of atmospheric gases at rates which are faster than atmospheric changes in temperature and pressure due to turbulence.”

The technique was used in a controlled laboratory environment using a small sample chamber for ground-based measurements, but the researchers say that if the scanner is mounted on a vehicle, an aircraft or a satellite, the method could work also at great distances.

The technique could be used to search for hidden explosives and to monitor chemical processes in industry and the environment, the investigators said. They have applied for a patent.

Findings were reported in Nature Photonics (doi: 10.1038/nphoton.2013.98).

For more information, visit: www.nist.gov