Sensing trace gases with quantum cascade lasers

Quantum cascade lasers have dramatically affected the field of trace-gas sensing, increasing the sensitivity of detectors that are becoming smaller and more convenient almost monthly. Sensitive photodetection has been reported by photoacoustic spectroscopy and quartz-enhanced photoacoustic spectroscopy. Quantum cascade lasers applied to cavity ringdown spectroscopy led to detection limits of 0.7 ppb of NO in N2. Integration of these laser sources into cavity output spectroscopy yielded detection of less than 1 ppb of NO in human breath samples. Quantum cascade lasers have also been applied in gas sensing by transmission spectroscopy. Perhaps the newest development in the field concerns the use of hollow waveguides coupled to quantum cascade lasers for gas sensing. Scientists in Austria detected ethylyne gas at concentration levels of 250 ppm. A collaboration of researchers in Tucson, Arizona and Denver, Colorado demonstrated hollow waveguide quantum cascade laser sensing with a 1-mm-inner-diameter waveguide. More recently, a team in Atlanta, Georgia demonstrated gas sensing with a quantum cascade laser coupled to a silica hollow waveguide. All these three gas-sensing systems are routinely capable of measuring concentrations in the range of parts per million to parts per billion.