Wavelength modulation spectroscopy with a pulsed quantum cascade laser for the sensitive detection of acrylonitrile

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 similar to 2.5 cm(-1) frequency scan. This allowed the measurement of spectroscopic features of acrylonitrile with absorption line widths of similar to 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 similar to 10(-5), corresponding to the sub 10(9) levels of acrylonitrile, was achieved with less than a minute averaging time. (C) 2011 Optical Society of America