Off-axis cavity enhanced spectroscopy based on a pulsed quantum cascade laser for sensitive detection of ammonia and ethylene.

A pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm(-1) was used in combination with an off-axis cavity enhanced absorption (CEA) spectroscopic technique for the detection of ammonia and ethylene. Here, the laser is coupled into a high-finesse cavity with an optical path length of ∼76 m. The cavity is installed into a 53 cm long sample cell with a volume of 0.12 L. The laser is excited with short current pulses (5-10 ns), and the pulse amplitude is modulated with an external current ramp, resulting in a ∼0.3 cm(-1) frequency scan. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We demonstrated detection limits of ~15 ppb and ∼20 ppb for ammonia and ethylene, respectively, with less than 5 s averaging time.