Monitoring of ethylene by a pulsed quantum cascade laser

We report on the development and performance of a gas sensor based on a quantum cascade laser operating at a wavelength of ∼10 μm to measure ethylene (C_2H_4) concentrations by use of a rotational component of the fundamental ν_7 band. The laser is thermoelectrically cooled and operates in a pulsed mode. The influence of pulse-to-pulse fluctuations is minimized by use of a reference beam and a single detector with time discriminating electronics. Gas absorption is recorded in a 100-m optical path-length astigmatic Herriott cell. With a 10-kHz pulse repetition rate and an 80-s total acquisition time, a noise equivalent sensitivity of 30 parts per billion has been demonstrated. The sensor has been applied to monitor C_2H_4 in vehicle exhaust as well as in air collected in a high-traffic urban tunnel.