Improved Technology Leads to a First for Flying Spectrometers
Difference frequency generation provides alternative to nitrogen-cooled laser source.
Scientists at the National Center for Atmospheric Research in Boulder, Colo., have reported a breakthrough in airborne spectrometers that was achieved through the careful application of laser technology. The group has deployed the first airborne mid-IR difference frequency spectrometer system for highly sensitive measurements of formaldehyde, thanks in part to advances in the implementation of distributed feedback fiber lasers and nonlinear optical crystals.
In a novel aircraft-borne spectrometer, a mid-IR difference frequency generated (DFG) laser beam is sent through a multipass cell and into a detector, where the absorption resulting from trace atmospheric gases is detected. Courtesy of Dirk Richter, National Center for Atmospheric Research.
Team member Petter Weibring said that the goal of the work was to build a difference frequency laser source on which a modular instrument could be based to target various groups of atmospheric chemical species. “The atmospheric science goal is to understand hydrocarbon oxidation processes and their influence on HO radical formation and ozone chemistry.”
Information on hydrocarbon oxidation could help increase knowledge of the effect that industry, cars and power plants have on local ozone pollution, and an important intermediate product of the hydrocarbon oxidation process is formaldehyde. The difference frequency laser source would replace light sources based on liquid-nitrogen-cooled tunable diode lasers, enabling room temperature and more robust spectrometer operation.
High-sensitivity measurements require a good signal-to-noise ratio, which implies that the output power from the laser is well above the detector noise floor. Although this has been possible with difference frequency laser sources for a few years, optical noise originates from the many components of the laser setup. In surmounting this problem, the investigators applied optical noise cancellation techniques with a twist: They suppressed the common-mode optical noise using a computer, which gave more precise control over the signals.
Although that technique corrected the noise problems arising from the use of the laser outside a laboratory, there were other challenges that they had to overcome. A prop-driven aircraft, which is the type typically used for this work, is full of vibrations and subject to temperature and pressure swings. Management of the vibration and temperature was vital to making the spectrometer work in such an environment. The group — which also included Alan Fried, Dirk Richter and James G. Walega — redesigned the instrument, including the mechanical, electrical and optical systems.
Their laser system was based on difference about 3.5 μm, which they achieved by mixing a distributed feedback diode laser at 1562 nm and a distributed feedback fiber laser at 1083 nm in a periodically poled nonlinear optical crystal of lithium niobate. The instrument captured outside air in an optical multipass cell through which the beam passed 182 times, creating a 100-m absorption path and boosting the signal. A detector on the other side determined the absorbance and, from that, software derived the concentration of the chemical species being measured.
The researchers flew the instrument on three airborne missions in 2006, improving the system each time. In the end, advanced lock-in software, dual-beam optical noise subtraction, thermal control and active wavelength stabilization produced a spectrometer with a sensitivity for formaldehyde of about 20 parts per trillion by volume. This was after 30 s of signal integration time and was about half the performance of the system in the laboratory.
The results thus far are promising, Weibring noted, especially because the instrument did not require adjustment during the last aerial campaign. The device, however, is only the beginning, he said. “The goal in the next two years or so is to build a lightweight, fully autonomous instrument capable of measuring several species simultaneously.”
Optics Express, Oct. 17, 2007, pp. 13476-13495.
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