Abstract
A pulsed, distributed feedback (DFB) quantum cascade laser centered at 957 cm−1 was used in combination with a wavelength modulation spectroscopic technique for the detection of acrylonitrile. The laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with a linear subthreshold current ramp at 20 Hz resulting in a ∼2.5 cm−1 frequency scan. This allowed the measurement of spectroscopic features of acrylonitrile with absorption line widths of ∼1 cm−1. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We then superimposed a 10 kHz sine wave current modulation on top of the 20 Hz current ramp. The resulting high frequency temperature modulation of the DFB structure results in wavelength modulation. A minimum detectable absorbance of ∼10−5, corresponding to the sub 109 levels of acrylonitrile, was achieved with less than a minute averaging time.
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