Quantum Cascade Infrared Laser Spectroscopy for Real-Time Cigarette Smoke Analysis

Non-symmetrical line broadening effects using short-pulse QCL spectrometers as determined with sub-nanosecond time-resolution

“…After the sign of the slope was reversed, i.e. decreasing laser frequency and intensity during the sweep, the shadow was observed at higher channels (see also [9,11]). A detailed analysis of the shift of a selected spectral position reveals an uncertainty of at least 8-10 channels.…”

“…Firstly, although Nelson et al worked at low pressure (4 Torr) no rapid passage signatures were found. Secondly, the asymmetric absorption lines in the inter pulse spectra exhibited both a high-frequency tail [9,11] and a low-frequency tail [12]. If just a single frequency-down chirped QCL pulse is considered, the rapid passage structures are always observed after the absorption feature, i.e.…”

“…Extensions originally developed for TDLAS, e.g., the sweep integration method [7,8], were adapted to pulsed QCL spectrometers [9] and enable a time-resolution of milliseconds. Employing this method is not only of interest for on-line trace gas measurements, on which most of the experiments carried out so far were focussing [5,6,[9][10][11][12][13][14][15][16][17], but also for monitoring species in plasma diagnostics [18][19][20]. Later, the inherent frequency-down chirp of the lasers was exploited by using long pulses to acquire absorption spectra during the pulse [21,22].…”

“…On the other hand, increased pulse widths lead to a chirp-limited resolution (70 ns, 2.79 GHz [6]). Furthermore, asymmetric line shapes of absorption features have been detected in several studies employing both the short-pulse [9,11,24] and the long-pulse mode [25]. Additionally, the latter method suffers from non-linear absorption effects [26].…”

“…Another approach abstracts correction factors by comparing measurements and simulated spectra [e.g., 6, 9, 13]. Only in a few studies the results were validated with independent counter-measurements by means of TDLs [9,11] or cw QCLs [12,29]. Recently, an intercomparison between the short-and long-pulse method has been in the centre of interest [14].…”