high power of the OPO allow vibrationaltransitions to be excited into saturation(Figure 4) where approximately 50 percentof the molecules are in a single quantumstate. The wide tunability of the OPOmakes it a highly versatile source for thisstudy, and the researchers anticipate usingit in the future to extend their studies to avariety of molecular species and absorption features.
Applications in chemical sensing
Professor Frans Harren leads the LifeScience Trace Gas Facility at RadboudUniversity Nijmegen in the Netherlands.The group has developed state-of-the-artlaser-based trace gas detectors that areused for research in areas from plant physiology to medicine and human health. Harren and his colleagues use a variety oflasers combination with sensitive detectionschemes, including photoacoustic spectroscopy and cavity ring-down spectroscopy.
The team’s recent experiments havechallenged the findings of a publishedstudy that asserted that emission of significant amounts of methane from plants mayplay a significant role in the global atmospheric concentration of this powerfulgreenhouse gas.
By growing plant samples in the presence of carbon dioxide molecules based onthe stable isotope of carbon 13C, anymethane emitted by the plants could beidentified by the presence of this isotope inthe methane molecule. Researchers performed trace methane detection with thephotoacoustic technique using a singly resonant CW OPO pumped by a diode-pumped solid-state laser operating at approximately 3. 24 μm (Figure 5). TheOPO’s high power level allowed a 13CH4detection limit of three parts per billion tobe attained using the OPO spectrometer.This is well below the naturally occurringlevel of 13CH4 in the atmosphere. Monitoring the 13CH4 level showed that there wasnot a statistically significant level ofmethane emission from the plants, contradicting the original published finding.
It appears likely that, with time, newtypes of semiconductor lasers may emergeas the lowest cost pathway for laser emission in the 3- to 4-μm region. However,the high power, wide tuning and robustphysical characteristics of OPOs, especially those pumped by fiber lasers, makethese sources important spectroscopictools in today’s research laboratories.
These same attributes also make OPOspromising for in-the-field applications inindustry, medicine and sensing.
Meet the author
Angus Henderson is a principal scientist atLockheed Martin Aculight. He can be reachedat firstname.lastname@example.org.
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