Quantum Cascade Laser Spectroscopy
375
A room-temperature operated EC-QCL with a mode-hop-free tuning range of
1257–1341 cm
−1 was exploited in this study to probe the rotational R (27) line of
N 2 O centered at 1306.929 cm
−1 arising from fundamental ν 1 band. The potential of
QCL along with its coupling with high finesse cavity enabled to achieve enabled them
to get high-precision and highly sensitive N 2 O breath concentration measurement
for discrimination between infected and non-infected subjects.
5 QCL in High-Resolution Spectroscopy
The emergence of external-cavity (EC) continuous-wave (cw) Quantum Cascade
lasers (QCL) paved the way for high-resolution spectroscopic research. Its access to
the MIR region can probe the fundamental and combinational bands of several important molecules selectively. However, those bands were previously not accessible with
diode lasers. Important spectroscopic parameters e.g. line-strength, temperaturedependant exponent, broadening coefficients can be measured with greater accuracy by QCL based spectroscopic techniques that provide high-precision and highresolution measurements. In 2004, Joly et al. did the investigation of spectroscopic
studies of the ν 1 band of SO 2 by employing continuous-wave DFB QCL at 9.1 μm
[72]. In this study, line-intensities and self-broadening coefficient values of transition
lines were experimentally studied and subsequently were validated by previous study
and theoretical prediction. In another work, researchers also studied the pressureinduced line-shift of transition lines belonging from the n 3 fundamental band of
ozone by analyzing high-resolution spectra using DFB-QCL at 9.54 μm in presence of other foreign gases: oxygen (O 2 ), air and the noble gases helium (He),
argon (Ar), and xenon (Xe) [73]. However, QCL-based spectrometer has also been
exploited to investigate the high-resolution infrared spectroscopic studies of longchain molecule isoprene (2-methyl-1,3-butadiene) near 992 cm
−1 [74]. The potential of QCL enabled to assign transition lines originated from ν 17 fundamental band
and several hot bands of ν 26 and ν 17 by recording highly resolved spectra . Moreover, Maithani et al. exploited a widely tuneable EC-QCL in the mid-infrared region
around 6.2 μm to perform a high-resolution analysis of the ro-vibrational fine spectra
of the gaseous 1,3-butadiene molecule for experimentally determination of important
spectroscopic parameters [75]. Furthermore, the potential of QCL to record highly
resolved spectra were utilised in Dutta Banik et al. by measuring l-type doubling
spectra of the weak hot band of OCS near 5.2 μm spectral region [76] (Fig. 7).
Recently, the group of Pradhan and Biswajit used a mid-IR cw-QCL-based CRD
spectrometer at 7.8 μm to evaluate important spectroscopic parameters such as
temperature-dependent exponent (n) along with line-intensity, air-broadening coefficient of transition lines of
13 C-isotopomer of CH 4 by probing ν 4 fundamental band
[77]. In another work, the same room-temperature operating EC-QCL coupled CRDS
spectrometer has been exploited to record high-resolution spectra of several P and Q
branch lines of combination band of C 2 H 2 molecule [78]. Transition lines from both
375
A room-temperature operated EC-QCL with a mode-hop-free tuning range of
1257–1341 cm
−1 was exploited in this study to probe the rotational R (27) line of
N 2 O centered at 1306.929 cm
−1 arising from fundamental ν 1 band. The potential of
QCL along with its coupling with high finesse cavity enabled to achieve enabled them
to get high-precision and highly sensitive N 2 O breath concentration measurement
for discrimination between infected and non-infected subjects.
5 QCL in High-Resolution Spectroscopy
The emergence of external-cavity (EC) continuous-wave (cw) Quantum Cascade
lasers (QCL) paved the way for high-resolution spectroscopic research. Its access to
the MIR region can probe the fundamental and combinational bands of several important molecules selectively. However, those bands were previously not accessible with
diode lasers. Important spectroscopic parameters e.g. line-strength, temperaturedependant exponent, broadening coefficients can be measured with greater accuracy by QCL based spectroscopic techniques that provide high-precision and highresolution measurements. In 2004, Joly et al. did the investigation of spectroscopic
studies of the ν 1 band of SO 2 by employing continuous-wave DFB QCL at 9.1 μm
[72]. In this study, line-intensities and self-broadening coefficient values of transition
lines were experimentally studied and subsequently were validated by previous study
and theoretical prediction. In another work, researchers also studied the pressureinduced line-shift of transition lines belonging from the n 3 fundamental band of
ozone by analyzing high-resolution spectra using DFB-QCL at 9.54 μm in presence of other foreign gases: oxygen (O 2 ), air and the noble gases helium (He),
argon (Ar), and xenon (Xe) [73]. However, QCL-based spectrometer has also been
exploited to investigate the high-resolution infrared spectroscopic studies of longchain molecule isoprene (2-methyl-1,3-butadiene) near 992 cm
−1 [74]. The potential of QCL enabled to assign transition lines originated from ν 17 fundamental band
and several hot bands of ν 26 and ν 17 by recording highly resolved spectra . Moreover, Maithani et al. exploited a widely tuneable EC-QCL in the mid-infrared region
around 6.2 μm to perform a high-resolution analysis of the ro-vibrational fine spectra
of the gaseous 1,3-butadiene molecule for experimentally determination of important
spectroscopic parameters [75]. Furthermore, the potential of QCL to record highly
resolved spectra were utilised in Dutta Banik et al. by measuring l-type doubling
spectra of the weak hot band of OCS near 5.2 μm spectral region [76] (Fig. 7).
Recently, the group of Pradhan and Biswajit used a mid-IR cw-QCL-based CRD
spectrometer at 7.8 μm to evaluate important spectroscopic parameters such as
temperature-dependent exponent (n) along with line-intensity, air-broadening coefficient of transition lines of
13 C-isotopomer of CH 4 by probing ν 4 fundamental band
[77]. In another work, the same room-temperature operating EC-QCL coupled CRDS
spectrometer has been exploited to record high-resolution spectra of several P and Q
branch lines of combination band of C 2 H 2 molecule [78]. Transition lines from both