Researchers at the National Center for Atmospheric Research (NCAR; Boulder, CO), Earth Observing Laboratory have developed an ultrasensitive laser-based trace-gas-absorption sensor for airborne atmospheric chemistry research based on mid-IR absorption spectroscopy. It uses an optical-fiber-pumped difference-frequency generation (DFG) tunable laser source operating at a wavelength of 3.5 µm and consists of commercially available fiber-optic components, diode and fiber lasers, and optical-fiber amplifiers that pump a 5-cm-long periodically poled lithium niobate (PPLN) crystal to generate several hundred microwatts of tunable mid-IR radiation.
The DFG spectrometer was deployed in three consecutive airborne field campaigns in which heavy vibration, shock, temperature changes of more than 15°C, and aircraft-cabin temperatures in excess of 40°C were experienced. Nonetheless, very sensitive measurements of formaldehyde (an important intermediate constituent in the production of atmospheric ozone) were achieved. The design can also be expanded to other molecular species, limited only by the transparency range of the conversion crystal (about 2 to 5 µm for PPLN). Advances in quasi-phase-matched gallium arsenide crystals may extend the wavelength coverage of DFG laser sources to 16 µm. Contact Dirk Richter at [email protected] or Petter Weibring at [email protected].
