Comparison of cw and pulsed operation with a TE-cooled quantum cascade infrared laser for detection of nitric oxide at 1900 cm-1

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

“…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].…”

“…While a theoretical description for the non-linear absorption phenomena based on optical Bloch equations exists [25,26,28], the origin of distorted line shapes in the short-pulse mode is not yet completely understood [9]. The problem is often empirically minimised by finding a compromise between a reasonable signal-to-noise ratio (SNR) and a narrow spectral width [12]. The sensitivity of spectrometers or quantitative results are typically verified by calibrating an effective laser line width.…”

“…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].…”

“…Consequently, Nelson et al concluded that a QCL should be operated close to its threshold with short pulses close to the uncertainty broadening limit with rise times faster than 8 ns achieved with the original Alpes Lasers operation unit. The investigation of asymmetrically broadened absorption lines was continued by McManus et al since such absorption features revealed another phenomenon: the peak absorption signal decreased as the distortion became more pronounced [12]. In these experiments the operating voltage was varied at constant pulse width.…”