Frequency combs measure greenhouse gases over a two-kilometre path

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Laser frequency combs have been used to monitor the concentrations of greenhouse gasses over a distance of two kilometres. The technique, which was demonstrated by the United States’ National Institute of Standards and Technology (NIST) in collaboration with the National Oceanic and Atmospheric Administration (NOAA), could be used in the future to support research on atmospheric gasses associated with climate change.

Frequency combs are laser-generated tools made up of a large number of very precisely defined frequencies that are evenly spaced. They can be compared to teeth on a pocket comb, with each ‘tooth’ representing an individual colour or frequency; which allows for very accurate measurements of the characteristic absorption signatures of different gas molecules of interest. 

In the recent demonstration, NIST's pair of laser frequency combs measured the simultaneous signatures of several greenhouse gases − including carbon dioxide, methane and water vapour − along a two-kilometre path between a NIST laboratory roof in Colorado, and a nearby mesa.

The team collected data continuously for three days under varied weather conditions, and the measurements obtained were very precise - for example, the carbon dioxide measurement, which was taken over five minutes, had an uncertainty level of less than one part per million. That's precise enough to ensure detection of small increases in trace gases due to large, distributed sources such as cities. Future systems should be able to achieve even better sensitivities over shorter timescales.

The researchers identified gases in the atmosphere by measuring the amount of comb light absorbed at different frequencies during its trip from the NIST lab roof to a mirror on the mesa and back to a detector in a lab. Because the optical frequencies are too high to be measured directly, they created two combs with slightly different spacing between the teeth. Mixing light from these dual frequency combs together creates a ‘beat’ frequency shifted down to the radio band, which is low enough to be measured. This was the first demonstration of the technique over long distances outdoors.

Presently, there are no portable sensors that can measure multiple gases at long range with consistent results. Remote sensing of atmospheric gases − from a satellite, for instance − can be performed with spectrometers, but while satellite instruments have global coverage, they sample specific regions on Earth infrequently. Therefore, regional measurements are made with ground-based point sensors that have a range that can be measured in metres, and which varies depending on wind speeds.

Although the NIST comb system was built to detect gases over two kilometres, in principle, the dual-comb technique could detect an even wider range of gases over many kilometres. Accuracy in the measured atmospheric transmission is assured by the well-defined position of each frequency comb tooth. Because the technique makes repeated measurements rapidly over the same path, it is immune to signal distortions caused by atmospheric turbulence. And, because the comb measurements can be averaged over the entire path length rather than relying on a few spot measurements, the comb method is better matched to the scale of atmospheric transport models.

The NIST researchers are currently working on optimising the comb system to improve sensitivity and expand spectral coverage to identify additional gases. According to the researchers, portable frequency comb systems could eventually support regional gas monitoring at costs comparable to point sensors, but over the kilometre scales relevant to many transport models and to monitoring of distributed sources such as large cities.

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