A pulsed distributed feedback quantum cascade laser (QCL) operating near 957 cm-1 was employed in wavelength
modulation mode for spectroscopic trace gas sensing applications. The laser was excited with short current pulses (5-10
ns) with < 2% duty cycle. The pulse amplitude was modulated with a linear sub-threshold current ramp at 20 Hz
resulting in a ~ 2.5 cm-1 frequency scan, which is typically wider than what has been reported for these lasers, and would
allow one to detect molecular absorption features with line widths up to 1 cm-1. A demodulation approach followed by
numerical filtering was utilized to improve the signal-to-noise ratio. We then superimposed a sine wave current
modulation at 10 kHz onto the 20 Hz current ramp. The resulting high frequency temperature modulation of the
distributed feedback (DFB) structure results in wavelength modulation (WM). The set-up was tested by recording
relatively weak absorption lines of carbon dioxide. We demonstrated a minimum detectable absorbance of 10-5 for this
spectrometer. Basic instrument performance and optimization of the experimental parameters for sensitivity
improvement are discussed.
We investigated the use of a pulsed, distributed feedback quantum cascade laser centered at 957 cm-1 in combination
with an astigmatic Herriot cell with 250 m path length for the detection of acrolein and acrylonitrile. These molecules
have been identified as hazardous air-pollutants because of their adverse health effects. The spectrometer utilizes the
intra-pulse method, where a linear frequency down-chirp, that is induced when a top-hat current pulse is applied to the
laser, is used for sweeping across the absorption line. Up to 450 ns long pulses were used for these measurements which
resulted in a spectral window of ~2.2 cm-1. A room temperature mercury-cadmium-telluride detector was used, resulting
in a completely cryogen free spectrometer. We demonstrated detection limits of ~3 ppb for acrylonitrile and ~6 ppb for
acrolein with ~10 s averaging time. Laser characterization and optimization of the operational parameters for sensitivity
improvement are discussed.
We investigated the use of a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm-1 in
combination with cavity ring-down spectroscopy (CRDS) and cavity enhanced spectroscopic (CES) techniques for the
detection of ethylene. In these techniques, the laser is coupled to a high-finesse cavity formed by high reflectivity
mirrors. In the CRDS application, the laser frequency was tuned at a rate of ~0.071 cm-1/K by changing the heat sink
temperature in the range between -20 and 50°C. For off-axis CES, the laser was excited with short current pulses (5-10
ns), and the pulse amplitude was modulated with an external current ramp which gave a frequency scan of ~0.3 cm-1. We
utilized a demodulation approach followed by numerical filtering to improve the signal-to-noise ratio. Basic instrument
performance and optimizations of the experimental parameters for sensitivity improvement are discussed. We
demonstrated a detection limit of ~130 ppb with CRDS and ~15 ppb with off-axis CES for ethylene.
We investigated the use of a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm-1 in
combination with an astigmatic Herriot cell with 150 m path length for the detection of ammonia and ethylene. This
spectrometer utilizes the intra-pulse method, where a linear frequency down-chirp, that is induced when a top-hat current
pulse is applied to the laser, is used for sweeping across the absorption line. This provides a real time display of the
spectral fingerprint of molecular gases, which can be a few wave numbers wide. A 200 ns long pulse was used for these
measurements which resulted in a spectral window of ~1.74 cm-1. A room temperature mercury-cadmium-telluride
detector was used, resulting in a completely cryogen free spectrometer. We demonstrated detection limits of ~3 ppb for
ammonia and ~5 ppb for ethylene with less than 10 s averaging time.
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