Abstract

A pulsed, distributed feedback (DFB) quantum cascade laser centered at 957cm1 was used in combination with a wavelength modulation spectroscopic technique for the detection of acrylonitrile. The laser was excited with short current pulses (510ns), and the pulse amplitude was modulated with a linear subthreshold current ramp at 20Hz resulting in a 2.5cm1 frequency scan. This allowed the measurement of spectroscopic features of acrylonitrile with absorption line widths of 1cm1. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We then superimposed a 10kHz sine wave current modulation on top of the 20Hz current ramp. The resulting high frequency temperature modulation of the DFB structure results in wavelength modulation. A minimum detectable absorbance of 105, corresponding to the sub 109 levels of acrylonitrile, was achieved with less than a minute averaging time.

© 2011 Optical Society of America

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